Doc Brown's Advanced A Level Organic Chemistry Revision Notes - Help in Revising Advanced Organic Chemistry

PART 14 ORGANIC ISOMERISM and Stereochemistry Revision Notes

Part 14.4 Notes on protein structure, enzyme structure and function and metalloenzymes

Case studies of structure, formation, properties and consequences

INDEX of isomerism & stereochemistry of organic compounds notes

Forward to section 2b.6 and glimpse of the world of biochemistry! E. coli bacteria contain about over 5000 different compounds, most of which are macromolecules, including over 3000 proteins/polypeptides (over 100000 in humans) and about 1000 different nucleic acids in the form of RNA and DNA.   All bar a small % of proteins are built from the same 'pool' of 20 alpha amino acids in the cytoplasm of cells, despite the fact that 150 other amino acids have been found in living organisms too.   Over 2000 enzymes are known and several hundred proteins have been crystallised and their detailed 3D structure determined by x-ray crystallography.  It has estimated that a polypeptide of M_r 34000, built from 12 different amino acids can have 10³⁰⁰ different sequences, now that's what you call isomerism!  All of the DNA and RNA nucleic acids are built from just 8 nucleotides (phosphate-sugar-base 'monomers') and are all constructed with the help of enzymes.  All the thousands of different reactions keeping an organism going are specific and efficient from cell division to respiration.  So, to finish where we started, what can be the cause of a very nasty, and potentially fatal, tummy bug, is a very different chemical world from refluxing a halogenoalkane with sodium hydroxide to make an alcohol in the laboratory, but its a bit too much to expect us to intellectually marvel at the wonders of biochemistry when we are feeling very sick!

The importance of proteins in living organisms cannot be over-emphasised e.g. their existence as the hormone insulin and enzyme molecules ('biological catalysts') or in muscle/membrane/hair structure and even the oxygen transporter haemoglobin which is a protein-Fe-O₂ complex when red!, and now the growing area of biotechnology to manufacture a whole range of chemical products. In the form of enzymes, they are highly reactive and selective towards substrate molecules, catalyzing a multitude of specific chemical reactions often in complex multi-stage sequences.

The specificity and sensitivity of enzymes: The structure of enzyme-protein molecules is described in various ways below and the very complex and specific nature of each enzyme structure explains why they will only interact with specific substrate molecules. The descriptions below also explain an enzymes sensitivity to increased temperature or pH changes. Please remember, they only increase the kinetics of the reactions, and, although they decrease the activation energy, they CANNOT change the enthalpy of a reaction or the % yield if an equilibrium is formed.

An example of the stereospecificity of enzymes is the action of the fumarase in converting trans-butenedioic acid (fumaric acid) into 2-hydroxybutanedioic acid (malic acid).

HOOC-CH=CH-COOH + H₂O == fumarase ==> HOOC-^*CH(OH)-CH₂-COOH

2-hydroxybutanedioic acid has a chiral or asymmetric carbon (^*C), so two optical isomers (enantiomers) are possible, BUT only L enantiomer is formed, and non of the D enantiomer. Not only is the product a stereospecific optical isomer, the reactant is it self a stereospecific geometrical isomer!

Protein Structure

Primary structure: The order of the amino acids in the polypeptide chain (the sequence of residues). Polypeptide synthesis-formation is illustrated in a later section. The primary structure sequence only involves covalent bonding i.e. the condensation reaction involves the formation of the polypeptide or polyamide linkage -NH-CO- to give a natural condensation polymer (since a small molecule, water, is eliminated in the process).

Secondary structure: The coiling of the polypeptide chain into an a-helix or the formation of a b-pleated sheet of protein is stabilised by hydrogen bonding (see below).

Tertiary structure: This is the full 3D shape (conformation) of the protein is produced by the folding of the secondary structure (alpha-helix or beta-sheet) and stabilised by four types of inter-molecular force or chemical bonding usually between side chain 'R' groups (see examples below).

Quaternary structure: In some cases, single protein molecules can join together to give a larger unit e.g. , insulin can exist as a monomer, dimer (insulin)₂ or a hexamer (insulin)₆. The quaternary structure of dimers, trimers etc. is usually held together by complexing with a metal ion (in the insulin hexamer, the insulin acts as a ligand forming dative covalent bonds with a Zn²⁺ ion) or intermolecular forces e.g. hydrogen bonding (haemoglobin has 4 components).

Four types of interaction are important in holding together the secondary and tertiary structure. In order of strength they are instantaneous dipole-induced dipole (Van der Waals) < permanent dipole-permanent dipole (usually hydrogen bonding) < ionic or covalent bonding. Examples are briefly described below AND comments added on how these interactions may be changed to affect enzyme function and efficiency with respect to increase in temperature or pH change. See also a discussion of enzyme kinetics - rates of reaction

Secondary or tertiary structure of proteins - denaturing (and enzyme inhibition)

Examples of secondary or tertiary structures are described below and references to denaturing 

1. Instantaneous/transient dipole-induced dipole forces

Non-polar Van der Waals forces, hydrophobic interactions in the presence of water.

These forces arise from the random behaviour of the electron fields in atoms producing minute transient dipoles ^δ+molecule^δ- which in turn induce ^δ+molecule^δ- dipoles in neighbouring molecules. They are the weakest of the intermolecular forces, but here we are not talking molecule-molecule interaction (inter-molecular forces), but within one big molecule we are talking molecular section-section interaction, and the same argument applies for 2. below.

The centres of protein molecules often have amino acids with hydrocarbon sections (e.g. the side chains of leucine, isoleucine and phenylalanine shown below). These non-polar side chains in the polypeptide do not affect hydrogen bonding of the protein with surrounding water molecules or polar substrate molecules.

The residues (Leu, Ile etc.) are the parts of the original amino acids which are used to form the protein chain i.e. after loss of H₂O from -COOH + H₂N- to give linked adjacent amino acid residues.

(a) leucine, 2-amino-4-methylpentanoic acid,

  residue Leu, (CH₃)₂CHCH₂CH(NH-)CO-, 

The red - covalent bonds connect one residue via the CO/NH link to the next amino acid residue, the polypeptide link between two residues is NH-CO and the non-polar side chain is shown in blue.

(b) isoleucine, 2-amino-3-methylpentanoic acid,

residue Ile, CH₃CH₂CH(CH₃)CH(NH-)CO-

(c) phenylalanine, 2-amino3-phenylpropanoic acid,

residue Phe, C₆H₅CH₂CH(NH-)CO-

and remember the -NH₂ and -COOH on the right are used in forming the -NH-CO- peptide linkage, so concentrate on the left-hand side i.e. the 'side chain', and do the same for the other examples below.

These weak instantaneous dipole-induced dipole force interactions will be the first to be weakened and disrupted on increase in temperature and beginning the thermal 'denaturing' or 'degradation' of the tertiary 3D structure of an protein, including enzymes. Once the conformation is changed the enzyme is catalytically inactive.

Note: Most enzymes are 'globular' in shape and the hydrophobic non-polar side-chain groupings (above in section 1.) tend to align alongside each other towards the centre of the 3D conformation. If not involved in holding the tertiary structure, the hydrophilic polar side-chain groupings (below in section 2.) tend to be directed towards the outer 'surface' to interface with the aqueous media the enzyme operates in. So each type of intermolecular force does not interfere with the others function.

2. Polar bonds and hydrogen bonding

Polar permanent dipole-permanent dipole intermolecular forces, hydrophilic interactions in the presence of water.

Permanent dipoles arise when two atoms of a bond in a molecule have different electronegativities. Electronegativity is a measure of the electron attracting power of an atom in a covalent bond situation, and in polar bond, the more electronegative element attracts more of the bonding electron cloud to give an asymmetric distribution i.e. a permanent dipole. These dipoles occur between the carbonyl group ^δ+C=O^δ- and the secondary amide group ^δ+H-N^δ- of one peptide linkage (-NH-CO-) and the peptide linkage in the same chain or another chain. There are also H-bond interactions between proteins and water in an aqueous media. The hydrogen bond,, is the strongest of the intermolecular forces arising from the big electronegativity difference between O/N and H in these cases (electronegativity of O/N > H). It is NOT a chemical bond like an ionic or covalent bond but it is the strongest of the permanent dipole - permanent dipole interactions between molecules OR sections of a molecule, i.e. hydrogen bonding is the strongest of the intermolecular forces.

[1] Hydrogen bonding between an alcohol group in the side chain of an amino acid residue and water surrounding a protein molecule.

[2] Hydrogen bonding between an alcohol group in the side chain of an amino acid residue and an amine group in a side-chain.

[3] Hydrogen bonding between a carbonyl group and water with a protein in an aqueous media.

[4] Hydrogen bonding between the carbonyl group of one peptide linkage and the secondary amide group of another polypeptide linkage. The link could be between 'folds' in the same polypeptide chain e.g. the alpha-helix form (H-bond every 3-4 residues holding the coils of the helix in place, see denaturing diagram) OR between two separate polypeptide chains held together e.g. beta-sheets of protein material, structure shown schematically below.

Polypeptide chains held together to produce the '2D' arrangement of a beta-sheet of protein. The structural formula, the bond polarities and the hydrogen bonds are shown. The variable R side-chain grouping is alternately above and below the '2D planarity' of the sheet which extends in the dimensions of the screen/paper printout (left-right covalent bonding, up-down hydrogen-bonding). The sheets are quite strong and you would find this sort of arrangement in collagen or muscle tissue.

A simpler diagram of the hydrogen bonding via the skeletal formula (the H's to the N must be shown in the skeletal formula to show the hydrogen bonds).

The ....^δ+C=O^δ-....^δ+H-N^δ-.... intermolecular force interaction is the most important hydrogen bond that holds together the secondary structure of proteins to give the sheet and helical structures AND about 50% of the hydrogen bonds that hold together the single helix structure of RNA molecules and the double helix structure of DNA molecules.

These permanent dipole interactions can also occur between polar side-chain groups as well as the polypeptide -NH- and -CO- links (see alpha-helix below and beta-sheet above) and usually involve hydrogen bonding between neighbouring highly polar groups. For example, ^δ-O-H^δ+, ^δ-NH₂^δ+ or ^δ+C=O^δ- groups in the side chains of serine, threonine, asparagine and glutamine.

look for the possibility of  ^δ-O-H^δ+||||^δ-O=C^δ+, ^δ-O-H^δ+||||^δ+H-N^δ-, ^δ-N-H^δ+||||^δ-O=C^δ+, ^δ-O-H^δ+||||etc.

(a) serine, 2-amino-3-hydroxypropanoic acid

residue Ser, -O^δ-||||H^δ+-O^δ-CH₂CH(NH-)CO-

the purple |||| shows just one exemplar hydrogen bond, H^δ+-O^δ-|||H^δ+-O^δ-, and it could involve any of the amino acid residues described below, H-O in another part of the polypeptide on a side chain.

The red - covalent bonds connect one residue via the CO/NH link to the next amino acid residue, the polypeptide linkage between two residues is NH-CO.

(b) threonine, 2-amino-3-hydroxybutanoic acid

residue Thr, CH₃CH(OH)CH(NH-)CO-, with an H-O on a side chain

(c) asparagine,

residue Asn, H₂N-C(=O)CH₂CH(NH-)CO-, with a H₂N or C=O on a side chain

(d) glutamine,

residue Gln, H₂N-C(=O)CH₂CH₂CH(NH-)CO-, with a H₂N or C=O on a side chain

(e) aspartic acid, 2-aminobutanedioic acid,

residue Asp, HOOCCH₂CH(NH-)CO-, with an OH or C=O on a side chain

(f) lysine, 2,6-diaminohexanoic acid,

residue Lys,  H₂NCH₂CH₂CH₂CH₂CH(NH-)CO-, with a H₂N on a side chain

Again, like the transient dipole-induced dipole forces, these stronger, BUT still relatively weak hydrogen bonding inter-molecular forces, will be increasingly weakened/broken as the temperature is raised. The 'effective' 3D protein structure (conformation) is 'denatured' as more hydrogen bonds get broken. Both secondary and tertiary structure is destroyed but not the strong covalent bonds of the primary structure.

The possible effect of pH change via acids and alkalis (other 'denaturing agent') on hydrogen bonding and ionic bonds, is conveniently described in section 3. below, but please remember the above hydrogen bonds are NOT chemical bonds!

The 'schematic' diagram below shows the denaturing effect on the secondary and tertiary structure of proteins, particularly by the disruption of hydrogen bonds. The increased thermal agitation of the molecule is sufficient to break most of the hydrogen bonds holding together the 3D tertiary protein structure, but NOT the primary structure of the amino acid residues, which is dependant on the strong covalent bonding of the polypeptide -NH-CO- linkages. The denaturing of a protein can lead to its precipitation or coagulation (cooking egg white). However in some cases on mild heating, the 3D tertiary structure is 'unwound' and reforms on heating so the randomised and stereospecific conformations are reversible. This means that the primary 1D structure of amino acid residue sequence offers the 'blueprint' of a thermodynamically stable and specific 3D tertiary conformation at lower temperatures, typically <50^oC.

1. Alpha-helix conformation in protein-polypeptide structure

2. Enzyme tertiary structure destroyed by e.g. heating which breaks disulphide linkages and hydrogen bonds

3. The full 3D structure of enzyme intact and active site 'enabled'.

Other denaturing 'agents', which particularly affect the hydrogen bonding of the tertiary structure e.g. hydrogen bonded alcohol competes with side-chain hydrogen bonding of the proteins structure, hence its use as a disinfectant by denaturing the enzymes in bacteria. Other tertiary structure denaturing 'agents' include detergents and acids/alkalis and pH change are discussed below in sections 3. ionic bonding, and 4. covalent bonding and reference back to this diagram will help follow the ideas.

3. Ionic bonding

This can occur between ionisable side chains e.g. a carboxylic group in one side chain can donate a proton to an amino group in another side-chain so that ionisation occurs and the formation of an ionic bond via the attraction of the positive and negative ions. It is a strong bond and contributes significantly to the maintenance of the 3D tertiary structure i.e.

R'-NH₂ + HOOC-R  R'-NH₃⁺ + ^-OOC-R

However if the pH is changed by making the enzyme's medium more acid or more alkaline, then these ionic bonds can be disrupted i.e. the protein can be 'denatured' becoming less effective. The equations below illustrate the acid-base behaviour of side-chains (and polypeptide chain terminal functional groups) and the possible disruption of ionic bonds or hydrogen bonding will affect the protein structure, hence affect an enzymes function. In passing, they also illustrate how proteins can act as buffering agents (buffers minimise pH change if small amounts of acids/alkalis are added to a system). Note: the equations/structures only show a single residue and the chemical changes of the side-chain e.g. 

 (a) aspartic acid, 2-aminobutanedioic acid, residue Asp

which with alkali, HOOCCH₂CH(NH-)CO- + OH^{- -}OOCCH₂CH(NH-)CO- + H₂O

so increase in pH (more alkaline) could disrupt a hydrogen bond involving the HOOC group,

or with acid, ^-OOCCH₂CH(NH-)CO- + H⁺ HOOCCH₂CH(NH-)CO-

so decrease in pH could disrupt an important ionic bond.

The red - covalent bonds connect one residue via the CO/NH link to the next amino acid residue, the polypeptide link between two residues is NH-CO and the -COOH or -NH₂ in the side chain.

(b) glutamic acid, 2-aminopentanedioic acid, residue Glu,

with alkali, HOOCCH₂CH₂CH(NH-)CO- + OH^{- -}OOCCH₂CH₂CH(NH-)CO- + H₂O

so increase in pH could disrupt a hydrogen bond involving the HOOC group,

or with acid, ^-OOCCH₂CH₂CH(NH-)CO- + H⁺ HOOCCH₂CH₂CH(NH-)CO-

so decrease in pH could disrupt an important ionic bond.

(c) lysine, 2,6-diaminohexanoic acid, residue Lys, 

with acid, H₂NCH₂CH₂CH₂CH₂CH(NH-)CO- + H^{+ +}H₃NCH₂CH₂CH₂CH₂CH(NH-)CO-

so decrease in pH could disrupt a hydrogen bond involving the H₂N group.

with alkali ⁺H₃NCH₂CH₂CH₂CH₂CH(NH-)CO- + OH^- H₂NCH₂CH₂CH₂CH₂CH(NH-)CO- + H₂O

so increase in pH could disrupt an important ionic bond.

Salt bridge: This term should only be used in the context of ion solution of e.g. KNO_3(aq) or NH₄NO_3(aq) to complete the circuit in an electrochemical cell. Unfortunately, the term is also used in biochemistry to describe the ionic bond above. Perhaps the term salt bridge bond or an ionic bridge bond  would be better? I've also come across the term used to describe cations like Na⁺_(aq) loosely holding together proteins with a negative group in a side chain but I can't find any specific examples? Perhaps it is has been used to describe the zinc ion, Zn²⁺_(aq), holding six insulin protein units together to form the quaternary hexamer. Here, a polar group atom (lone pair donor) from each insulin molecule ,forms a dative covalent bond with the zinc ion to form a complex ion, [Zn(:insulin)₆]²⁺_(aq).

4. Covalent bonding (usually disulphide linkage)

One common example is the S-S bond and occurs three times in the hormone insulin, where two S-S bonds connect two polypeptide chains together. The amino acid containing the original S-H bond is cysteine and the sulphur-hydrogen groups of two neighbouring cysteine residues can be oxidised to give the S-S bond. 

cysteine, residue Cys, HS-CH₂-CH(NH)-C=O

so in terms of the two residues, Cys, the disulphide bridge link is formed on oxidation ...

2 HS-CH₂-CH(NH-)-CO- + [O] ==>  -CO-CH(NH-)-CH₂-S-S-CH₂-CH(NH-)-CO- + H₂O

The red - covalent bonds connect one residue via the CO/NH link to the next amino acid residue, the polypeptide link between two residues is NH-CO and SH (thiol) and -S-S (disulphide link) via the side chain.

Cofactors and enzyme function

Although some enzymes can function without additional chemical species, many enzymes (E) require one or more extra components known as co-factors (C) to be catalytically active when the enzyme is bound to the substrate (S) to convert it into products (P). There are different types of cofactors, non of them are proteins and most are either regenerated if chemically changed in the process, or chemically unchanged at the end of the reaction.

The terms cofactor and coenzyme seem to be loosely interchangeable on the web and in textbooks.

Incidentally, C can also be an inhibitor/deactivator as well as an 'promoter/activator/effector' for an enzyme and this duality in behaviour can be used to regulate the chemistry in living organisms using cofactors.

(i) Coenzymes: These cofactors are non-protein organic molecules which bind reversibly to the enzyme, unlike prosthetic groups. They are often vitamin or nucleotide derivatives and function as carriers to transfer atoms or functional groups from the enzyme to a substrate.

in principle: E + C EC + S ECS EC + P

Metal ions such as Zn²⁺, Mg²⁺, Mn²⁺, Fe^2+/3+, Cu^+/2+, are often described as co-enzymes themselves. These ions may be needed to provide an electrical charge to hold a substrate (via dative covalent/ionic bonds)  or participate in an electron transfer process involving a change in oxidation state (Cu or Fe). They can serve as (a) part of the principal catalytic site, (b) a bridging group to bind enzyme and substrate together covalently/ionically, or (c) an agent stabilizing the conformation of the enzyme protein in its catalytically active form, again via covalent/ionic bonds. Enzymes/proteins requiring metal ions are sometimes called metalloenzymes/metalloproteins.

(ii) Prosthetic groups: These non-protein organic cofactor groups which are strongly bound (effectively irreversibly) to the protein.

in principle: E-C + S E-CS E-C + P

Sometimes the prosthetic groups can reversibly or irreversibly form complexes with a metal ions such as those listed above. e.g. The enzyme catalase is very effective in decomposing hydrogen peroxide to water and oxygen. The active site partly consists of a cofactor combination of the a prosthetic group (porphyrin) co-ordinated with a metal ion of iron. Iron is a classic transition metal and the reaction involves changes in ligand structure/co-ordination, oxidation state as well as HO-OH bond breaking. Other catalysts promote this decomposition without the enzyme BUT the enzyme-cofactor complex is far more efficient. The uncatalysed reaction has an activation energy (E_a)  of about 80kJmol^-1, catalysts like iron salts/MnO₂/Pt reduce it to about 40-50kJmol^-1 but catalase reduces E_a to <10 kJmol^-1 and it works remarkably fast at room temperature Catalase and other peroxidases save our cells from the perils of a free and unwelcome powerful oxidising agent! Remember a catalyst provides a different reaction pathway that lowers the activation energy, helps bond breaking and so speeds up the reactions considerably at room temperature  but anything we create artificially in the laboratory ain't a patch so far on 'mother nature'!

The inhibition of enzyme activity

(These are GREATLY SIMPLIFIED DESCRIPTIONS, and do not cover all possibilities e.g. many enzyme catalytic processes involve two substrates but they only get the briefest of mention. A full analysis and description requires up to 2nd year undergraduate level knowledge, but I hope to ILLUSTRATE via EXAMPLES the use of the terms irreversible, reversible, non-competitive, uncompetitive and allosteric in an enzyme inhibition, any comments, as ever, would be appreciated.)

Enzymes inhibitors (I) in some way reduce, or completely stop, the catalytic activity of an enzyme. Various scenarios are described below and their descriptors may well overlap to describe a particular inhibition situation completely. To appreciate the points below, its advisable to revise the structure and function of an enzyme. Many successful pharmaceutical products are enzyme inhibitors.

KEY: E = enzyme (including any co-factor), S = substrate, P = product(s), ES = enzyme-substrate complex, EP = enzyme-product complex, I = inhibitor bound to active centre or some other part of the enzyme, EI = enzyme-inhibitor complex and EIS = enzyme-inhibitor-substrate complex. The active site/centre is the part of the enzyme where the catalyzed reaction takes place. A non-active site (with respect to S==>P) site may allow binding of an inhibitor or regulator molecule.

• Competitive inhibition: ES versus EI(active site) complex formation.

  • (i) E + S ES EP E + P
    • Product formed.
  • (ii) E + I EI_{(active site)}
    • The competitive and reversible binding of inhibitor to the active site.
  • A competitive inhibitor is a similar shaped molecule^* compared to the substrate. It binds reversibly to the enzymes active site-centre via (ii), and so preventing the binding of the substrate molecule in sequence (i). Therefore the substrate and inhibitor compete for the active site of the enzyme i.e. sequence (i) versus reaction (ii). Increasing the concentration of a substrate can increase its chances of binding with the active site and decrease the effect of the inhibitor (Le Chatelier's equilibrium principle). This is an example of reversible inhibition. ^*A competitive inhibitor can be described as 'substrate analogue'.
  • Example 1: In the aerobic tricarboxylic acid (citric acid/Krebs) respiratory cycle, the enzyme succinate dehydrogenase helps oxidise (by H loss) [1] the  butanedioate ion (succinate) into the trans-butenedioate ion (fumarate).
    • ^-OOC-CH₂-CH₂-COO^- -2[H] ==> ^-OOC-CH=CH-COO^-
    • (its actually: succinate + E-FAD <==> fumarate + E-FADH₂ where FAD is a cofactor-coenzyme functioning as a hydrogen acceptor or 'oxidiser' and getting reduced in the process)
    • In the catalytic process the butandioate ion binds to the active centre of E via the two negative oxygens and succinate dehydrogenase is inhibited by many ions which have a similar stereochemical structure to it e.g.
    • [2] propanedioate ion (malonate),  [3] ethanedioate ion (oxalate, well know poison!), and [4] the pyrophosphate(V) ion.
    • 
  • Example 2: ?
    • ?

• Non-competitive/uncompetitive/irreversible inhibition:

  • (i) E + S ES EP E + P
    • Sequence (i) leads to product formation.
  • The formation of EI or EIS complex instead of EP.
  • (ii) E + I EI_{(non-active site)}
    • In reaction (ii) the inhibitor binds in reversible non-competitive inhibition, NOT competitive.
  • (iii) ES + I ESI_{(non-active site)}
    • Reaction (iii) is non-competitive and uncompetitive reversible inhibition.
  • (iv) E + I E-I_{(active/non-active site)}
    • Reaction (iv) is irreversible inhibition, the inhibitor strongly binds strongly to the active or non-active site on the enzyme.
  • Noncompetitive inhibition involves reactions (ii) and (iii) versus sequence (i), where I binds to a site (NOT the 'active site') on the enzyme to give EI AND can also bind to ES to give ESI. There is NO competition between the substrate and inhibitor for the active site of the enzyme. In other words the inhibitor does not have to be stereochemically like the substrate molecule to bind to the active site (no need to be the key for the lock!) but its effect on the conformation* prohibits the transformation of ESI into products. * essentially the tertiary structure.
    • Note (a) In two substrate systems the binding of I and S₁ to give ES₁I can prevent the binding of S₂ to form ES₁S₂ needed to form the product,
      • OR ES₁ might be formed and I and S₂ compete for the 2nd active site to give ES₁I (no product) or ES₁S₂ (can give product). I'm afraid descriptors can get quite mixed in inhibition.
    • Note (b): In an example of so called uncompetitive inhibition, ONLY involves  reaction (iii) versus sequence (i). I can bind to the ES complex (but NOT to the free enzyme on its own) to give an ESI enzyme-substrate-inhibitor complex that cannot breakdown to form products. It seems that the binding of the substrate changes the enzyme conformation to allow the binding of the inhibitor. The inhibitor doesn't stop the substrate binding BUT the conformation changes prevent the S => P transformation in the ESI complex.
      • In a two substrate reaction, ESI formation can prevent the binding of the 2nd substrate to produce ES₁S₂, required to breakdown into E and P.
  • The term allosteric inhibition applies to the situation where an inhibitor binds to an enzyme somewhere other than the active site (but another stereospecific or allosteric site), and in doing so, changes the 3D conformation around the active site. This prevents either the substrate binding to the active site and being converted into product or allowing ESI to form products. So non-competitive inhibition is a form of allosteric inhibition.
    • Note - this situation can work in a proactive way i.e. an activator or modulator molecule (C), a  cofactor, rather than an inhibitor (I), can bind with the enzyme changing the conformation of the active site to enable an ECS complex to form that can breakdown to give products. These activation/deactivation processes are very important in regulating many reactions in living organisms.
  • In irreversible inhibition (iv) only, the inhibiting molecules or ions bind, usually covalently (E-I), with the active site, or to some other part of the enzyme structure and is not easily removed. Therefore, the enzyme is permanently changed/deactivated by completely 'blocking' the active site OR changing the enzymes conformation so it cannot perform its 'lock' function (the allosteric site effect). The rate of the permanent inhibition reaction (iv) will initially compete with the rates of sequence (i) but progressively the enzyme system will cease to function. Therefore the total enzyme function is inhibited and cannot be restored by adding excess substrate as in competitive inhibition.
• • EXAMPLES to ILLUSTRATE inhibition, mainly non-competitive and irreversible.
  • Example 1: Heavy metal ions.
    • Cations such as lead (Pb²⁺), silver (Ag⁺) or mercury (Hg²⁺) can strongly bind irreversibly with the main polypeptide chain and permanently change the enzyme conformation.
      • Catalase catalyses the decomposition of hydrogen peroxide.
      • 2H₂O_2(aq) ==> 2H₂O_(l) + O_2(g)
      • Redox analysis: H only ox. state (+1), but for oxygen the changes are: (-1) in H₂O₂ to (-2) in H₂O and (0) in O₂.
      • Catalase is a heme enzyme involving an iron(III) ion co-ordinated in the active centre. The mechanism is not completely understood but is reported as involving iron(III) and iron(IV) complexes in a redox catalytic cycle typical of homogeneous transition metal catalysis. Addition of iron salts to the enzyme-peroxide mixture can enhance the rate of reaction (promotion) but addition of copper(II) ion salts reduce the reaction rate (inhibition). Whether the Cu²⁺ ions bind to the -NH- in the primary polypeptide chain or a side chain  COO^- group, I don't know?, but whatever, some part of the protein structure will be acting as a ligand coordinating with the Cu²⁺ ion to change the conformation of the enzyme. This change in shape renders the enzyme ineffective. It is non-competitive inhibition because it doesn't bind with the active site, but I'm not sure whether it is reversible or not?
      • Its also possible that metal ions can replace the 'essential ion' at the active site which themselves cannot perform the essential electron transfers and oxidation state changes.
    • Zn²⁺, Ca²⁺, Hg²⁺, Pb²⁺ can react irreversibly with the thiol group (-SH) in the side-chain R group of polypeptides in a non-'active site' on an enzyme, particularly those in the nervous system.
    • Heavy metal ions can form insoluble salts with proteins via anionic carboxylic groups in the side-chains (or terminal) which would not only change their conformation but precipitate them from cell fluids. In doing so they may well break ionic bridging bonds (polypeptide-NH₃^{+ -}OOC-polypeptide) important for maintaining secondary and tertiary structure i.e. the enzyme protein is denatured.
      • This reaction can be used to treat heavy metal poisoning e.g. if a heavy metal ion solution was accidentally ingested, you could be given (hopefully rapidly!) a protein solution like milk or egg-white to form the heavy metal ion-salt and then swallow an emetic to make you vomit the offending material.
    • Silver nitrate and mercury salts are used as a disinfecting agent because they inhibit/denature enzymes in bacteria.
    • Cadmium (Cd²⁺) and mercury (Hg²⁺) can replace Zn²⁺ in the cofactor group in of alkaline phosphatase which hydrolyses esters of phosphoric acid (not sure whether this is a reversible competitive example?).
  • Example 2: The enzyme cytochrome c oxidase is important in electron transfer (redox) systems of cell respiration. The enzyme contains a co-factor (prosthetic complex ion group) involving the oxidation state change of Cu(I) <==> Cu(II) + e^-.
    • Its function is inhibited by cyanide ion (CN^-) which acts a ligand and covalently binds irreversibly to the copper ion group interfering with the function of the active site and inhibiting the electron transfer via the Cu oxidation state change. Hence it interferes with respiration and rather fatally "My dear Watson!" If ingested and a suitable anti-dote is rapidly taken, which traditionally in laboratories was (is?), I gather, a horrible tasting iron solution which forms a complex with the cyanide ion before reaching the enzymes, you may survive!
  • Example 3: Aspirin acting as a non-steroid anti-inflammatory agent
    • Aspirin inhibits an enzyme called cyclooxygenase-2 (also known as COX-2 or PHG₂ synthase) which is partly responsible making prostaglandins which stimulate the inflammatory responses of pain and fever etc. The chemistry, in principle!, is quite simple. 2-ethanoylhydroxybenzoic acid (acetylsalicylic acid or aspirin!) is hydrolysed in such a way that the ethanoyl group (CH₃CO) is transferred from the aspirin to esterify the HO- (ol or hydroxy) group in the side-chain of a serine amino acid residue (Ser).
    • HOOCC₆H₄OCOCH₃ + HO-Ser-E ==> HOOCC₆H₄OH + CH₃COO-Ser-E
    • The reaction is irreversible and the change in conformation* of the enzyme makes it inactive, so reducing the sequence of chemical events that leads to feelings of pain etc., but don't worry, your body eventually re-synthesises the necessary enzymes to maintain your sensory warning systems! *The change in conformation could arise from disruption of a hydrogen bond or the more bulky ethanoyl group 'pushing' neighbouring groups of atoms further apart.

  • Example 4: Certain organic molecules used as nerve poisons for warfare and insecticides.
    • Some organofluorophosphorus molecules like diisopropylfluorophosphate, covalently bind with acetylcholinesterase and completely deactivate an enzyme essential for the function of the nervous system, e.g. nerve impulses are blocked and you lose body control. The now banned nerve gas Sarin, binds with the -OH group on a serine amino acid residue (Ser) on the active site, so deactivating the enzyme, BUT it does not bind to 27 other Ser residue sites on the enzyme! You die from respiratory failure in a few minutes via fatally accurate stereochemistry, Sarin is a biochemical 'sniper'!
      • basically: Sarin-F + H-O-Ser-E ==> Sarin-O-Ser-E + HF
  • Example 5: Fungal toxins.
    • Chemicals from fungi can irreversibly deactivate RNA polymerase and inhibit crucial synthesis reactions in cell chemistry.
  • Example 6: Complex or chelating agents can bind reversibly to a metal ion essential for catalytic activity.
    • e.g. EDTA, a polydentate ligand, can form a complex with Mg²⁺ (or probably most other M²⁺ ion) and is a case of reversible non-competitive inhibition.
    • [E-Mg]²⁺_(active) + [EDTA]^4-_(aq) + [E]_(inactive) + [Mg-EDTA]^2-_(aq)
    • The Mg²⁺ was itself part of a metalloprotein complex ion, but the removal of the metal ion and change in enzyme ligand status, deactivates the enzyme.
    • The cyanide ion, CN^-, is a monodentate ligand and binds to the iron(III) ion of the active site of catalase (see also above) in competitive inhibition.
  • Example 7: Oxidising and reducing agents.
    • Cells do not generally like ingested 'foreign' strong oxidising or reducing agents as they produce irreversible effects and don't normally form a food constituent.
    • One protein-enzyme structure that is easily affected is the disulphide link (-S-S-) or the thiol group (-SH) in the cysteine amino-acid residue (Cys).
    • Oxidation will form a disulphide bond which would inevitably change the conformation of an enzyme.
      • 2 polypeptide-Cys-SH + [O] ==> polypeptide-Cys-S-S-Cys-polypeptide + H₂O
    • Or, a reducing agent can break a necessary disulphide link needed to maintain the secondary or tertiary structure of the enzyme protein.
      • polypeptide-Cys-S-S-Cys-polypeptide + 2[H] ==> 2 polypeptide-Cys-SH

The effects of temperature increase and pH change and protein denaturing effects are discussed elsewhere on the page.

A very simplified discussion of enzyme kinetics is on the advanced rates of reaction page.

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