Advanced level chemistry kinetics notes: Enzyme kinetic factors explained

AQA Advanced A level Chemistry Edexcel Advanced A level Chemistry OCR Advanced A level Chemistry A OCR Salters Advanced A level Chemistry B

Doc Brown's Advanced A Level Chemistry

Advanced A Level Chemistry

Kinetics-Rates Part 7

Selected Case Studies of a variety of chemical reactions and their rate expressions

7.3 Enzyme kinetics - biological catalyzed reactions

(enzymes being biological catalysts)

Advanced A Level Kinetics Index

Notes on the uses of enzymes and optimum conditions

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced level physical theoretical chemistry students US K12 grade 11 grade 12 physical theoretical chemistry courses topics including kinetics rates of reaction speeds


Case study 4.3 Enzyme kinetics – Biological Catalysts

Explanation and derivation of orders of reactants and how to write the rate expression

doc b

  • KEY in sequence: E = free enzyme, S = free substrate reactant molecule, ES = enzyme–reactant complex, EP = enzyme–product complex, E = free enzyme, P = free product



  • The mechanism of enzyme catalysed:

    • Step (1) E + S ==> ES

      • The 'docking in' of the substrate S of the onto the 'active site' of E to form the enzyme–substrate complex ES. This is often quoted as the rate determining step (rds), but it isn't (see step (2) below!).

    • Step (2) ES ==> EP

      • The conversion of the substrate–enzyme complex ES into the enzyme–product EP, is the rate determining step. The docking in of the substrate to be held e.g. by inter molecular forces is likely to have a low activation energy and a relatively high probability initially,  so E + S ==> ES cannot be the rds. However, ES ==> EP involves bond breaking and will have a much higher activation energy giving a much lower probability of ES  ==> EP transformation.

      • However, [ES] cannot be measured directly, so a simplified kinetics approach is to treat the formation and transformation of ES/EP through to P as a function of the concentrations of E and S (see the rate expressions below, which is a big bad fudge seen here and in most textbooks!). The situation should be dealt with via the Michaelis–Menten equation, but this goes way above AS–A2 chemistry demands.

    • Step (3) EP ==> E + P

      • The breakdown of the EP complex leading to the departure of the product P and leave the enzyme E, as a free 'active site' completing the catalytic cycle.

A possible detailed reaction profile is shown below.

The uncatalysed profile would be a single, and much higher 'hump' (see 2nd simplified diagram below).

doc b

  • Here Ea1 is small for the formation of the enzyme–substrate complex or intermediate ES. The energy of ES is lower than the reactants because it involves an energy releasing interaction whether it be via intermolecular forces or chemical bonding.

  • Ea2 is much larger for the transformation of the ES intermediate into the EP intermediate because it involves bond breaking in the substrate molecule.

    • Ea2 is still much smaller than Ea for the uncatalysed reaction, which is not shown here, but is shown in the simplified diagram below e.g. the uncatalysed decomposition of hydrogen peroxide solution has an Ea of 76kJmol–1, but with an enzyme like catalase/peroxidase, the Ea is reduced to 30kJmol–1 AND the rate of reaction is increased by a factor of 108, impressive biochemistry!!!

    • Ea3 is relatively small for the breakdown of the EP intermediate into the free product and free enzyme.

  • However, the simplified reaction profile diagram below should be good enough in an exam and shows the relevant activation energies.

    • Here, Ea1 is for E+S ==> ES, Ea2 is for ES ==> EP ==> E + P

      • (Ea for EP ==> E + P isn't shown separately)

    • and Ea3 for the uncatalysed reaction, which with no enzyme is significantly greater.

      • My thanks to Professor Martin Chaplin of London South Bank University for his most helpful emails concerning reaction profiles of enzymes.

doc b

  • Treating step (2) as the rds, gives the rate expression ...

    • rate = kES[ES],

    • that is the rate of reaction = rate of transformation of ES into EP .

    • However, since you cannot measure the concentration of the enzyme–substrate complex, the rate is treated as a function of the concentrations of the enzyme and substrate, [E] and [S], which form the ES intermediate, assuming a steady state situation, giving

      • rate = k1[E][S]

      • (Which is a considerably simplified approach if you care to research further in a thick undergraduate textbook of biochemistry and researching the Michaelis–Menten equation!)

    • So we are treating the reaction as 1st order with respect to both enzyme and substrate, 2nd order overall.

    • For a set of experiments, using a fixed amount of enzyme, the initial rate should be proportional to the concentration of substrate at relatively low concentrations, so the effective rate expression is pseudo 1st order ... 

      • rate = k2[S]

    • If you keep the substrate concentration constant and vary the quantity of enzyme, the effective rate expression is also pseudo 1st order ...

      • rate = k3[E]

  • However, if the substrate concentration is very high, the maximum number of active centres on the enzymes are occupied and the rate is completely controlled by the transformation of the ES complex at its maximum possible concentration and independently of the high substrate concentration. This means you are now dealing with the maximum possible rate, which only depends on the amount of enzyme present,

    • so the rate expression becomes zero order with respect to the substrate,

      •  rate = k3[E]

    • and for a constant [E], and varying [S] at high concentrations of S the rate = k4,

    • The two situations are summed up by graph (1) below for a constant amount of enzyme. The graph moves from 1st order kinetics to zero order.

    • Graph (2) is a 'simple' comparison of the effects of no inhibitor, a constant amount of a competitive or a non–competitive inhibitor for a constant amount of enzyme.

      • Vo is the initial reaction rate for a constant amount of enzyme and varying the substrate concentration.

      • Vmax is the maximum possible rate when the maximum number of active centres are occupied with S or P.

      • Competitive inhibition: Although the inhibitor initially decreases the reaction rate, the substrate can increasingly compete with the inhibitor to occupy the 'active sites' as the substrate concentration increases (Le Chatelier's equilibrium concentration principle) so that the maximum reaction rate Vmax can eventually equal Vmax for the non inhibited enzyme.

      • Non–competitive inhibition: Since the inhibitor molecule does not bind to the 'active site' of the enzyme it cannot be 'replaced' by increasing the substrate concentration. However, because it constantly interferes with a fraction of the enzymes, the reaction rate will be reduced giving the lower the Vmax which can never reach Vmax however high the substrate concentration.

      • See also enzyme structure and function notes for more details on inhibition

(1) doc b, (2) doc b

  • The remarkable efficiency of enzymes - biological catalysts are superb!

    • For example the decomposition of hydrogen peroxide - an unwanted harmful chemical produced in the body!

    • 2H2O2(aq)  ==> 2H2O(l)  +  O2(g)

    • Three activation energies (Ea) are quoted

      • (i) uncatalysed: 75 kJmol-1

      • (ii) catalysed with colloidal platinum: 49 kJmol-1

      • (iii) catalysed by the enzyme catalase in you body: 23 kJmol-1

    • The numbers speak for themselves!

    • Both catalysts are effective in considerably reducing the activation energy and hence greatly increasing the speed of the decomposition reaction.

    • BUT, the enzyme catalase is far superior to a class transition metal catalyst!

  • On the (c) doc b GCSE page describing enzyme uses and rates graphs for concentration, pH and temperature changes.

  • On the Isomerism and Stereochemistry Part II page there is a discussion of enzyme structure and function.






Advanced A Level Kinetics Index

Notes on the uses of enzymes and optimum conditions

Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre–university students (equal to US grade 11 and grade 12 and AP Honours/honors level courses) case studies in kinetics: orders of reaction and rate expressions for enzyme kinetics of key and lock mechanism


KS3 BIOLOGY QUIZZES ~US grades 6-8 KS3 CHEMISTRY QUIZZES ~US grades 6-8 KS3 PHYSICS QUIZZES ~US grades 6-8 HOMEPAGE of Doc Brown's Science Website EMAIL Doc Brown's Science Website
GCSE 9-1 BIOLOGY NOTES GCSE 9-1 CHEMISTRY NOTES and QUIZZES GCSE 9-1 PHYSICS NOTES GCSE 9-1 SCIENCES syllabus-specification help links for biology chemistry physics courses IGCSE & O Level SCIENCES syllabus-specification help links for biology chemistry physics courses
Advanced A/AS Level ORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level INORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level PHYSICAL-THEORETICAL Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level CHEMISTRY syllabus-specificatio HELP LINKS of my site Doc Brown's Travel Pictures
Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial.

Advanced A Level Kinetics Index

 Doc Brown's Chemistry 


best gift deals latest video game release, best gift deals best bargains in shop sales latest pop music releases, best sales deals download free music, latest film releases, best gifts for teenagers latest high street fashion in clothes, fashionable trending in clothing, best gift deals best bargains in shop salesgirls buy clothes, spend a lot of money on clothes, best sales deals shoes, sweets and chocolates, magazines and make-up best gifts for teenagers Boys buy food and drink, computer games best gift deals best bargains in trainers shop sales DVDs and CDs, girls and boys spend a lot of money on credit for mobile phones best sales deals best bargains in shop sales buses and trains to transport them going out best gifts for teenagers best bargains in shoes shop sales Girls spend a lot of money on clothes best gift deals color colour preferences in clothes, cool sunglasses best sales deals boys buy expensive thins like best gifts for teenagers designer sports clothes and trainers, teenagers save for holidays, best sales deals clothes, mobile phones, birthday presents and electronic goods, teenage accessories, Favourite style of jeans. best gifts for teenagers A typical girl’s shopping list includes mobile phone credit deals best shoes gift deals fashionable quality cool airpods, air pods, fashionable clothes best sales deals the most popular favourite sneakers best gifts for teenagers fancy shoes, sweets, chocolates, magazines, best trainers gift deals best bargains in shop sales lip moisturizer best bargains in  shop sale slots on make-up, well being, teenage decor decorating their room best sales deals teenagers like LED string lantern lights, best gifts for teenagers beauty products for teenagers, denim jackets, scrunchies coolness, fashionable back packs, typical boy’s shopping list includes mobile credit deals, eating out takeaway food and drinks, what teenagers like to buy in clothes best gifts for teenagers computer games, DVDs, CDs, what teenagers talk about best gift deals what teenagers worry about, what teenagers like to do for fun sports best sales deals what cool things do teenagers buy, resale websites like eBay Teenager best high street shop or best online deals currys pc world argos amazon john lewis dell acer samsung raycon bose sony asus huawei HP microsoft in-ear headphones earbuds ipad desktop computer laptop computer for school college university students latest video games consoles apple iphone google high end mobile phones cell phone bargain smartphone xiaomi oppo high tech products latest fashion in trainers latest fashion in shoes latest fashion in mobile phones cell phones