1. Introduction - metabolism, what are enzymes? why are they so important to living organisms?

Examples of biological molecules mentioned in conjunction with enzymes

Carbohydrates like sugars and starch are a combination of the elements carbon, hydrogen and oxygen (C, H and O).

Lipids (vegetable oils/animal fats) are mainly a combination of the elements carbon, hydrogen and oxygen (C, H and O), though sometimes phosphorus (P) too.

Proteins are mainly a combination of the elements carbon, hydrogen, nitrogen and oxygen (C, H, N and O), sometimes sulfur (S) too, enzyme proteins might contain metal ions like Zinc Zn²⁺ which form part of the active site.

Enzymes are involved in all the reactions to synthesise these larger molecules or break them down into smaller molecules.

 

Metabolism is the scientific term used for all the chemical reactions that go on inside an organism's body - mainly cell chemistry.

Metabolic reactions:

(i) synthesise molecules of all shapes and sizes for specific uses in an organism,

(ii) break larger molecules down into smaller ones (e.g. in digestion),

(iii) supply the energy needs for the chemistry of every cell from respiration in the mitochondria.

Most metabolic reactions are controlled by specific enzymes.

What are enzymes? and why are they so important in living systems?

Enzymes are complex protein molecules made of chains of linked amino acids that catalyse most chemical reactions that go on in cells.

Each of these biological catalysts has a unique complex 3D protein structure, particularly the shape of the active site, into which the specific substrate molecule's shape 'fits in' to give the enzyme substrate complex, and be chemically changed (key and lock mechanism).

In forming this enzyme-substrate complex, the enzymes provide a chemical reaction pathway of lower activation energy, so, for any given temperature, a greater proportion of reactant molecules have sufficient kinetic energy to change when they collide with the active site on the enzyme.

The activation energy for an enzyme catalysed reaction (top of the green hump shown on the diagram above) is the minimum kinetic energy the reactant molecules must have to break bonds and undergo a chemical change.

The much higher black higher activation energy curve represents the non-catalysed reaction pathway.

For more details see GCSE chemistry - rates of reaction - effect of a catalyst

Because of their unique molecular structure, each enzyme catalyses a specific reaction (rarely catalyse more than one reaction).

Most enzyme catalysed reactions involve:

Breaking large molecules down into smaller ones.

Building small molecules into large molecules.

Overall, always changing one molecule into another.

(see later in the details of 'key and lock' mechanism theory).

Reminder - catalysts are substances that increase the speed of reactions by lowering the activation energy needed, BUT, they are not chemically changed overall or been used up after the reaction has taken place.

Most biochemical reactions have and require, a specific enzyme to catalyse it.

This is referred to as the specificity of an enzyme.

Enzymes are true biological catalysts, speeding up reactions without being used up in the chemical processes they facilitate.

Every protein an organism requires, including enzymes, is coded for by a different gene in the DNA genome.

This specific gene not only determines the sequence of the amino acids in an enzyme protein, but also its unique shape, and it is the shape that largely determines what an enzyme can do.

The enzyme shape is created by the folding and coiling of chains of amino acids joined together in the protein molecule.

The 3D shape of the enzyme determines the unique 3D shape of the active site.

Only one type of molecule can fit into the active site - if denatured, it becomes inactive.

The rate of chemical reactions are increased by increase in temperature, but higher temperatures may harm the structure and function of complex biological molecules like the enzyme protein molecules.

 

Therefore the catalytic power of enzymes speeds up the thousands of different chemical reactions which enable most organisms to live specifically at relatively low temperatures - in fact, to keep them alive at any temperature!

Extremophiles e.g. like bacteria growing near hot volcanic undersea hydrothermal vents still have enzymes but their activity is much less than in organisms at the typical lower temperatures of the Earth's surface.

 

Without enzymes there would be no life - no photosynthesis in plants, no protein synthesis and respiration in plants and animals, so without these processes there would be no life!

e.g. mitochondria contain all the enzymes needed for the chemical reactions involved in respiration - the source of energy to power cells,

and plant cells have all the enzymes needed for photosynthesis - chlorophyll (not an enzyme) is just one molecule in the many required for the process.

Moreover, all the chemical processes of life must well controlled to keep things in balance so any organism can function properly.

This involves enzymes and hormones for example e.g. the right levels of sugar in the blood (hormone control), what the organism can do with the sugars (enzyme controlled).

Hormones may control the appropriate concentrations of substrates and products as well as temperature - all important variables that need controlling.

BUT, if the rates of so many chemical reactions are not controlled in harmony with each other, then cells may be damaged beyond repair hence endangering the whole organism,

AND many of the reactions involved are facilitated by enzymes.

 

All of the descriptions of enzyme action on this page are greatly simplified, and most biochemical reactions are multi-stage process i.e.

initial substrate reactant 1 == enzyme 1 ==> product 1  == enzyme 2 ==> product 2 etc. == last enzyme ==> final product

so, please remember this when studying my 'pretty' simplified diagrams!

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Key points Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Key ideas on what enzymes can do and their importance

What Are Enzymes?

• Definition: Enzymes are biological catalysts—proteins that speed up chemical reactions in living organisms without being used up.

• Made of: Long chains of amino acids folded into a specific 3D shape.

• Each enzyme has an active site—a region with a specific shape that binds to a particular substrate (reactant).

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Why Enzymes Are Vital in Human Biology

1. They Enable Metabolic Reactions to happen more easily

• All life processes rely on enzyme-controlled reactions—collectively called metabolism.

• Without enzymes, these reactions would occur far too slowly to sustain life.

• Example: The breakdown of glucose during respiration requires multiple enzymes (like amylase, maltase, and ATP synthase).

2. They Digest Our Food

• Enzymes in the digestive system break down large, insoluble food molecules into small, soluble ones.

• e.g. Amylase converts starch into maltose in saliva

• Protease in the small intestine breaks down proteins into amino acids

• The enzyme lipase breaks down lipids (fats) into fatty acids + glycerol

• These products are small enough to be absorbed into the bloodstream through the small intestine.

3. They Support Homeostasis

• Enzymes are essential in regulating internal conditions (e.g. blood glucose, temperature, pH).

• Example: Insulin-regulated enzymes help convert glucose to glycogen in the liver.

4. They Are Highly Specific

• "Lock and key" and induced fit models explain why enzymes only work on specific substrates.

• This specificity is crucial for controlling biochemical pathways and avoiding unwanted reactions.

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Enzyme Function and Conditions

• Enzymes work best at an optimum temperature and optimum pH.

  • Human enzymes usually work best at around 37°C.

  • Different enzymes have different pH optima (e.g., pepsin in the stomach prefers acidic conditions).

• Denaturation: High temperature or extreme pH can damage the enzyme's active site, stopping it from working.

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Real-Life Examples in the Human Body

• DNA replication – DNA polymerase helps copy genetic material during cell division.

• Blood clotting – Thrombin catalyses the conversion of fibrinogen to fibrin to form clots.

• Immune response – Enzymes break down pathogens in white blood cells (e.g., lysozymes).

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Summary: Why Understanding Enzymes Matters

• They are fundamental to life—every breath, heartbeat, and meal depends on them.

• Help explain diseases (e.g., enzyme deficiencies like lactose intolerance or PKU).

• Crucial for biotechnology, medicine, and diagnostics (e.g., enzyme therapy, pregnancy tests).

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Summary of learning objectives and key words or phrases

Be able to explain what an enzyme is and they do as biochemical catalysts.

Be able to draw and understand a diagram showing the profile for an uncatalysed reaction, and catalysed reaction, and showing the 'hump' of the activation energy and its significance.

Using a diagram for an enzyme reaction explain the importance of lowering the activation energy for the biochemistry of living organisms.

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