Interactions and Inhibitors Revision Notes for OCR A-Level Biology A

Interactions and Inhibitors

Enzymes do not work in isolation. Their activity can be influenced by various substances that interact with them in different ways. Understanding these interactions is essential for comprehending how cells regulate metabolic processes and how certain drugs and poisons affect biological systems.

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The substances that interact with enzymes fall into two main categories: helper molecules that enable enzyme function (cofactors, coenzymes, and prosthetic groups) and inhibitors that reduce or prevent enzyme activity. Both play crucial roles in cellular regulation.

Cofactors, coenzymes and prosthetic groups

Many enzymes require additional non-protein substances to function properly. Without these helper molecules, the enzyme remains inactive or works inefficiently.

Cofactors

A cofactor is a non-protein substance required by an enzyme to carry out its catalytic function. Cofactors can be either permanently attached to the enzyme molecule (prosthetic groups) or temporarily associated during the reaction.

Inorganic cofactors

Some enzymes require inorganic ions to function correctly. These metal ions help to stabilise the enzyme's tertiary structure or participate directly in the catalytic reaction at the active site.

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Cofactor Examples in Action

Amylase: Requires calcium ions ( $\text{Ca}^{2+}$ ) and chloride ions ( $\text{Cl}^-$ ) to break down starch into maltose.

Carbonic anhydrase: Found in red blood cells, uses a zinc ion ( $\text{Zn}^{2+}$ ) as part of its active site to catalyse the conversion of carbon dioxide and water to carbonic acid.

Coenzymes

Larger organic molecules that act as cofactors are called coenzymes. These are small organic non-protein molecules that participate in enzyme-catalysed reactions by transferring chemical groups or hydrogen ions between different reactions.

Coenzymes can bind to enzymes in two ways:

Some bind permanently to the enzyme, often in or near the active site
Others bind temporarily during the reaction and then dissociate

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Coenzymes play a vital role in linking different enzyme-catalysed reactions within metabolic sequences such as respiration and photosynthesis. This makes them essential for coordinating complex metabolic pathways.

Many coenzymes are synthesised from B-group vitamins:

Pantothenic acid (vitamin B₅) is a component of coenzyme A
Nicotinic acid is used to synthesise the coenzymes NAD and NADP
Riboflavin (vitamin B₂) forms the basis of the coenzyme FAD

These coenzymes serve specific functions in metabolism:

NAD and FAD are alternately reduced and oxidised during respiration, transferring energy in the form of hydrogen ions
NADP fulfils a similar role in chloroplasts during photosynthesis
Coenzyme A transfers acetyl groups ( $-\text{CH}_3\text{CO}$ ) from glucose and fatty acids during respiration
ATP transfers phosphate groups between respiration and energy-consuming processes in cells

The diagram above shows how a cofactor (coenzyme) activates an enzyme. In the non-active state, the enzyme's active site cannot accept the substrate. When the coenzyme binds, the enzyme becomes active and can convert substrate molecules into products.

Prosthetic groups

Any cofactor that becomes a permanent part of an enzyme molecule is called a prosthetic group. These groups contribute to the three-dimensional shape of the enzyme and are therefore essential for its catalytic function.

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Prosthetic Group Examples

The zinc ion in carbonic anhydrase forms an essential part of its active site
FAD acts as the prosthetic group for the mitochondrial enzyme succinate dehydrogenase

Enzyme inhibitors

An inhibitor is any substance that reduces the rate of or completely stops an enzyme-catalysed reaction by interacting with the enzyme molecule. Inhibitors play crucial roles in regulating metabolic pathways, and some are used as medicinal drugs or act as poisons.

Non-reversible inhibitors

Non-reversible inhibitors (also called irreversible inhibitors) bind permanently to enzymes by forming strong covalent bonds. This binding completely inactivates the enzyme. Once inhibited, the cell must synthesise new enzyme molecules through gene transcription and translation to restore enzyme activity.

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Heavy metal ions such as lead ( $\text{Pb}^{2+}$ ) and mercury ( $\text{Hg}^{2+}$ ) are examples of non-reversible inhibitors. These ions form covalent bonds with sulfur-containing R groups in proteins, causing permanent damage. They are serious metabolic poisons that can have fatal consequences.

For instance, lead permanently inhibits the enzyme ferrochelatase, which is involved in synthesising haem for haemoglobin production.

Reversible inhibitors

Reversible inhibitors bind temporarily to enzymes. Their inhibitory effect can be overcome by changes in the enzyme's environment, such as alterations in substrate concentration or removal of the inhibitor. There are two main types of reversible inhibitors, classified according to where they bind on the enzyme.

Competitive inhibitors

Competitive inhibitors are molecules that possess a similar three-dimensional structure to the substrate molecule, or have at least one region with a complementary shape. These molecules compete directly with substrate molecules for access to the enzyme's active site.

When a competitive inhibitor collides with an enzyme and binds to the active site, it effectively blocks substrate molecules from entering. This forms an enzyme-inhibitor complex that cannot catalyse any reaction, reducing the overall reaction rate.

The effectiveness of competitive inhibition depends on the relative concentrations of inhibitor and substrate molecules:

When inhibitor concentration is low compared to substrate concentration, the reaction rate is only slightly reduced
When inhibitor concentration is high, more inhibitor molecules successfully occupy active sites, significantly reducing enzyme-substrate complex formation

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Competitive inhibition can be overcome by increasing the substrate concentration. With more substrate molecules present, they can outcompete the inhibitor for active sites, allowing the reaction rate to increase.

The graph above demonstrates the effect of a competitive inhibitor on reaction rate. Notice that:

Both curves (with and without inhibitor) eventually reach the same maximum rate of reaction
The curve with inhibitor requires a higher substrate concentration to reach the maximum rate
This is because substrate molecules must outcompete the inhibitor molecules

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Product Inhibition by Galactose

The enzyme β-galactosidase catalyses the breakdown of lactose:

$\text{lactose} + \text{water} \xrightarrow{\text{β-galactosidase}} \text{glucose} + \text{galactose}$

Galactose, one of the products, acts as a competitive inhibitor. This provides a natural regulatory mechanism - when sufficient product molecules accumulate, they slow down further enzyme activity.

Non-competitive inhibitors

Non-competitive inhibitors do not compete with substrate molecules for the active site. Instead, they bind to a different region of the enzyme called an allosteric site. This binding causes a conformational change in the enzyme's tertiary structure, which distorts the shape of the active site.

Once the active site shape has been altered, substrate molecules can no longer bind effectively, preventing enzyme-substrate complex formation and reducing the reaction rate.

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Because competitive and non-competitive inhibitors work through different mechanisms, their effects differ significantly:

Increasing substrate concentration has no effect on non-competitive inhibition
The maximum rate of reaction is reduced when a non-competitive inhibitor is present
Once all allosteric sites are occupied by inhibitor molecules, the reaction stops completely, regardless of substrate concentration

The graph in the figure above shows how non-competitive inhibitors reduce the maximum rate of reaction, unlike competitive inhibitors which only delay reaching the maximum rate.

Applications of enzyme inhibitors

Controlling metabolic processes

Enzyme inhibitors provide essential control mechanisms for metabolic pathways. Most intracellular enzymes work as part of sequential pathways, where the product of one reaction becomes the substrate for the next.

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In many metabolic sequences, the final product molecules act as non-competitive inhibitors of the first enzyme in the pathway. This creates a feedback mechanism called product inhibition - when sufficient product has been synthesised, it automatically reduces its own production by inhibiting the initial enzyme. This prevents wasteful overproduction of metabolic products.

Since enzymes can accelerate reaction rates by factors of up to ten million, such regulatory mechanisms are vital for maintaining metabolic balance within cells.

Inhibitors as poisons

Some inhibitors are highly toxic metabolic poisons that prevent essential metabolic reactions from occurring. These substances can be fatal because they block critical biochemical pathways.

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Cyanide is a non-reversible inhibitor of cytochrome oxidase, the mitochondrial enzyme that catalyses the final step of aerobic respiration. Because this inhibition is permanent and cannot be reversed except by synthesising new enzymes (a time-consuming process), cyanide poisoning is often fatal.

Malonate is a competitive inhibitor of the mitochondrial enzyme succinate dehydrogenase. It is also a metabolic poison, though its effects can potentially be overcome by increasing substrate concentration.

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Researchers use metabolic poisons as experimental tools to understand enzyme function and metabolic pathways, particularly in processes like cellular respiration.

Inhibitors as medicinal drugs

Many enzyme inhibitors have beneficial medical applications because they selectively block enzymes that contribute to disease processes or unwanted physiological responses.

Medicinal drug	Inhibitory action	Benefit of the inhibitor
Penicillin	Non-reversible inhibition of transpeptidase enzyme responsible for forming cross-links in bacterial cell walls	Bacteria are destroyed
Aspirin (acetylsalicylic acid)	Non-reversible inhibition of the COX enzyme involved in producing prostaglandins for stimulating inflammation and pain	Reduces inflammation and provides pain relief
Eflornithine	Non-reversible inhibitor of ornithine decarboxylase, an enzyme essential for cell growth	Used for treatment of African trypanosomiasis (sleeping sickness)
Statins	Competitive inhibition of HMG-CoA reductase, an enzyme involved in cholesterol synthesis	Reduces the concentration of cholesterol in the blood

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These examples demonstrate how understanding enzyme inhibition mechanisms has led to the development of effective treatments for bacterial infections, pain management, parasitic diseases, and cardiovascular conditions.

Remember!

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Key Points to Remember:

Cofactors are non-protein substances (inorganic ions or organic coenzymes) that enzymes require to function properly
Prosthetic groups are cofactors permanently attached to enzymes, contributing to their three-dimensional structure
Competitive inhibitors compete with substrates for the active site and can be overcome by increasing substrate concentration
Non-competitive inhibitors bind to allosteric sites, changing the active site shape, and cannot be overcome by adding more substrate
Enzyme inhibitors serve important roles in regulating metabolism, act as poisons, and form the basis of many medicinal drugs

Interactions and Inhibitors (OCR A-Level Biology A): Revision Notes