Chapter Review

Enzymes

Enzyme Action and Factors

Active Site Structure

The active site is composed of two distinct regions: the binding site recognises and attaches the correct substrate, and the catalytic site carries out the chemical transformation. Substrate binding activates the catalytic site, forming a transient enzyme-substrate complex.

Key Points

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    Binding site: recognises substrate via complementary shape and charge
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    Catalytic site: converts substrate into product(s) upon activation
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    Enzyme-substrate complex: transient intermediate formed at the active site
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    Enzyme reuse: enzyme detaches unchanged after releasing products
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    Most enzymes are organised on cellular membranes, not freely floating in the cytoplasm

Lock and Key Model

Proposed by Emil Fischer (1890), this model treats the active site as a rigid, pre-formed structure. Only a substrate with the exact complementary shape can bind — like a specific key fitting only one lock.

Key Points

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    Active site is a fixed, inflexible structure
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    Explains enzyme-substrate specificity (one enzyme, one substrate)
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    Does not account for enzymes that change shape upon substrate binding
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    Enzyme acts merely as a passive template

Induced Fit Model

Proposed by Koshland (1959), this model states that substrate binding induces a conformational change in the enzyme's active site. The enzyme moulds around the substrate, enabling more effective catalysis than the lock and key model predicts.

Key Points

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    Active site is flexible, not rigid
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    Substrate binding triggers a structural change in the enzyme
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    Results in a tighter enzyme-substrate fit during catalysis
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    Maintains specificity while explaining the catalytic mechanism

Effect of Temperature on Enzyme Activity

Reaction rate increases with temperature up to the optimum (37°C for human enzymes), as heat provides kinetic energy and activation energy. Beyond the optimum, violent atomic vibrations destroy the globular structure, causing irreversible denaturation.

Key Points

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    Rate roughly doubles for every 10°C rise (below optimum)
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    Optimum temperature: 37°C for human body enzymes
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    Higher temperature → more kinetic energy → more effective enzyme-substrate collisions
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    Denaturation: permanent loss of 3D structure and catalytic activity above optimum
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    Denaturation is irreversible under normal conditions
Formula

Effect of pH on Enzyme Activity

Each enzyme has a narrow optimum pH. Deviation alters ionisation of amino acids at the active site and the substrate, reducing binding and catalysis. Extreme pH causes denaturation by breaking bonds within the enzyme.

Key Points

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    Optimum pH varies per enzyme (pepsin: 2.0, salivary amylase: 6.8, catalase: 7.6, pancreatic lipase: 9.0)
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    pH changes alter the charge of amino acid residues at the active site
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    Substrate ionisation can also be affected, reducing binding affinity
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    Extreme pH breaks enzyme bonds → irreversible denaturation

Enzyme Concentration and Reaction Rate

At unlimited substrate, rate is directly proportional to enzyme concentration because more active sites are available. Beyond a threshold, substrate becomes the limiting factor and rate plateaus.

Key Points

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    Doubling enzyme doubles the rate (at unlimited substrate)
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    More enzyme molecules → more active sites available for catalysis
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    After a limiting concentration, substrate becomes the bottleneck

Substrate Concentration and Saturation

At low substrate concentration, rate is directly proportional to S. As S increases with fixed enzyme, rate rises until all active sites are occupied. The enzyme is then saturated, and the rate plateaus at Vmax.

Key Points

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    Low S: rate increases linearly with substrate (many free active sites)
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    High S: rate plateaus at when all active sites are occupied
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    Further increase in substrate does not increase rate after saturation
Formula

Irreversible Inhibition

Irreversible inhibitors permanently inactivate enzymes by forming covalent bonds with active site residues or destroying the globular structure. Their effect cannot be reversed by increasing substrate concentration.

Key Points

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    Form strong covalent bonds with amino acid residues at the active site
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    May destroy the enzyme's entire globular structure
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    Effect cannot be overcome by increasing substrate concentration
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    Cyanide: classic example — irreversibly inhibits cytochrome c oxidase

Competitive Inhibition

A reversible inhibitor with structural similarity to the substrate competes for the active site. It binds but cannot activate the catalytic site, so no product forms. Increasing substrate concentration overcomes this inhibition.

Key Points

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    Inhibitor closely resembles the substrate in shape and structure
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    Competes with substrate for the same active site
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    Cannot activate the catalytic site — no product formation
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    Can be overcome by increasing substrate concentration
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    Malonic acid: competitively inhibits succinic dehydrogenase

Non-competitive Inhibition

A reversible inhibitor binds at an allosteric site, altering the enzyme's 3D structure so the catalytic site fails even if the substrate binds. Increasing substrate does NOT overcome this inhibition.

Key Points

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    Binds at a site distinct from the active site (allosteric site)
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    Alters the enzyme's overall shape, distorting the active site
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    Even with substrate bound, catalysis cannot occur
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    Cannot be overcome by increasing substrate concentration

Formulas

Q10 Temperature Coefficient

Reaction rate approximately doubles for every 10°C rise below the optimum temperature

Rate vs Substrate Concentration

Direct proportionality at low [S], plateau at Vmax when all active sites are saturated

Enzyme Concentration Proportionality

At unlimited substrate, reaction rate is directly proportional to enzyme concentration