Enzymes Enzymes Enzymes are proteins that catalyse specific

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Enzymes

Enzymes

Enzymes • Enzymes are proteins that catalyse specific chemical reactions without themselves being altered

Enzymes • Enzymes are proteins that catalyse specific chemical reactions without themselves being altered • ‘Catalyse’ means to break down Model of an enzyme (pyruvate dehydrogenase ) PDA – US Fed Gov

Features of enzymes 1. All enzymes are proteins 2. All enzymes speed up chemical

Features of enzymes 1. All enzymes are proteins 2. All enzymes speed up chemical reactions 3. Enzymes are not used up in the reaction 4. All enzymes are reaction-specific 5. Each enzyme works under specific conditions e. g. p. H & temperature

Features of enzymes 6. Enzymes work by lowering the energy of reaction or lowering

Features of enzymes 6. Enzymes work by lowering the energy of reaction or lowering the activation energy 7. Enzymes are thought to work on a lock-key principle 8. Enzymes often work in systems with co-enzymes & co-factors 9. Enzymes can be denatured by heating

Activation energy • Activation energy is the energy needed to start a chemical reaction.

Activation energy • Activation energy is the energy needed to start a chemical reaction. • Enzymes function by lowering this ‘activation energy’

Activation energy with NO enzyme The stairs in this diagram represent the activation energy

Activation energy with NO enzyme The stairs in this diagram represent the activation energy needed for a chemical reaction to take place without an enzyme. Reduced activation energy with an enzyme The stairs in this diagram represent the reduced activation energy needed for a chemical reaction to take place with an enzyme.

Activation energy

Activation energy

Two Models • Lock & Key – Active site of the enzyme fits the

Two Models • Lock & Key – Active site of the enzyme fits the substrate exactly • Induced Fit – Binding of the substrate causes change in the active site so it fits the substrate more closely – Explains why some enzymes can act on more than one substrate

Lock-key model The lock-key model is an analogy put forward to explain the specific

Lock-key model The lock-key model is an analogy put forward to explain the specific action of an enzyme with a single substrate

The lock-key model • The active site on the enzyme attaches to a substrate

The lock-key model • The active site on the enzyme attaches to a substrate molecule forming an enzymesubstrate complex. • The enzyme causes a weakening of chemical bonds, resulting in the substrate breaking down into two smaller product molecules. • The enzyme is unaltered during the reaction and is free to catalyse the breakdown of another substrate molecule.

Induced Fit • Assumes that substrate plays a role in determining the final shape

Induced Fit • Assumes that substrate plays a role in determining the final shape of the enzyme & that the enzyme is partially flexible • Explains why certain compounds can bind to the enzyme but do not react because the enzyme has been distorted too much • Other molecules may be too small to induce the proper alignment & therefore can not react • Only the proper substrate is capable of inducing the proper alignment of the active site

Denaturing enzymes • The active site is the specific portion of an enzyme that

Denaturing enzymes • The active site is the specific portion of an enzyme that attaches to the substrate. • The specific shape of the active site must be maintained if the protein is to function properly. • If the shape of the active site is altered (e. g. by heating over the optimum temperature & by raising & lowering the p. H) or blocked (e. g. by a poison) the enzyme will no longer function and is said to be denatured.

Factors affecting the rate of enzyme controlled reactions

Factors affecting the rate of enzyme controlled reactions

 • Also affected by the presence of inhibitors • The activity of most

• Also affected by the presence of inhibitors • The activity of most enzymes is regulated by feedback inhibition – where the product of a reaction can inhibit enzyme activity • When large amount of products they will bind to the enzyme (not to the active site), altering the active site, thereby slowing the reaction rate. This is a non-competitive inhibitior

 • A competitive inhibitor competes with the substrate for space in the active

• A competitive inhibitor competes with the substrate for space in the active site