cofactors and coenzymes

Cofactors

When a small, simple molecule or inorganic ion is the non-protein component of an enzyme

Coenzyme

When an organic molecule is the non-protein element of an enzyme

Function of cofactors and coenzymes

Some enzymes can only catalyse a reaction if another non-protein substance binds to the enzyme

• e.g. inorganic ion cofactors may combine with either the enzyme or substrate

• presence of the ion means that the ESC forms more easily

• Some reactions require atoms to be carried from one reaction to another in a multistep pathway

difference between cofactors and coenzymes

• coenzymes are organic molecules

• cofactors are inorganic molecules (dont contain carbon)

Example of a cofactor

Cl- ions are cofactors for salivary amylase enzymes

Example of coenzyme

coenzyme NAD used in respiration to carry H atoms between different enzyme controlled reactions

Prosthetic groups

• prosthetic groups on enzymes are a type of cofactor- differ if they are permanently attached by covalent or non covalent forces

• prosthetic groups contribute to overall 3D shape of the enzyme

What is an inactive precursor?

• An enzyme that requires a change in tertiary structure to activate the enzyme.

• this prevents enzymes from causing damage within cells and ensures they are only used when needed

These are called;

1. zymogens

2. proenzymes

How is a change in tertiary structure brought about for inactive precursors?

• action of another enzyme

• change in conditions (e.g pH or temp)

what is the precursor protein ( inactive enzyme) known as

apoenzyme

what is an activated enzyme due to binding of a cofactor known as

holoenzyme

how is an enzyme activated by a cofactor

• they bind to enzyme and causes change in shape of tertiary structure

• active site becomes complemnetary in shape to substrate

What is an enzyme inhibitor?

Any substance or molecule that slows down or stops an enzyme-controlled reaction.

Two types of enzyme inhibitors

1. Competitive

2. Non-competitive

Competitive inhibitors

• similar shape and structure to substrate

• complimentary in shape to active site

• bind with the active site of the enzyme

• form enzyme-inhibitor complex

• blocks active site and prevents enzyme from catalysing reaction

• lowers rate of reaction

Are competitive inhibitors reversible?

Yes, they are usually reversible and have no permanent effect on enzyme

How can competitive inhibition be overcome?

Increasing the substrate concentration

• more likely for enzyme active site to collide with substrate rather than inhibitor

• can knock out the inhibitor

Non-competitive inhibitors

• bind to an allosteric site

• causes tertiary structure of the enzyme to change

• active site changes shape

• active site is no longer complementary to substra

• enzyme substrate complexes form less frequently and rate of reaction is MUCH lower

What is an allosteric site?

A part of the enzyme other than the active site

Are non-competitive inhibitors reversible?

No, they often bind permanently to the enzyme. Their action is often irreversible.

Can non-competitive inhibition be overcome by increasing substrate concentration?

No. The level of inhibition is determined by the number of inhibitor molecules present.

Roles of inhibitors

Some metabolic poisons are enzyme inhibitors

• e.g potassium cyanid is an irreversible non-competitive inhibitor of a vital enzyme in aerobic respiration called cytochrome c oxidase

• e.g. organophosphates in insecticides are non-competitive inhibitors of an enzyme present at synapses

Some medicinal drugs work by inhibiting the activity of enzymes

• e.g. aspirin is a competitive inhibitor of COX enzymes which synthesis prostaglandins (which cause pain and inflammation)

• e.g. statins are competitive inhibitors of the enzymes responsible for cholesterol production

what is end product inhibition

• the products of some reactions are reversible inhibitors for enzymes involved in controlling reaction

• this enables reactions to be controlled

• if there is a lot of product present, it will inhibit the enzymes and cause reaction to slow or stop

• prevents resources being wasted