2.1.4 Enzymes

What are the role of enzymes?

• Biological catalysts →they speed up the rate of metabolic reactions, without being used up themselves

• Globular proteins, with complex teriary structures

What is an intracellular enzyme?

An enzyme that is produced and functions inside the cell e.g. catalase

What is an extracellular enzyme?

An enzyme that is secreted by cells and catalyse reactions outside the cells e.g. amylase

What is catalase’s function?

Converts hydrogen peroxide (H₂O₂) into water and oxygen, preventing any damage to cells or tissues

What is amylase’s function?

It is involced in carbohydrate digestion and hydrolyses starch into simple sugars.

It is secreted by salivary glands and pancreas, for digestion of starch in the mouth and small intestine.

What is the mechanism of enzyme reaction?

• Enzymes have an active site where specific substrates bind, forming an enzyme-substrate complex

Explain the lock-and-key hypothesis

Each enzyme has its own special shape, with an area, the ACTIVE SITE, onto which the substrate molecules bind/fits exactly

Explain the induced-fit hypothesis

The enzyme’s active site and (sometimes) the substrate can change shape slightly as the substrate enters the enzyme, to give it an overall good fit

What is the activation energy?

The amount of energy needed to break existing chemical bonds between molecules

What factors affect the rates of enzyme-controlled reactions?

• Temperature

• pH

• Substrate concentration

• Enzyme concentration

• Presence of inhibitors

How does pH affect enzyme activity?

pH - measure of H+ ion concentration

Below/Above optimum pH:

• Hydrogen & ionic bonds hold the shape of the enzyme

  → Excess H+ ions cause these bonds to break, changing the shape of the active site, so ESC form less easily (lowers RoR)

How does temperature affect enzyme activity?

Optimum temperature:

• Heated → gains KE → increases change of successful collisions, so rate of formation of ESC & the RoR up to a certain point

Above:

• Temperature increases → rate the molecules vibrate also increases, which can break hydrogen & ionic bonds

• Tertiary structure starts to change, so does the active site so substrates don’t fit as well → RoR decreases & reaction cannot continue → denatures

How does enzyme concentration affect enzyme activity?

Increasing enzyme concentration:

• Increases active sites available to interact with the substrate

→ Increase in formation of ESC & RoR increases

• Increase will continue as long as there is sufficient substrate available

How does substrate concentration affect enzyme activity?

Increasing substrate concentration:

• Increases substrates available to interact with the active sites

→ Increase in formation of ESC & RoR increases

What are coenzymes?

Small organic non-protein molecules that bind temporarily to the active site of the enzyme, before or at the same time as the substrate

e.g. vitamins (nicotinic acid → coenzymes NAD & NADP)

What are cofactors?

Non-protein molecules that assist an enzyme in its function (stabilise structure or take part in the reaction at the active site)

e.g. chloride ions act as a cofactor for amylase

What are competitive inhibitors?

• Similar shape to substrate

• Compete with substrate to bind active site

What are non-competitive inhibitors?

• Binds to the enzyme at an alternative site (NOT active site)

• Changes the shape of active site, so substrate no longer fits

Are inhibitors irreversible or reversible?

• Irreversible if inhibitor is bound permanently

• Reversible if inhibitor is bound temporarily

What is end-product inhibition?

Enzyme inhibition is important in regulating metabolic pathways

• Amount of end product is high → binds non-competitively to an enzyme in the pathway, blocking further production of itself

• Amount of end product falls → inhibition ends & pathway restarts