Enzymes (2.5, 8.1) Flashcards
Define an enzyme.
An enzyme is a globular protein which acts as a biological catalyst by speeding up the rate of a chemical reaction. It is not changed or used up during the reaction.
What is the active site of an enzyme?
The active site is the region on the surface of the enzyme which binds to the substrate molecule. The active site and the substrate complement each other in terms of shape and chemical properties. Only a specific substrate is capable of binding to an enzyme’s active site.
Explain enzyme-substrate interactions.
When a substrate binds to an enzyme’s active site, an enzyme-substrate complex is formed. The enzyme catalyses the conversion of the substrate into product, creating an enzyme-product complex. The enzyme and product then dissociate – as the enzyme was not consumed, it can continue to catalyse further reactions.
What is the induced fit hypothesis?
When binding occurs, the active site undergoes a conformational change to optimally interact with the substrate. This change destabilises chemical bonds within the substrate, lowering the activation energy. So the substrate is converted into product at an accelerated rate.
Give the factors that affect the function of enzymes.
1) Temperature
2) pH
3) Enzyme and substrate concentration
Give an example of the use of enzymes in industry.
Lactose is a dissacharide of glucose and galactose that can be broken down by the enzyme lactase.
Lactose-free milk can be produced by treating the milk with lactase. The lactase is purified from yeast or bacteria and then bound to an inert substance (such as beads). Milk is then repeatedly passed over the immobilised enzyme, becoming lactose-free.
Define an enzyme inhibitor.
A molecule that disrupts the normal reaction pathway between an enzyme and a substrate. Can be competitive or non-competitive.
How does competitive inhibition of an enzyme work?
Competitive inhibitors bind to the enzyme’s active site and prevent substrate binding. They are able to bind to the active site because they are structurally and chemically similar to the substrate. Because the inhibitor is in competition with the substrate, its effects can be reduced by increasing the concentration of the substrate.
Give an example of competitive inhibition of an enzyme.
Ethanol (alcohol) is metabolised in the body to form acetaldehyde which is further oxidised by the aldehyde oxidase enzyme to form acetic acid. The second reaction is rapid to prevent the accumulation of acetaldehyde. The drug Antabuse inhibits the enzyme causing the accumulation of acetaldehyde. It is used to help people overcome drinking problems as it causes unpleasant side effects when alcohol is consumed.
How does noncompetitive inhibition of an enzyme work?
Noncompetitive inhibitors are not similar to the substrate so cannot bind to the enzyme’s active site. Instead they bind to a different site on the enzyme and change the shape of the active site. The substrate may still be able to bind to the active site however the enzyme is unable to catalyse the reaction so it occurs at a slower rate.
Give an example of noncompetitive inhibition of an enzyme.
Arsenic inhibits the functioning of pyruvate dehydrogenase which is an essential enzyme in the metabolic reaction of cell respiration. It causes a disruption to the cells and they die.
Define end product inhibition.
A form of negative feedback by which metabolic pathways can be controlled.
How does end product inhibition work?
The final product from a series of reactions inhibits an enzyme from an earlier step in the sequence, acting as a noncompetitive inhibitor. The product binds to an allosteric site and temporarily inactivates the enzyme. As the enzyme can no longer function, the reaction sequence is halted and the rate of product formation is decreased
Give an example of end product inhibition.
Isoleucine is synthesised from threonine in a reaction pathway in bacteria and animals. It is an essential amino acid (i.e. can not be synthesised by humans and must be ingested).
Isoleucine binds to the allosteric site of the threonine deaminase enzyme, acting as a noncompetitive inhibitor. As excess production of isoleucine inhibits further synthesis, it functions as an example of end-product inhibition.
