Which of the following statements best explains the reaction specificity of an enzyme?

One of the properties of enzymes that makes them so important as diagnostic and research tools is the specificity they exhibit relative to the reactions they catalyze. A few enzymes exhibit absolute specificity; that is, they will catalyze only one particular reaction. Other enzymes will be specific for a particular type of chemical bond or functional group. In general, there are four distinct types of specificity:

• Absolute specificity - the enzyme will catalyze only one reaction.
• Group specificity - the enzyme will act only on molecules that have specific functional groups, such as amino, phosphate and methyl groups.
• Linkage specificity - the enzyme will act on a particular type of chemical bond regardless of the rest of the molecular structure.
• Stereochemical specificity - the enzyme will act on a particular steric or optical isomer.

Though enzymes exhibit great degrees of specificity, cofactors may serve many apoenzymes. For example, nicotinamide adenine dinucleotide (NAD) is a coenzyme for a great number of dehydrogenase reactions in which it acts as a hydrogen acceptor. Among them are the alcohol dehydrogenase, malate dehydrogenase and lactate dehydrogenase reactions.

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Question 1

Which of the following statements is false with respect to an enzyme's ability to catalyse a reaction?

a) An enzyme provides a reaction surface and a suitable environment for the reaction to take place.

b) An enzyme binds reactants such that they are positioned correctly and can attain their transition-state configurations.

c) An enzyme allows the reaction to go through a less stable transition state than would normally be the case.

d) An enzyme can weaken bonds in reactants through the binding process.

Question 2

Which of the following statements is not true regarding the active site of an enzyme?

a) An active site is normally a hollow or cleft on the surface of an enzyme.

b) An active site is normally hydrophilic in nature.

c) Substrates fit into active sites and bind to functional groups within the active site.

d) An active site contains amino acids which are important to the binding process and the catalytic mechanism.

Question 3

Which of the following statements best describes an allosteric binding site?

a) It is a binding site containing amino acids with aliphatic side chains.

b) It is a binding site that can accept a wide variety of differently shaped molecules.

c) It is a binding site, which is separate from the active site, and affects the activity of an enzyme when it is occupied by a ligand.

d) It is a description of an active site which has undergone an induced fit.

Question 4

Which of the following descriptions best describes an induced fit?

a) The process by which an active site alters shape such that it is ready to accept a substrate.

b) The process by which a substrate adopts the correct binding conformation before entering an active site.

c) The process by which a substrate binds to an active site and alters the shape of the active site.

d) The process by which an active site alters the shape of the substrate such that it can adopt the necessary active conformation for binding.

Question 5

Which of the following statements is incorrect with respect to an enzyme's ability to catalyse a reaction?

a) an enzyme provides a reaction surface and a hydrophilic environment for the reaction to take place

b) an enzyme binds reactants such that they are positioned correctly and can attain their transition-state configurations

c) an enzyme allows the reaction to go through a more stable transition state than would normally be the case

d) an enzyme can weaken bonds in reactants through the binding process

Question 6

Some enzymes require the presence of a non-protein substance if they are to catalyse a reaction. Which of the following terms is the best general term for such a substance?

a) prosthetic group

b) cofactor

c) co-enzyme

d) modulator

Question 7

What term is used for a non-protein organic molecule that is required by some enzymes in order to catalyse a reaction on a substrate?

a) prosthetic group

b) cofactor

c) co-enzyme

d) modulator

Question 8

Some enzymes covalently bind a non-protein organic molecule to the active site. The organic molecule concerned is required if the enzyme is to catalyse a reaction on a substrate. What is the term used for such a molecule?

a) prosthetic group

b) cofactor

c) co-enzyme

d) modulator

Question 9

Which of the following molecules is involved in the feedback control of the enzyme phosphorylase a?

a) glucose-1-phosphate

b) adrenaline

c) glycogen

d) AMP

Question 10

What secondary messenger is generated as a result of the action of nitrous oxide?

a) GTP

b) cyclic GMP

c) ATP

d) cyclic AMP

Question 11

From which amino acid is nitrous oxide generated?

a) arginine

b) aspartic acid

c) asparagine

d) lysine

Question 12

The Michaelis-Menton equation relates the rate of an enzyme-catalysed reaction to which of the following?

a) Substrate concentration

b) Product concentration

c) Activation energy

d) Inhibitor concentration

Which best explains enzyme specificity?

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