Enzymes Regulation Questions and Answers

Answer: B

Correct answer: Covalent

Covalent modification - the attachment or removal of a group such as phosphate, methyl, acetyl or ADP-ribose - changes an enzyme’s activity by altering its charge and shape. It is a true chemical bond, so the change persists until another enzyme reverses it, which is what makes it a switch rather than a rheostat.

Answer: B

Correct answer: Induction

Induction is an increase in the rate at which an enzyme is synthesised, usually because a substrate or signal molecule has relieved the repression of its gene. The lac operon is the standard example: lactose induces the enzymes that digest it.

Answer: D

Correct answer: Repression

Repression is a decrease in the synthesis of an enzyme - typically because the end product of its pathway has accumulated and, acting through a repressor protein, shut the gene down. It saves the cell the cost of making an enzyme it does not need.

Answer: D

Correct answer: Both A and C

Isoenzymes catalyse the same reaction, so they do not differ in function. What differs is their structure - different subunit compositions or different genes - and, as a consequence, their kinetic properties: \(K_m\), \(V_{max}\), optimum pH, response to inhibitors. That is why the heart and muscle forms of an enzyme suit their different tissues.

Answer: B

Correct answer: Allosteric enzyme do not obey double-reciprocal plot

Three of the statements hold. Effectors and modifiers are used interchangeably for a molecule that binds an enzyme at a regulatory site and changes its activity, positively or negatively. Lactate dehydrogenase exists in five forms, built from two kinds of subunit, H and M, in a tetramer - H4, H3M, H2M2, HM3 and M4 - whose relative abundance differs between heart and skeletal muscle and is used diagnostically. Phosphorylation is the classic covalent modification: a kinase attaches a phosphate group to a serine, threonine or tyrosine residue and a phosphatase removes it, switching activity on or off through an actual change in the enzyme’s chemistry rather than a reversible binding.

The statement about allosteric enzymes is the one that fails, and the reason is worth being precise about. An allosteric enzyme does not follow Michaelis–Menten kinetics: because the subunits cooperate, its velocity against substrate concentration traces a sigmoidal curve rather than a hyperbola. A double-reciprocal plot of \( 1/v \) against \( 1/[S] \) can still be drawn from the same data - the transformation applies to any pair of measurements - but the result is a curve rather than the straight line the Lineweaver–Burk method assumes, so \( K_m \) and \( V_{max} \) cannot be read off its intercepts. Allosteric enzymes are described instead by the Hill equation, whose coefficient measures the degree of cooperativity.

Answer: A

Correct answer: Induction and repression

Only induction and repression change the amount of enzyme, by acting on transcription of its gene. Covalent modification and allosteric control alter the activity of enzyme molecules that already exist, and isoenzymes are alternative forms rather than a mechanism of control over synthesis.

Answer: D

Correct answer: None of the above

Creatine kinase has three isoenzymes, made from two subunit types: CK-MM in skeletal muscle, CK-BB in brain, and the hybrid CK-MB in heart muscle. Since three is not among the options, the answer is none of them. The MB form’s rise in blood was long used to diagnose myocardial infarction.

Answer: A

Correct answer: Induction and repression

Enzymes controlled by induction and repression are those needed only in particular circumstances - when an unusual substrate appears, or at one stage of the cell cycle. Making them only when required saves the cell the expense of maintaining enzymes it would rarely use; the price is that the response takes minutes to hours rather than seconds.

Answer: C

Correct answer: allosteric enzymes

Allosteric enzymes are the control points of metabolism. Because a modulator binding at a regulatory site can change the activity of the catalytic site, such an enzyme responds to signals other than its own substrate - which is what lets a pathway be regulated by conditions elsewhere in the cell.

Answer: C

Correct answer: allosteric modulators

A molecule that binds at the regulatory site, is not the substrate, and changes the enzyme’s activity is an allosteric modulator (or effector). Positive modulators increase activity and negative ones reduce it; the commonest negative modulator is the end product of the pathway itself.