Enzymes Regulation Questions and Answers

Answer: A

Correct answer: Bacterial glutamine synthase

Glutamine synthetase is the point at which ammonia enters organic combination, being attached to glutamate to make glutamine. From there the nitrogen passes to every other nitrogenous compound the cell makes, which is why the enzyme is among the most elaborately regulated known - by cumulative feedback from nine end products and by adenylylation.

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: B

Correct answer: Apoenzyme and Co-enzyme

Holoenzyme = apoenzyme + coenzyme - the inactive protein plus the small organic molecule or metal ion it needs. Only the complete assembly has catalytic activity.

Answer: C

Correct answer: larger and more complex than simple enzyme

An allosteric enzyme is larger and more complex than a simple one, because it needs more than an active site: it is built of several polypeptide subunits and carries one or more regulatory sites as well. That extra structure is what allows a modulator to bind at one place and change the shape of the catalytic site elsewhere.

Answer: B

Correct answer: have different structural forms but identical catalytic properties

Isozymes are different molecular forms of an enzyme catalysing the same reaction. They differ in amino acid sequence and often in kinetic detail - which is what lets the same reaction be tuned differently in different tissues - but the reaction itself is identical, and that is what makes them isozymes rather than separate enzymes.

Answer: A

Correct answer: not pass through semipermeable membrane

Enzymes are large protein molecules, far too big to cross a semipermeable membrane. That is what keeps them inside the cell or the organelle where they belong, and it is why a cell must secrete an enzyme deliberately if it is to act outside.

Answer: B

Correct answer: enzymes

Enzymes are catalysts: they speed a reaction by lowering its activation energy and emerge from it unchanged, ready to act again. That is why a small quantity turns over a great deal of substrate. Coenzymes assist enzymes but are altered in the reaction and must be regenerated; substrates and reactants are consumed.

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: C

Correct answer: Sigmoid curve

An allosteric enzyme has several subunits, and binding substrate to one of them changes the shape of the others - cooperativity. The result is that activity rises slowly at low substrate concentration, then steeply, then levels off, giving a sigmoid curve rather than the hyperbola of Michaelis-Menten kinetics. The steep middle section is what makes such enzymes good switches: a small change in substrate produces a large change in rate.

Answer: A

Correct answer: Kuhne

Wilhelm Kühne coined the word enzyme in 1877, from the Greek for “in yeast”, to name the catalytic agents that acted outside living cells - as against the “ferments” thought to require them. Buchner later showed there was no such distinction, by fermenting sugar with a cell-free yeast extract.