Enzymes and Enzyme Kinetics Questions and Answers

Answer: C

Glutathionation is a conjugation reaction.

Drug metabolism runs in two phases. Phase I reactions (oxidation, reduction, hydrolysis) add or expose a functional group; cyclophosphamide is itself activated this way by liver cytochrome P450 enzymes. Phase II reactions conjugate the drug or its metabolite with an endogenous molecule to make it more water-soluble for excretion. Attaching glutathione, catalysed by glutathione S-transferases, is one of these, and it detoxifies reactive metabolites such as acrolein and phosphoramide mustard.

Reduction and oxidation are phase I reactions. Glucuronidation is also a conjugation, but with glucuronic acid, not glutathione, so it is not the name of this reaction.

Answer: B

In non-competitive inhibition, the inhibitor binds at a site other than the active site and may bind to either E or ES complex.

Because the non-competitive inhibitor does not occupy the active site, substrate binding is not blocked, and in the pure case the inhibitor binds the free enzyme and the enzyme-substrate complex equally well. The inhibited complexes cannot make product, so \(V_{max}\) falls, while \(K_m\) is unchanged. Raising substrate concentration cannot overcome it.

Binding at the active site describes competitive inhibition. Binding only to the ES complex is uncompetitive inhibition, which lowers both \(V_{max}\) and \(K_m\). An inhibitor binding only free E away from the active site, the last option, is not the textbook definition of the non-competitive type.

Answer: C

k_cat is the Turn over number.

k_cat is the first-order rate constant for conversion of the enzyme-substrate complex to product when the enzyme is saturated: \(k_{cat} = V_{max}/[E]_{t}\). It gives the number of substrate molecules one active site converts to product per second. Carbonic anhydrase, for example, has a k_cat of about \(10^{6}\) per second.

The Michaelis-Menten constant K_m is the substrate concentration at half of V_max, a different quantity. There is no single substrate concentration 'at V_max', because V_max is only approached as the substrate concentration rises. The total number of active sites is [E]_t, the term k_cat is divided by, not k_cat itself.

Answer: C

A glucometer measures the rate of electron flow from glucose to an electrode surface.

Most test strips are amperometric biosensors. Glucose oxidase (or glucose dehydrogenase) on the strip oxidises glucose, taking electrons from it; a mediator such as ferricyanide carries those electrons to the electrode. The current produced is proportional to the glucose concentration in the drop of blood, and the meter converts it to a reading.

The meter does not track the disappearance of glucose or the appearance of gluconolactone as such, which would need a separate chemical measurement. Electrons flow from glucose, which is oxidised, so 'electron flow from a donor to glucose' has the direction reversed.

Answer: D

Enzyme efficiency is best measured by \(k_{cat}/K_{m}\), the specificity constant.

\(K_{m}\) reflects how much substrate is needed for half-maximal rate, and \(k_{cat}\) is the turnover number, the number of substrate molecules converted per active site per second. Their ratio is the apparent second-order rate constant at low substrate concentration and measures both binding and catalysis together. Its upper limit is the diffusion rate, about \(10^{8}\) to \(10^{9}\) per molar per second, reached by 'catalytically perfect' enzymes such as triose phosphate isomerase.

\(K_{m}\) or \(k_{cat}\) alone captures only one side of performance, and \(K_{m}/V_{max}\) is the slope of a Lineweaver-Burk plot, not a measure of efficiency.

Answer: B

The trigger is enterokinase.

Pancreatic proteases are secreted as inactive zymogens. Enterokinase (enteropeptidase), made by the duodenal mucosa, cleaves trypsinogen to active trypsin, and trypsin then activates the other zymogens: chymotrypsinogen, procarboxypeptidase and proelastase, as well as more trypsinogen.

Trypsin does activate the others, but only after enterokinase has formed it, so enterokinase starts the cascade. Carboxypeptidase and chymotrypsin are among the enzymes activated, not the trigger.

Answer: A

Allosteric enzymes show sigmoidal kinetics.

Allosteric enzymes usually have several subunits whose binding sites interact. Binding of substrate at one site raises the affinity of the others (positive cooperativity), so the plot of rate against substrate concentration is S-shaped, and activity switches sharply over a narrow range of substrate.

A rectangular hyperbola is the Michaelis-Menten curve of non-allosteric enzymes. Allosteric activators and inhibitors shift the substrate concentration for half-maximal rate, so the apparent K\(_{m}\) is not fixed. Competitive inhibition can occur with any enzyme and is not a property of allostery.