1. The alteration of enzyme structure on binding of a substrate to an active sit

ID: 60614 • Letter: 1

Question

The alteration of enzyme structure on binding of a substrate to an active site is referred to as

enzyme denaturation.

induced fit.

enzyme inhibition.

What common mechanistic feature do metalloproteases, carbonic anhydrase, and the EcoRV restriction enzyme have in common?

All three use metal ions to activate water molecules.

All three active sites contain Zn(II).

All three perform hydrolytic reactions.

Carbonic anhydrase contains water coordinated to a Zn(II) in its active site. The pH dependence of the enzyme activity has an apparent pKa near 7.0 that is attributed to the ionization of this bound water molecule. Which of the following is true?

The pKa near 7.0 indicates that there is no significant effect of the Zn(II) on the ability of the bound water molecule to gain or lose a proton.

The Zn(II) activates the water, raising its pKa and making it more difficult to lose a proton.

The Zn(II) activates the water, lowering its pKa and making it easier to lose a proton.

The role of the Mg(II) ion in the reaction catalyzed by myosin is

to produce a conformational change in the enzyme such that ATP can bind.

to activate a water molecule in the active site.

to form a complex with ATP which acts as the true substrate for the enzyme.

You have isolated a new protease that cleaves peptide bonds on the carboxyl side of Asp and Glu. Based on the enzyme's inactivation by DIFP, you suspect that it may utilize a mechanism similar to chymotrypsin. The difference in specificity might be explained by

the presence of a negatively charged residue in the S1 binding pocket.

replacement of Serine-195 with a positively charged residue.

the presence of a positively charged residue in the S1 binding pocket.

In addition to the three histidinel residues involved in binding of the Zn(II) in carbonic anhydrase, a fourth histidine residue is also important in the function of the enzyme. The role of this fourth histidine residue (His-64) is

to neutralize the negative charge on the hydroxide ion bound to the Zn(II).

to shuttle protons to and from the active site.

to provide a fourth ligand for the Zn(II) when the hydroxide reacts with carbon dioxide to form carbonic acid.

Diisopropylfluorophosphate (DIFP) inactivates chymotrypsin by covalently modifying Serine-195. This occurs because:

DIFP looks like the substrate for chymotrypsin and binds in the active site as a competitive inhibitor.

DIFP randomly modifies all serine residues on the protein and if enough is added the one in the active site will eventually be modified.

Serine-195 is in an environment which gives it a higher than normal reactivity with respect to DIFP.

Although restriction enzymes are quite efficient at digestion of foreign DNA, they are prevented from acting on the DNA of host cells by

the methylation of bases in recognition sequences in host cell DNA.

the presence of competitive inhibitors which prevent the enzymes from binding to the host DNA.

the inability of host cell DNA to be bind to the active site of the enzyme.

The mechanism of chymotrypsin can be viewed as a two-step process - acylation of the enzyme active site followed by a deacylation reaction. The observation of "burst" kinetics in rapid kinetic studies of the hydrolysis of p-nitrophenylphosphate by chymotrypsin is due to

the rate of the acylation reaction being slower than the deacylation reaction.

the rates of acylation and deacylation being equal.

the rate of the acylation reaction being faster than the deacylation reaction.