BIOLOGY - NMAT

A competitive inhibitor

• binds at a site other than the active site.

• cannot be processed by the enzyme.

• irreversibly binds and inactivates the enzyme.

• does not inhibit enzyme activity, but lowers substrate concentration.

• binds to and inactivates the substrate.

Answer explanation

(B) Some inhibitors bind enzymes at other than the active site, but these are identified

as allosteric inhibitors. A poison is an inhibitor that irreversibly binds and permanently

deactivates an enzyme. While option B does not describe what a competitive inhibitor

does, the fact remains that it cannot be processed at the active site, making this choice the

correct one.

A person with a bacterial infection usually develops a fever. This fever helps

protect the person by inhibiting the growth of bacteria because

• bacteria reproduce more rapidly at higher body temperatures.

• fever blocks the synthesis of proteins in bacteria.

• the higher temperature increases the metabolic rate of bacteria.

• sweating removes cofactors required by bacteria from the blood.

• enzymes do not function as well at temperatures other than that

  which is optimal.

Answer explanation

(E) Bacteria normally do not reproduce or metabolize faster at higher body tem-

peratures. Fever does not block bacterial protein synthesis; if it did, fever would always

be effective in halting bacterial growth. Option E is correct because an elevated body

temperature causes all bacterial enzymes to function at less than the optimal rate, thus

reducing the growth rate of the bacteria and giving the body a better chance to clear the

infection.

If a small molecule that acts as a substrate for a specific enzyme was modi-

fied by being coupled to a larger molecule before the reaction could occur,

what would be the most likely result?

• The reaction catalyzed by the enzyme would progress in the nor-

  mal fashion because the part of the molecule that interacts with the

  enzyme remains substantially unchanged.

• The rate of the reaction would increase because of the additional mass

  of the reactant.

• The rate of the reaction would increase because the additional molecular

  structure would act as a cofactor.

• The enzyme would be permanently disabled by the larger portion of

  the substrate molecule.

• The enzyme would not be able to interact with the modified

  substrate.

Answer explanation

(E) Cofactors are normally separate molecules from the enzyme. While the additional

molecular size would affect the enzyme, it would not permanently disable it because, as

soon as the modified substrate is removed from the reaction, the enzyme could act upon

the unmodified substrate.

Complement is a series of blood proteins best associated with

• initiation of blood clotting.

• regulation of blood clotting.

• platelets.

• cellular lysis.

• apoptosis.

Answer explanation

(D) Complement is a complex of normally inactive proteins circulating in the blood.

When activated, a complex known as C1 begins a cascading series of reactions that cleave

the inactive forms of these proteins to active forms one after another, ultimately producing

an attack complex that drills holes through the cell membrane, causing lysis.

If a person undergoes a process whereby selected plasma proteins are

removed (plasmapheresis), then that person

• is attempting to help someone with chronic infections.

• is attempting to help someone with hemophilia.

• probably has a chronic infection.

• has a large fluid regulatory problem.

• is undergoing therapy for leukemia.

Answer explanation

(B) Plasmapheresis is the process of removing plasma from the body, separating out

protein fractions, and returning the remaining plasma components to the blood. The pro-

teins normally sought for removal are those that are associated with clotting the blood, to

be administered to hemophiliacs.

If a person had a C4 protein deficiency, what problems would they

demonstrate?

• Hemophilia

• Recurrent infections

• Frequent episodes of shock

• Pernicious anemia

• Heart arrhythmias

Answer explanation

(B) When the letter “C” is followed by a number and is used in relation to the blood,

it refers to the protein components of the complement cascade, identified as C1–C9. The

primary function of complement is the targeting and lysis of invading cells such as bacteria.

This means that the lack of C4, one of the key proteins in the cascade, would prevent the

lysis and destruction of bacteria producing an infection.

If whole blood was collected by venipuncture into a tube containing

EDTA or citrate, which of the following could NOT be conducted on the

resulting material in the tube?

• Complete blood count

• Hematocrit

• Differential stain

• Quantitation of proteins C3 or C4

• Blood clotting time

Answer explanation

(E) Assays that require free uncoagulated cells include a complete blood count, hema-

tocrit, and a differential stain. Measuring complement proteins can be done on either

plasma or serum. However, you could not measure clotting time on blood collected in a

material that would prevent clotting.

Which of the following fit within the active site of an enzyme?

• Noncompetitive inhibitors

• Competitive inhibitors

• Coenzyme

• Vitamins

• Cofactors

Answer explanation

(B) The active site of an enzyme is the location that physically interacts with the sub-

strate and product molecules. Other molecules that physically resemble these can also fit

into the active site and, when doing so, inhibit proper enzyme function.