BCSE Veterinary Biochemistry and Molecular Biology

Question 1

A cat with hepatic lipidosis has severely elevated plasma triglycerides. Which key enzyme in the TCA cycle is inhibited by the accumulation of NADH during excessive fat oxidation, perpetuating the cycle of hepatic fat accumulation?

Accepted Answer

Isocitrate dehydrogenase

Answer

Isocitrate dehydrogenase is allosterically inhibited by high NADH/NAD+ ratios. During excessive β-oxidation (as occurs in feline hepatic lipidosis), NADH accumulates, inhibiting this enzyme and slowing the TCA cycle. Acetyl-CoA then cannot enter the cycle efficiently and is diverted toward ketone body and fatty acid synthesis, worsening fat accumulation. This NADH feedback inhibition is a central mechanism.

Question 2

Which base-pairing rule is violated in RNA but not DNA?

Accepted Answer

Adenine pairs with uracil instead of thymine

Answer

In RNA, uracil replaces thymine. Thymine pairs with adenine in DNA (via two hydrogen bonds). RNA uses uracil instead, which also pairs with adenine but lacks the 5-methyl group of thymine. Guanine–cytosine pairing (three hydrogen bonds) occurs in both DNA and RNA. This distinction is fundamental to understanding transcription and RNA secondary structures.

Question 3

The Km of an enzyme refers to:

Accepted Answer

The substrate concentration at which reaction velocity equals ½ Vmax

Answer

Km (Michaelis constant) is the substrate concentration at which the reaction velocity is half of Vmax. A low Km indicates high affinity (the enzyme reaches half-maximal velocity at low substrate concentrations). Vmax is the maximum velocity at enzyme saturation. Km is not the equilibrium dissociation constant (Kd), though it approximates it when the product release step is rate-limiting.

Question 4

In the polymerase chain reaction (PCR), what is the purpose of the denaturation step?

Accepted Answer

To separate the double-stranded DNA template into single strands

Answer

Denaturation (typically 94–98°C) breaks the hydrogen bonds between complementary DNA strands, converting double-stranded DNA to single-stranded templates. This must precede primer annealing. Primer annealing occurs in the next step (lower temperature). Extension by Taq polymerase occurs at ~72°C. Taq is thermostable and does not require activation by heat.

Question 5

Ammonia toxicity in ruminants fed high non-protein nitrogen (NPN) diets occurs because excess ammonia overwhelms the liver's ability to convert it to urea via the urea cycle. Which enzyme catalyzes the rate-limiting, first committed step of the urea cycle?

Accepted Answer

Carbamoyl phosphate synthetase I (CPS I)

Answer

Carbamoyl phosphate synthetase I (CPS I), located in the mitochondrial matrix, catalyzes the ATP-dependent conversion of ammonia + CO2 → carbamoyl phosphate. This is the rate-limiting step of the hepatic urea cycle and is allosterically activated by N-acetylglutamate. When ammonia influx exceeds CPS I capacity (as with rumen NPN overload), blood ammonia rises, causing neurological signs.

BCSE - Basic and Clinical Sciences Examination Practice Test

BCSE - Basic and Clinical Sciences Examination Practice Test

BCSE Veterinary Biochemistry and Molecular Biology