Question

Glycogenic amino acids enter TCA cycle except:

• A. $ \text{Alanine} $
• B. $ \text{Glycine} $
• C. $ \text{Glutamate} $
• D. $ \text{Aspartate} $

Hint:

Memorize amino acid catabolic fates — important for Biochemistry questions in GPAT.

Answer 1

The Correct Option is B

- Glycogenic amino acids are converted into intermediates of the TCA cycle, which ultimately lead to glucose formation via gluconeogenesis.
- Glycine is considered conditionally glucogenic, but it doesn't directly feed into a TCA intermediate without further conversion, making it an exception in this context.

Answer 2

This question asks which glycogenic amino acid does not feed directly into the TCA cycle as an intermediate. Let's look at how each amino acid is broken down.

1. Alanine: Alanine undergoes transamination with alpha-ketoglutarate to form pyruvate. Pyruvate can be carboxylated by pyruvate carboxylase to oxaloacetate, a TCA cycle intermediate, so alanine feeds the cycle through a short, direct route.
2. Glycine: Glycine cannot enter the TCA cycle on its own. It is first converted to serine by serine hydroxymethyltransferase, and serine is then converted to pyruvate by serine dehydratase before pyruvate can be carboxylated to oxaloacetate. Glycine can also be broken down by the glycine cleavage system into carbon dioxide, ammonia, and a one-carbon unit that is handed to tetrahydrofolate, which again does not hand a carbon skeleton straight to the cycle. Because glycine needs this extra conversion through serine, or loses its carbons to the folate pool, rather than becoming a cycle intermediate directly, it stands apart from the other three amino acids.
3. Glutamate: Glutamate is deaminated by glutamate dehydrogenase, or transaminated, directly to alpha-ketoglutarate, which is itself a TCA cycle intermediate. This is the most direct entry of any amino acid into the cycle.
4. Aspartate: Aspartate undergoes transamination with alpha-ketoglutarate to form oxaloacetate directly, another TCA cycle intermediate. Aspartate also feeds the cycle in one step.

Alanine, glutamate, and aspartate all reach a TCA cycle intermediate in one or two well-defined steps that keep the full carbon skeleton in the cycle. Glycine's main route sends most of its carbon out through the glycine cleavage system, and its minor route via serine needs an extra conversion step before it reaches pyruvate.

Therefore, the correct answer is Glycine.