Question

Which of the following sets of amino acids are not both glucogenic and ketogenic?
A. Tryptophan and tyrosine
B. Methionine
C. Phenylalanine
D. Lysine and isoleucine

Answer 1

Hint : Amino acids are the building block of the human body. Amino acids that can be converted into glucose through gluconeogenesis are called Glucogenic amino acids. In the human body there are thirteen amino acids which are glucogenic. Hence its tendency is to produce a pyruvate residue in metabolism which undergoes conversion to a carbohydrate and is stored as a complex carbohydrate. Examples of Glucogenic amino acids – Alanine, histidine, glutamate, glycine, cystine, threonine, valine and aspartate etc.

Complete step by step solution:
A ketogenic amino acid can be degraded into acetyl-CoA which is a precursor of ketone bodies. These amino acids can not be converted into glucose because both carbon atoms in the ketone body are degraded to carbon dioxide. Ketogenic term is used for low-carb diet. It takes more calories from protein and fat and less from carbohydrates. Example of Ketogenic amino acid – L- leucine
Amino acids that can give rise to both glucose and ketone bodies are very essential. Examples of amino acids that are both glucogenic and ketogenic – Tryptophan, tyrosine, iso-leucine, lysine and phenylalanine.
Hence in the given option Methionine is neither glucogenic and not ketogenic. It is an essential amino acid in the human body. It acts as a substrate for other amino acids. So option B is the correct answer to this problem.

Note : The chemical formula of methionine is $C_5{H}_{11}N{O}_{2}S$. Methionine is important for protein synthesis. It contains an $\alpha$ amino group, a carboxylic group and an S-methyl thioether side chain. It can be classified as a nonpolar, aliphatic amino acid. Methionine increases the urinary exertion of quinidine by acidifying the urine.