Question

Perhaps the most central concept in amino acid degradation is that some amino acids are glucogenic, some are ketogenic and some are both glucogenic and ketogenic. Which of the following amino acids is both glucogenic and ketogenic?

A. Arg

B. Ile

C. Leu

D. Asn

E. Cys

Answer 1

Amino acids are biologically important organic compounds composed of amine (NH₂), and carboxylic acid (COOH) functional groups, along with a side chain specific to each amino acid. Amino acids, when deaminated,yield a keto -acids that directly or via additional reactions, feed into the major metabolic pathways (for an example- Kreb's cycle) depicts the ultimate products of dagradation of amino acid carbon skeletons.

Amino acids are grouped into two classes,based on whether or not their carbon skeleton can be converted into glucose:

1. Glucogenic ( aa that can be converted into glucose)- their carbon skeletons are degraded to pyruvate that are the precursors of Gluconeogenesis.This happens when gucose levels are low.

2. Ketogenic (aa that can be converted directly into acetyl CoA)- their carbon skeletons are degraded to acetyl CoA or acetoacetate.carbon skeleton of ketogenic aa can be catabolysed for energy in Kreb's Cycle,or converted to ketone bodies or fatty acids. They cannot be converted into glucose.

A. Arg (Arginine)- (2-amino-5-guanidinopentanoic acid) is a semi essential aa. It is Glucogenic. This aa is cleaved by arginase to produce ornithine that is subsequently converted to -ketoglutarate.

B. lle (Isoleucine)- (2-amino-3-methylpentanoic acid) is an essential aa.it is both glucogenic and ketogenic.this aa goes to transamination gives -keto--methylvaleric acid then after oxidative decarboxylation & further dehydrogenation with steps gives succinyl CoA.

C.Leu (leucine)-(2-amino-4-methylpentanoic acid) is also an essential aa. it is Ketogenic.this aa on transamination gives ketoisocaproic acid which on oxidative decarboxylation & dehydrogenation gives finally acetoacetate + Acetyl CoA.

D.Asn(asparagine)-(2-amino-3-carbamoylpropanoic acid) is non essential aa. it is Glucogenic. it contains an amide linkage with ammonia at b-carbonyl.reaction is driven by asparagine synthtase using glutamine as a amide donor.

E.Cys(cysteine)-(2-amino-3-sulphydrylpropanoic acid) is also a non essential aa. it is Glucogenic. this aa on desulfuration gives pyruvate.