Amino Acid Catabolism and Transamination

Amino acid catabolism is the set of pathways that dismantle amino acids when they are surplus to needs or when protein is being turned over. The first common step for most amino acids is transamination, the transfer of the amino group to a keto acid, which separates the nitrogen from the carbon skeleton so each can be handled independently.

Definition

Amino acid catabolism is the degradation of amino acids, beginning with removal of the alpha-amino group (typically by transamination) and its subsequent release as ammonia, followed by conversion of the carbon skeleton into glucogenic or ketogenic intermediates.

Scope

This entry covers how amino groups are removed and collected, principally by aminotransferases and glutamate dehydrogenase, and how the remaining carbon skeletons are routed into central metabolism. It treats the degradative side of amino acid metabolism; synthesis is covered in a sibling entry and nitrogen disposal in the urea cycle and nitrogen entries.

Core questions

• How is the amino group separated from the rest of the amino acid?
• How is the collected nitrogen funneled toward disposal?
• Into which central metabolites do the carbon skeletons feed?

Key concepts

• Transamination
• Aminotransferases (transaminases) and pyridoxal phosphate
• Glutamate as a central nitrogen carrier
• Oxidative deamination by glutamate dehydrogenase
• Glucogenic versus ketogenic amino acids
• Branched-chain amino acid catabolism

Mechanisms

In transamination, an aminotransferase moves the alpha-amino group from an amino acid to alpha-ketoglutarate, producing glutamate and the corresponding keto acid; the reaction depends on a pyridoxal phosphate cofactor that shuttles the amino group through a Schiff-base intermediate. This channels nitrogen from many amino acids onto glutamate. Glutamate can then undergo oxidative deamination by glutamate dehydrogenase, regenerating alpha-ketoglutarate and freeing ammonia for disposal. The deaminated carbon skeletons are classified as glucogenic, when they yield pyruvate or citric-acid-cycle intermediates that can form glucose, or ketogenic, when they yield acetyl-CoA or acetoacetate; some amino acids are both. Aspartate and alanine aminotransferases are clinically familiar because their leakage into blood marks tissue injury.

Clinical relevance

Serum aminotransferase activities, such as alanine and aspartate aminotransferase, are widely measured indicators of tissue and especially liver injury, and the catabolic fate of amino acids underlies how protein contributes to energy and glucose supply. This entry explains the underlying biochemistry and is not a basis for individual diagnosis or treatment.

Evidence & guidelines

The reactions described here are established enzymology consolidated in standard biochemistry texts and reviews; this is reference material rather than a clinical guideline domain.

History

Transamination was characterized in the 1930s by Alexander Braunstein and colleagues, and the role of pyridoxal phosphate as the cofactor of aminotransferases was clarified through mid-twentieth-century work by Esmond Snell and others, establishing transamination as the central entry step of amino acid degradation.

Key figures

• Alexander Braunstein
• Esmond Snell
• Hans Krebs

Seminal works

• wu-2009

Frequently asked questions

Why is transamination usually the first step in breaking down an amino acid?

It cleanly separates the nitrogen-bearing amino group from the carbon skeleton by transferring it to a keto acid, so the cell can dispose of the nitrogen and reuse the carbon independently.

What is the difference between glucogenic and ketogenic amino acids?

Glucogenic amino acids are degraded to intermediates that can be made into glucose, whereas ketogenic amino acids yield acetyl-CoA or ketone-body precursors; some amino acids are both.