Transamination

1. 1

   Amino acid + enzyme-bound pyridoxal phosphate (E-PLP) → corresponding α-keto acid + enzyme-bound pyridoxamine phosphate (E-PMP)

   Aminotransferase (transaminase), PLP half-reaction 1 ST-0133 Reversible PLP (B6)
   In Enzyme-bound pyridoxal phosphate

   Out Enzyme-bound pyridoxamine phosphate
   › Notes

   An aminotransferase (transaminase) forms an external aldimine with the amino acid and transfers its amino group to PLP. PLP (pyridoxal 5′-phosphate; vitamin B6) is essential; this reversible first half-reaction uses no ATP and no free ammonia is produced.

2. 2

   E-PMP + α-ketoglutarate → E-PLP + L-glutamate

   Aminotransferase (transaminase), PLP half-reaction 2 ST-0134 Reversible PLP (B6)
   In E-PMP Alpha-ketoglutarate

   Out E-PLP Glutamate
   › Notes

   The same aminotransferase transfers the amino group from PMP to α-ketoglutarate, regenerating PLP and forming glutamate. This reversible second half-reaction completes the usual net reaction, amino acid + α-ketoglutarate ⇌ α-keto acid + glutamate.

3. 3

   L-alanine + α-ketoglutarate ⇌ pyruvate + L-glutamate

   Alanine aminotransferase (ALT / GPT) 2.6.1.2 ST-0135 Reversible PLP (B6)
   In Alanine Alpha-ketoglutarate

   Out Pyruvate Glutamate
   › Notes

   Alanine aminotransferase (ALT/GPT) catalyzes this PLP-dependent, reversible reaction. In the glucose–alanine cycle, muscle exports alanine and liver converts it to pyruvate for gluconeogenesis while retaining the nitrogen as glutamate.

4. 4

   L-aspartate + α-ketoglutarate ⇌ oxaloacetate + L-glutamate

   Aspartate aminotransferase (AST / GOT) 2.6.1.1 ST-0136 Reversible PLP (B6)
   In Aspartate Alpha-ketoglutarate

   Out Oxaloacetate Glutamate
   › Notes

   Aspartate aminotransferase (AST/GOT) catalyzes this PLP-dependent, reversible reaction. In liver, aspartate supplies one of the two nitrogens of urea; mitochondrial AST also participates in the malate–aspartate shuttle.

5. 5

   Branched-chain amino acid + α-ketoglutarate ⇌ branched-chain α-keto acid + L-glutamate

   Branched-chain aminotransferase (BCAT) 2.6.1.42 ST-0137 Reversible PLP (B6)
   In Alpha-ketoglutarate

   Out Glutamate
   › Notes

   Branched-chain amino acid aminotransferase (BCAT; PLP-dependent) initiates leucine, isoleucine, and valine catabolism. This reaction is prominent in skeletal muscle, which expresses BCAT but has relatively little hepatic BCAT activity.

Showing all 5 steps.

Overview

Transamination is the reversible transfer of an α-amino group from an amino acid to an α-keto acid. It funnels nitrogen from most amino acids onto glutamate, while producing carbon skeletons that can enter energy metabolism, gluconeogenesis, ketogenesis, or biosynthetic pathways. The reactions do not release free ammonia and therefore provide a safe, readily reversible first stage of amino-nitrogen handling.

Cellular location

Aminotransferases are predominantly cytosolic, although important isoenzymes are mitochondrial; aspartate aminotransferase (AST/GOT) has both cytosolic and mitochondrial forms. They are active in essentially all tissues, with particularly high flux in liver, skeletal muscle, heart, kidney, and brain. Hepatocytes use transamination to channel amino nitrogen to glutamate and aspartate for urea production.

Net energetics

The canonical transamination has no direct ATP, GTP, NADH, FADH2, or NADPH cost or yield. It is near equilibrium; its direction is governed by substrate/product concentrations and by subsequent use of glutamate, aspartate, or the generated keto acid.

Clinical significance

Transamination links amino-acid turnover with central carbon metabolism and concentrates amino nitrogen on glutamate, the principal immediate donor for oxidative deamination. ALT is relatively enriched in hepatocytes, whereas AST is abundant in liver, cardiac muscle, skeletal muscle, and erythrocytes; increased plasma activity can indicate cellular injury but is not organ-specific in the case of AST. Severe vitamin B6 deficiency can reduce aminotransferase activity and disrupt neurotransmitter and amino-acid metabolism.