Transamination and Nitrogen Metabolism

Fourteen vocabulary flashcards covering essential terms from the lecture on transamination reactions, nitrogen handling, and related metabolic pathways.

Alanine Aminotransferase (ALT)

Enzyme that catabolizes the reversible reaction of alanine + a-KG → pyruvate and glutamate.

Aspartate Aminotransferase (AST)

Enzyme that catabolizes the reversible reaction of aspartate + a-KG → OAA + glutamate

Glucose-Alanine Cycle

Active muscle, using glycogen, frees up glucose, which is catabolized into pyruvate. Some pyruvate is used to form alanine, carrying the NH4+ from the deamination of AAs in a transamination reaction. Alanine goes through the blood and enters the liver, where is is converted back into pyruvate, forming glutamate in another transamination reaction. This pyruvate is converted back to pyruvate, then glucose, which is again released into the bloodstream and taken up by the muscle. The glutamate donates NH4+ to the urea cycle for excretion.

Need for Nitrogen Fixation

N2 is not metabolically available, so it is fixed by cyanobacteria and others in legume roots, turning it into NH3.

Need for nitrogen excretion

Ammino exists as ammonium ion (NH4+), which is produced by transamination then deamination, and the catabolism of nucleotides. Ammino disposal done via urea formation in hepatocytes, where it is moved from peripheral tissues to the liver. Ammonia levels in the blood must remain low to prevent hyperammonemia, which is toxic to the central nervous system, as the glutamine levels rise, changing brain neural osmotic gradients.

PLP-dependent transamination

Uses a ping-pong mechanism; AA binds to the enzyme (via PLP) and reacts, the second substrate, a-KG, cannot bind to form glutamate until the first product, a-keto acid, leaves.

PLP-dependent transamination step 1

PLP covalently binds to a lysine residue at the enzyme active site, releasing water and forming a double bond between the CH of PLP and the NH of Lysine.

PLP-dependent transamination step 2

The amine group from the AA temporarily binds to PLP to form pyridoxamine phosphate (PAP). Water is added, and the a-keto acid is released.

PLP-dependent transamination step 3

The amino group from PAP is donated to a-KG, forming glutamate with the addition of water, and PLP is regenerated.

Tryptophan to pyruvate transamination

Try → Alanine (via 4 steps), and a-ketoglutarate is added to form glutamate and Pyruvate, using PLP and alanine aminotransferase.

oxidative deamination of glutamate

glutamate + NAD(P)+ → a-iminoglutarate + NAD(P)H (leaves) + H2) → a-KG + NH4+ (leaves for urea cycle). Uses glutamate dehydrogenase to release the N group as ammonium and regenerates a-KG

glutamine synthase

Converts glutamate to glutamine in a reversable reaction using NH4+ and ATP, generating ADP + Pi. The reverse reaction uses glutaminase and water to form glutamate and NH4+

AST/ALT liver values

assessment of liver damage, where elevation can indicate fatty liver disease, excessive ethanol, biliary disease, viral hepatitis, acetaminophen overdose, shocked liver, and HELLP