29.9 Catabolism of Proteins: Deamination

Protein Catabolism: Deamination

Process of Transamination in Catabolism

Transamination is the first major step in amino acid catabolism. It shuttles amino groups from various amino acids to a common acceptor, funneling nitrogen toward excretion while producing carbon skeletons that can enter central metabolic pathways.

The reaction transfers an amino group from an amino acid to an $\alpha$-keto acid, catalyzed by enzymes called transaminases (also known as aminotransferases). The most common acceptor is $\alpha$-ketoglutarate, which becomes glutamate after receiving the amino group. Alanine transaminase (ALT) and aspartate transaminase (AST) are two clinically important examples of these enzymes.

Every transaminase requires pyridoxal phosphate (PLP), a derivative of vitamin $B_6$, as a cofactor. PLP acts as an electron sink, stabilizing the negative charge that develops on the $\alpha$-carbon during the reaction. Without PLP, the intermediate would be too unstable for the reaction to proceed.

The overall mechanism is a ping-pong (double-displacement) reaction with two half-reactions:

1. First half-reaction: The amino acid donates its amino group to PLP, forming pyridoxamine phosphate (PMP) and releasing an $\alpha$-keto acid.
2. Second half-reaction: PMP transfers that amino group to a different $\alpha$-keto acid, regenerating PLP and producing a new amino acid (typically glutamate).

Conversion of Amino Acids to Keto Acids

Here is the step-by-step mechanism for the full transamination cycle:

First half-reaction (amino acid → keto acid):

1. The $\alpha$-amino group of the amino acid attacks the aldehyde group of PLP, forming an external aldimine (Schiff base) and releasing the enzyme's lysine residue.
2. The Schiff base undergoes tautomerization: the $\alpha$-hydrogen is removed, and electrons shift through the conjugated PLP ring system, forming a quinonoid intermediate.
3. The quinonoid intermediate is reprotonated at the PLP carbon, then hydrolyzed. This releases the $\alpha$-keto acid product and leaves PMP bound to the enzyme.

Second half-reaction (keto acid → new amino acid):

1. PMP forms a new Schiff base with a second $\alpha$-keto acid substrate (commonly $\alpha$-ketoglutarate or pyruvate).
2. This Schiff base tautomerizes through a second quinonoid intermediate, reversing the direction of proton transfer.
3. Hydrolysis of the resulting external aldimine releases the new amino acid (glutamate or alanine) and regenerates PLP, completing the catalytic cycle.

Regeneration of Pyridoxal Phosphate

The second half-reaction is what regenerates PLP, so it's worth understanding why this matters for the catalytic cycle:

• After the first half-reaction, PMP holds the amino group that came from the original amino acid.
• PMP cannot participate in another first half-reaction until it offloads that amino group onto a keto acid acceptor.
• Once PMP transfers the amino group (via the Schiff base → quinonoid → hydrolysis sequence described above), free PLP is restored.
• The regenerated PLP re-forms its internal aldimine with the active-site lysine, ready for the next amino acid substrate.

This cycling is what makes the mechanism catalytic. If PLP were not regenerated, the enzyme would be stuck after a single turnover.

Protein Breakdown and Nitrogen Metabolism

Transamination is only one piece of the larger nitrogen disposal pathway. Here's how the full process fits together:

1. Proteolysis: Proteins are hydrolyzed into free amino acids by proteases (in the gut) and by lysosomal/proteasomal pathways (inside cells).
2. Transamination: Most amino acids transfer their amino groups to $\alpha$-ketoglutarate, producing glutamate as the central nitrogen-collecting amino acid.
3. Oxidative deamination: Glutamate dehydrogenase removes the amino group from glutamate as free $NH_4^+$ (ammonium), regenerating $\alpha$-ketoglutarate. This reaction occurs in the mitochondrial matrix and can use either $NAD^+$ or $NADP^+$ as the oxidant.
4. Urea cycle: Free ammonia is highly toxic to the central nervous system, even at low concentrations. The urea cycle (occurring in the liver) combines two nitrogen atoms, one from free $NH_4^+$ and one from aspartate, with $CO_2$ to produce urea ($H_2N\text{-}CO\text{-}NH_2$). Urea is water-soluble, non-toxic, and excreted by the kidneys.

The $\alpha$-keto acid carbon skeletons left behind after deamination are not wasted. Depending on the amino acid, they enter the citric acid cycle as acetyl-CoA, $\alpha$-ketoglutarate, succinyl-CoA, fumarate, or oxaloacetate, contributing to energy production or gluconeogenesis.