- Under the action of proteases and peptidases the proteins are divided into amino acids. Part of the amino acids are the starting point for the synthesis of new proteins. - Another part undergoes the degradation following several mechanisms.
Specific enzymes called decarboxylases remove $CO_2$ from the amino acid function: A common coenzyme for decarboxylases is pyridoxalphosphate (see after).
Specific enzymes often assisted by the coenzyme pyridoxalphosphate (PAL) remove $NH_2$ from the amino acid function (transforming it into a keto acid function) to transfer it to the ketoacid function of a second molecule (transforming it into a new amino acid function):
1 L -alanine 2 pyruvic acid 1 ' alpha-ketoglutaric acid 2 ' L -glutamicacid ALAT alaninetransaminase
1 L - aspartic acid 2 oxaloacetic acid 1 ' alpha-ketoglutaric acid 2 ' L -glutamicacid ASAT aspartate transaminase
An example is oxidative deamination transforming an amino acid into the corresponding keto acid. The reaction is catalyzed by the coenzyme NAD+ (nicotinamidedinucleotide) which removes $H_2$ of the amino acid converting it to imino acid (oxidation!) which in turn is hydrolysed spontaneously into the corresponding keto acid
Oxidative deamination of L -glutamic acid ( 1 ) in α-ketoglutaric acid( 2 )