Metabolism of Amino Acids
The critical first step of amino acid degradation is the removal of the amino group by oxidative deamination. Oxidative deamination requires an amino transferase and a dehydrogenase. Ammonium ion (NH4 +) is liberated. NH4 + is a product of the breakdown of amino acids. NH4 + is required by cells for synthesis of nitrogen containing compounds. Excess NH4 + is very toxic. Excess NH4 + is converted to urea via the urea cycle and excreted.
The carbon skeletons are processed by various pathways. They may end up as:
Lesions in these pathways can lead to accumulation of other products as in maple syrup urine disease
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Another important enzyme is phenylalanine hydroxylase which converts phenylalanine to tyrosine. Some newborns lack this enzyme. It is routinely measured in hospitals but, if neglected, the build-up of phenylalanine causes mental retardation (PKU). Children with this condition have to be on a special diet with very little phenylalanine.
The synthesis of many of the amino acids is a simple reversal of their degradation, utilizing a transamination reaction. Amino acids skeletons end up as major metabolic intermediates during degradation. Likewise, amino acids are also biosynthesized from major metabolic intermediates.
