Some clinical cases of inborn errors in amino acid metabolism:

• Phenylketonuria (PKU)
• Maple Syrup Urine Disease (MSUD)

Back to Clinical Cases in Biochemistry Homepage

Diet and amino acids

Amino acids serve several purposes in the body: they are of course building constituents of proteins; they are precursors to other biochemicals such as nucleotides and neurotransmitters; and their carbon skeletons may be used as fuel by the body. A proper diet contains adequate amounts of protein. Deficiencies in dietary protein can lead to severe metabolic diseases, as can upsets or blockages in the metabolism of amino acids.

Humans can make only 10 of the 20 standard amino acids used in protein synthesis; the other 10 must be obtained through the diet. Those amino acids that must be obtained through the diet are called essential; the other 10 amino acids are nonessential.

The essential and nonessential amino acids

Metabolism of amino acids (either their synthesis or breakdown) can be blocked because of a defect or absence of activity in an enzyme or its cofactor involved in the synthetic or degradative pathway. According to the crossover theorem of biochemistry, such a pathway blockage can result in the buildup of earlier metabolites in the pathway. Normally these metabolites will be present in very low levels, but in cases of metabolic blockage their levels can become quite appreciable, and they can then interfere with other metabolic processes and cause disease. Some of the possible defects in amino acid metabolism are relatively benign, but others can produce quite severe pathology.