Amino Acid Metabolism

Essential Amino Acids

Essential amino acids must be consumed in the diet.

Mammalian cells lack enzymes to synthesize their carbon skeletons (-keto acids). These include:

Isoleucine, leucine, & valine Lysine Threonine Tryptophan Phenylalanine (Tyr can be made from Phe.) Methionine (Cys can be made from Met.) Histidine (Essential for infants.)

Reductive amination catalyzed byglutamate dehydrogenase

Glutamine synthesis is coupled to hydrolysis of ATP

Pyruvate is an amphibolic intermediatein synthesis of alanine

Asparagine synthesis is energetically favorable due to coupling to ATP hydrolysis

Serine biosynthesis

Multistep pathway for glycine biosynthesis

Glycine is also synthesized from serine

Proline biosynthesis

Cysteine is not nutritionally

essential, however it is derived from methionine

+NH3

CH

C

H2C

O-

O

H2C S CH3

Tyrosine is formedfrom phenylalanine

Hydroxyproline is formed after protein synthesis

Selenocysteine is synthesized from serine and selenophosphate

Amino acids that are synthesized de novo in humans. All are related by a small number of steps to glycolysis or TCA cycle

intermediates.

Salvage pathways for formation of certain nonessential amino acids from other amino acids

Amino Acid formed Precursor Amino Acid

Arginine Proline

Cysteine Methionine

Tyrosine Phenylalanine