Molecular Action

The biochemical and molecular actions of amino acids are areas of increasing research interest. It is obvious that dietary amino acids will stimulate muscle protein synthesis in the postabsorptive period. This anabolic effect is partly due to increased substrate supply at the sites of protein synthesis. However, it has been proposed that individual amino acids may act as signalling molecules that serve to regulate mRNA translation. The binding of initiator methionyl-tRNA to the 40S ribosomal subunit is an important step subject to regulation in vivo (Table 1.8). However, Anthony et a I. (2000a) suggest that leucine may also act in a signalling role in the stimulation of

Table 1.8. Diverse functions of amino acids.

Amino acids

Products

Significance/functions

21 amino acids3

Polypeptides and proteins

Hormones, enzymes and other bioactive proteins

Methionine

Formylmethionine (fMet)

Initiator of protein synthesis

S-Adenosylmethionine

Donor of methyl groups

Homocysteine

Donor of S; indicator of vitamin B12 status

Tryptophan

Serotonin (5-hydroxytryptamine)

Neurotransmitter

Nicotinamide

B-complex vitamin

Tyrosine

Dopamine

Neurotransmitter

Noradrenaline

Neurotransmitter

Adrenaline

Hormone

Thyroxine

Hormone

Arginine

Nitric oxide

Involved in vasorelaxation; neurotransmission; male reproductive performance; gut motility

Polyamines

Regulation of RNA synthesis; maintenance of membrane stability

Histidine

Histamine

Potent vasodilator

Glutamate

Glutamine

Purine and pyrimidine synthesis; excretion of N in avian species

Glutathione

Reduced form involved in maintenance of cysteine residues of blood proteins in a reduced state

•y-Aminobutyrate (GABA)

Neurotransmitter

Energy

Energy source in some tissues (mucosa)

Glycine

Porphyrins

Part of haemoglobin structure

Purines

Components of nucleic acids

Serine

Sphingosine

Membrane structure

Cysteine

Important for activity of proteins

Aspartate

Urea, purines and pyrimidines

Donor of N

3-Methylhistidine

Component of actin and myosin

Index of muscle protein breakdown

includes selenocysteine.

includes selenocysteine.

muscle protein synthesis by enhancing availability of specific eukaryotic initiation factors. Further studies demonstrated that leucine is unique among the BCAA in its ability to stimulate muscle protein synthesis (Anthony et al., 2000b). It is thought that these effects are independent of the action of insulin. Clearly, additional work is required to elucidate the role of leucine and other amino acids as signalling molecules.

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