Transamination of amino acids

Another interesting example of metal-directed chemistry involving the stabilisation and reactivity of imines is seen in the reaction of pyridoxal with amino acids. This reaction is at the basis of the biological transamination of amino acids to a-ketoacids, although the involvement of metal ions in the biological systems is not established. The reaction of pyridoxal (5.27) with an amino acid generates an imine (5.28), which is stabilised by co-ordination to a metal ion (Fig. 5-55). 

The a-keto acids formed by transamination of amino acids are further broken down in the citric acid (Krebs) cycle. This process yields energy, and the body s energy needs can be met with protein if sufficient carbohydrates or fats are not available. 

All tissues have some capability for synthesis of non-essential amino acids, through the interconversion (transamination) of amino acids and their keto-acid carbon skeletons (Figure 1.10). 

N CH3 pyridoxamine phosphate Figure 9.3. Transamination of amino acids. 

Aldehydes, especially the longer chain saturated and branched chain aldehydes (i.e., propanal, butanal, 2-methyl-1-propanal, 2-methyl-1-butanal, and 3-methyl-1-butanal) are also intermediates in the formation of fusel oils. These pathways involve anabolic metabolism of sugars or transamination of amino acids. During ethanol fermentation, the aldehydes may be reduced to the corresponding alcohols by ADH enzymes and excreted into the media. Herraiz et al. (19) found that longer chain aldehydes were not as readily reduced and excreted by the yeast, e.g., 35% reduction was observed for pentanal compared to 3% reduction for decanal. 

Benuck M, Stem F, Lajtha A (1971) Transamination of amino acids in homogenates of rat brain. J Neurochem 78 1555-1567. 

Most of the vitamins and cofactors discussed in previous chapters of the text would be needed during starvation because many of the essential metabolic pathways must continue to operate. Among the most obvious vitamins needed for those pathways are pyri-doxal phosphate (for the transamination of amino acids), niacin and riboflavin (for electron transport), thiamin (for the oxidative decarboxylation of pyruvate, a-ketoglu-tarate, and the branched-chain amino acids), biotin (for the carboxylation of pyruvate), and cobalamin (for the conversion of methylmalonyl Co A to succinyl Co A). 

Like homogeneous catalysis, the removal of a-hydrogen of the amino acid fragment by OH ions, the local concentration of which is apparently high in the polymer phase, is probably the rate-determining step of heterogeneous racemization. Under similar conditions, the rate of a-amino acid racemization decreases in the sequence Ala = Ser>Phe>Nva>Lys>Val, and correlates with the rate of substrate racemization in the presence of Schiff bases and transamination of amino acids by pyridoxal phosphate. 

The inter-membrane space contains enzymes involved in nucleotide metabolism, transamination of amino acids (section 9.3.1.2) and a variety of kinases. 

Fusel alcohols A mixture of higher alcohols formed during yeast-based fermentations that are congeners of ethanol fermentation. The main fnsel alcohols are propanol, butanol, and amyl alcohols. These are produced by transamination of amino acids present in the wort. Their presence in alcoholic beverages is known to be a cause of headaches and hangovers. 

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