An Investigation of the Metabolism of Isoleucine to Active Amyl Alcohol in Saccharomyces cerevisiae

Abstract

The metabolism of isoleucine to active amyl alcohol (2-methylbutanol) in yeast was examined by the use of 13 C nuclear magnetic resonance spectroscopy, combined gas chromatography-mass spectrometry, and a variety of mutants. From the identified metabolites a number of routes between isoleucine and active amyl alcohol seemed possible. All involved the initial decarboxylation of isoleucine to ␣-keto-␤-methylvalerate. The first, via branched chain ␣-ketoacid dehydrogenase to ␣-methylbutyryl-CoA, was eliminated because abolition of branched-chain ␣-ketoacid dehydrogenase in an lpd1 disruption mutant did not prevent the formation of active amyl alcohol. However, the lpd1 mutant still produced large amounts of ␣-methylbutyrate which initially seemed contradictory because it had been assumed that ␣-methylbutyrate was derived from ␣-methylbutyryl-CoA via acyl-CoA hydrolase. Subsequently it was observed that ␣-methylbutyrate arises from the non-enzymic oxidation of ␣-methylbutyraldehyde (the immediate decarboxylation product of ␣-keto-␤-methylvalerate). Mutant studies showed that one of the decarboxylases encoded by PDC1, PDC5, PDC6, YDL080c, or YDR380w must be present to allow yeast to utilize ␣-keto-␤-methylvalerate. Apparently, any one of this family of decarboxylases is sufficient to allow the catabolism of isoleucine to active amyl alcohol. This is the first demonstration of a role for the gene product of YDR380w, and it also shows that the decarboxylation steps for each ␣-keto acid in the catabolic pathways of leucine, valine, and isoleucine are accomplished in subtly different ways. In leucine catabolism, the enzyme encoded by YDL080c is solely responsible for the decarboxylation of ␣-ketoisocaproate, whereas in valine catabolism any one of the isozymes of pyruvate decarboxylase will decarboxylate ␣-ketoisovalerate.

Biochemistry of apple aroma: A review

“…The catabolism of amino acids has been well documented in bacteria and yeasts (18,(142)(143)(144). In fruits, it has been demonstrated that the reactions for the derivation of volatile compounds from branched amino acids follow the pathway found in some of these microorganisms.…”

“…These aldehydes can be reduced to their corresponding branched alcohol (2-methyl-butanol, 3-methyl-butanol and 2-methyl-propanol) by action of the ADH enzyme, which uses NAD(P) + as a cofactor (30,140,143,145,147). The transformation of amino acids into alcohols is known as the Ehrlich pathway, in which the key enzymes are transaminases, decarboxylases and dehydrogenases (148).…”

“…In the fi rst, 2-oxo acids can be converted into branched aldehydes (2-methyl-butanal, 3-methyl-butanal, and 2-methyl-propanal, for Ile, Leu and Val, respectively) by oxidative carboxylation by an α-keto acid decarboxylase enzyme (for example pyruvate decarboxylase, PDC) (18,30,140,142,143,147). These aldehydes can be reduced to their corresponding branched alcohol (2-methyl-butanol, 3-methyl-butanol and 2-methyl-propanol) by action of the ADH enzyme, which uses NAD(P) + as a cofactor (30,140,143,145,147).…”

“…In apple and other fruits, the biosynthesis of volatile compounds from Ile, Leu and Val starts with the removal of the amino group by aminotransferases (BCAT), forming α-keto acids (2-oxo-3-methyl pentanoate and 2-oxo-3-methyl butanoate) (18,90,140,143). During this transamination, the amino acids use 2-oxo-ketoglutarate (2-oxoglutarate) as a receptor for the amino group (18,63,142,145), which can be recycled into glutamate by the enzyme glutamate dehydrogenase (GDH) (18,146).…”

Growth of the salt-tolerant yeast in microtiter plates: effects of NaCl, pH and temperature on growth and fusel alcohol production from branched-chain amino acids

“…K-Ketoisovalerate and K-keto-L-methylvalerate show similar patterns towards NaCl and pH as K-ketoisocaproate. Cross talk between catabolic and anabolic reactions of the branched-chain amino acid metabolism may explain the formation of the di¡erent keto acids and fusel alcohols, as described by Dickinson et al [6,7]. Van der Sluis [16] has not reported the accumulation of these K-keto acids, which are not derived from leucine.…”