An aminotransferase from Lactococcus lactis initiates conversion of amino acids to cheese flavor compounds

Abstract

The enzymatic degradation of amino acids in cheese is believed to generate aroma compounds and therefore to be involved in the complex process of cheese flavor development. In lactococci, transamination is the first step in the degradation of aromatic and branched-chain amino acids which are precursors of aroma compounds. Here, the major aromatic amino acid aminotransferase of a Lactococcus lactis subsp. cremoris strain was purified and characterized. The enzyme transaminates the aromatic amino acids, leucine, and methionine. It uses the ketoacids corresponding to these amino acids and alpha-ketoglutarate as amino group acceptors. In contrast to most bacterial aromatic aminotransferases, it does not act on aspartate and does not use oxaloacetate as second substrate. It is essential for the transformation of aromatic amino acids to flavor compounds. It is a pyridoxal 5'-phosphate-dependent enzyme and is composed of two identical subunits of 43.5 kDa. The activity of the enzyme is optimal between pH 6.5 and 8 and between 35 and 45 degrees C, but it is still active under cheese-ripening conditions.

Production of phenyllactic acid by lactic acid bacteria: an approach to the selection of strains contributing to food quality and preservation

“…These findings indicate that the direct correlation between Phe and PLA, Tyr and OH-PLA, previously reported for L. lactis subsp. cremoris [11,12], may also be established for L. plantarum ITM21B, since metabolites are produced only in the presence of these amino acids and neither PLA nor OH-PLA are detected in the absence of both amino acids (data not shown). Moreover, the reduction in OH-PLA concentration associated with the increase in PLA level supports the clear correlation observed between the two metabolites ( 1 and Fig.…”

“…Phenyllactic and hydroxy-phenyllactic acids have also been found as metabolites involved in the formation of cheese flavour and produced by LAB strains through phenylalanine (Phe) and tyrosine (Tyr) degradation, respectively [10][11][12]. In particular, Kieronczyk et al [10] studied the contribution of two lactobacilli (Lactobacillus paracasei and Lactobacillus casei) in the production of carboxylic acids by a Lactococcus lactis subsp.…”

Biochemical and molecular characterization of ?-ketoisovalerate decarboxylase, an enzyme involved in the formation of aldehydes from amino acids by

“…Amino acid catabolism is the final step of proteolysis. In lactic acid bacteria, it is mainly initiated by transamination that converts amino acids into a-keto acids [2][3][4], which are further converted into hydroxyacids or into flavor compounds such as carboxylic acids by an oxidative decarboxylation, or aldehydes by an enzymatic non-oxidative decarboxylation. Conversion of a-keto acids to aldehydes may also be chemically feasible [5,6].…”

Methylthioacetaldehyde, a possible intermediate metabolite for the production of volatile sulphur compounds from -methionine by Lactococcus lactis

“…Such a catabolic pathway was evidenced in lactococci by 13 C nuclear magnetic resonance (NMR) using [ 13 C]L L -methionine [8]. Other works [9][10][11] have demonstrated that the L Lmethionine transamination in L. lactis is catalysed by two aminotransferases, the aromatic aminotransferase (AraT) and the branched-chain aminotransferase (BcaT). Inactivation of both aminotransferases almost totally prevented the formation of VSCs in a cheese model [12], confirming that this pathway is the main route for VSCs formation by L. lactis.…”