bEnzymatic transamination of amino acids yields ␣-keto acids and is the initial step for the production of volatile compounds that contribute to the sensory perception of fermented foods such as salami. Lactobacillus sakei is one of the lactic acid bacterial strains commonly used in starter cultures. Although the genome sequence of L. sakei 23K lacks genes encoding typical branchedchain amino acid transaminases, transamination activity and the formation of amino acid-derived volatile metabolites could be demonstrated. A protein purified from L. sakei is held responsible for the transamination activity. By heterologous expression of the corresponding gene in Escherichia coli, we were able to characterize the transamination side activity of an enzyme annotated as a putative acylphosphatase (AcP). A transamination side activity of hen egg white lysozyme (HEWL) was also discovered. Both enzymes showed substrate specificity toward branched-chain and aromatic amino acids. AcP also accepted L-methionine. Activity was optimal at neutral pH for both enzymes, whereas AcP showed a significantly higher temperature optimum (55°C) than that of HEWL (37°C). Kinetic parameters revealed high affinity toward L-leucine for AcP (K m ؍ 1.85 mM) and toward L-isoleucine for HEWL (K m ؍ 3.79 mM). AcP seems to play a major role in the metabolism of amino acids in L. sakei.
The flavor of dry fermented sausages derives from the ingredients (meat, spices, and smoke) and the chemical changes occurring during the fermentation and drying process. Flavor formation in dry fermented sausage occurs by meat enzymes and bacterial fermentation, which are responsible for the metabolism of fats, carbohydrates, and proteins (1, 2). The autochthonous microbiota of fermented meat products is dominated by coagulase-negative staphylococci and lactic acid bacteria (LAB), mainly Lactobacillus sakei, Lactobacillus curvatus, and Lactobacillus plantarum (3, 4). L. sakei, formerly known as L. sake, is one of the dominating LAB in dry spontaneously fermented sausages (3, 5). Therefore, it is commonly used in starter cultures for the production of dry fermented sausages (6).Free amino acids, incorporated into LAB cells by amino acid transporters or released intracellularly by peptidase activity, are converted, among other metabolic pathways, to various volatile compounds by transamination and decarboxylation. Enzymatic transamination leading to the formation of the corresponding ␣-keto acids is the first step of this intracellular amino acid catabolism to volatile metabolites in LAB (Fig. 1) (2, 7). The transformation of branched-chain, aromatic, and sulfur-containing amino acids by these reactions yields volatile compounds, which contribute to the sensory perception of fermented food (8, 9). It has been calculated that 11.8% of the volatile compounds in salami probably originate from amino acid catabolism (10).Analysis of the genome sequence of L. sakei 23K delivered insights into the strain's potential concerning aroma formation from amino acid substrates. G...
