Peptidases and amino acid catabolism in lactic acid bacteria

The conversion of peptides to free amino acids and their subsequent utilization is a central metabolic activity in prokaryotes. At least 16 peptidases from lactic acid bacteria (LAB) have been characterized biochemically and/or genetically. Among LAB, the peptidase systems of Lactobacillus helveticus and Lactococcus lactis have been examined in greatest detail. While there are homologous enzymes common to both systems, significant differences exist in the peptidase complement of these organisms. The characterization of single and multiple peptidase mutants indicate that these strains generally exhibit reduced specific growth rates in milk compared to the parental strains. LAB can also catabolize amino acids produced by peptide hydrolysis. While the catabolism of amino acids such as Arg, Thr, and His is well understood, few other amino acid catabolic pathways from lactic acid bacteria have been characterized in significant detail. Increasing research attention is being directed toward elucidating these pathways as well as characterizing their physiological and industrial significance.     

Metabolism of dicarboxylic amino acids and their amides in bacteria of the genus Citrobacter]

In 58 Citrobacter strains the pathways of the utilization of dicarbonic amino acids and their amides were studied. These organisms were found to be incapable of decarboxylating glutaminic and asparaginic acids, as well as their amides. All the strains could actively desamidizate asparagine. Not all of these strains showed glutaminase activity. Aspartate-aminotransferase occurred twice as often as alanine-aminotransferase, the level of activity being approximately the same. The Citrobacter strains desamidizated asparaginic acid with great constancy, but only in 1/3 of them this reaction occurred via an aspartase route. The desamidization of asparaginic acid in Citrobacter seemed to proceed in different ways. The desamidization of glutaminic acid was observed only in a part of the strains, and the reaction proceeded less actively.     

Monosaccharide composition of the lipopolysaccharides of bacteria of the genus Citrobacter

The authors studied antigens obtained by Grasset's method from 13 strains of Citrobacter of the International collection. The strains possessed O- and H-antigens whose behaviur in the electric field differed. All the strains under study were divided into two groups (by the number of serologically-active components of their O-antigens); representatives of the second group had no cathode O-antigen component. Chemical composition of specific lipopolysaccharides (LPS) obtained by Westphal's method was determined. Fourteen different sugars were revealed. The strains under study were referred to the known chemotypes. Strain 16/52 (8a, 8c) was for the first time studied in respect to the monosaccharide composition of specific LPS, and was referred to chemotype designated as CC-L.     

Aromatic amino acid catabolism in trypanosomatids

Trypanosomatids cause important human diseases, like sleeping sickness, Chagas disease, and the leishmaniases. Unlike in the mammalian host, the metabolism of aromatic amino acids is a very simple pathway in these parasites. Trypanosoma brucei and Trypanosoma cruzi transaminate the three aromatic amino acids, the resulting 2-oxo acids being reduced to the corresponding lactate derivatives and excreted. In T. cruzi, two enzymes are involved in this process: a tyrosine aminotransferase (TAT), which despite a high sequence similarity with the mammalian enzyme, has a different substrate specificity; and an aromatic L-2-hydroxyacid dehydrogenase (AHADH), which belongs to the subfamily of the cytosolic malate dehydrogenases (MDHs), yet has no MDH activity. In T. cruzi AHADH the substitution of Ala102 for Arg enables AHADH to reduce oxaloacetate. In the members of the 2-hydroxyacid dehydrogenases family, the residue at this position is known to be responsible for substrate specificity. T. cruzi does not possess a cytosolic MDH but contains a mitochondrial and a glycosomal MDH; by contrast T. brucei and Leishmania spp. possess a cytosolic MDH in addition to glycosomal and mitochondrial isozymes. Although Leishmania mexicana also transaminates aromatic amino acids through a broad specificity aminotransferase, the latter presents low sequence similarity with TATs, and this parasite does not seem to have an enzyme equivalent to T. cruzi AHADH. Therefore, these closely related primitive eukaryotes have developed aromatic amino acid catabolism systems using different enzymes and probably for different metabolic purposes.     

Flavour formation by amino acid catabolism

Microbial catabolism of amino acids produces flavour compounds of importance for foods such as cheese, wine and fermented sausages. Lactic acid bacteria are equipped with enzyme systems for using the amino acids in their metabolism and are useful for flavour formation of foods. Branched-chain amino acids (Leu, Ile, Val) are converted into compounds contributing to malty, fruity and sweaty flavours; catabolism of aromatic amino acids (Phe, Tyr, Trp) produce floral, chemical and faecal flavours; aspartic acid (Asp) is catabolised into buttery flavours and sulphuric amino acids (Met, Cys) are transferred into compounds contributing to boiled cabbage, meaty and garlic flavours.     

Catabolism of amino acids in bacteria of the genus Klebsiella

Propagation and activity level of 18 enzymes catalyzing deamination reactions of dicarboxylic and oxyamino acids and enzymes of amino acid reamination and amino acid N-acyl-derivatives' deacylation have been studied in Klebsiella bacteria. Klebsiella the most actively utilizes serin, threonine, aspartic and glutamic acids and aromatic amino acids. The first three amino acids are utilized by deamination, aromatic acids- in aminotransferase reaction with alpha-ketoglutaric acid, glutamic acid--by deamination and decarboxylation. Besides, Klebsiella actively deacylates N-acyl-derivatives of amino acids.     

Nitrogen regulation of amino acid catabolism in Neurospora crassa

Neurospora crassa can utilize numerous compounds including certain amino acids as a sole nitrogen source. Mutants of the nit-2 locus, a regulatory gene which is postulated to mediate nitrogen catabolite repression, are deficient in the ability to utilize several amino acids as well as other nitrogen sources used by wild type. Various enzymes involved in amino acid catabolism were found to be regulated in distinct ways. Arginase, ornithine transaminase, and pyrroline-5-carboxylate dehydrogenase are all inducible enzymes but are not subject to nitrogen catabolite repression. By contrast, proline oxidase and the amino acid transport system(s) are controlled by nitrogen repression and their synthesis is increased markedly when nitrogen source is limiting. Unlike wild type, the nit-2 mutant cannot derepress amino acid transport, although proline oxidase is regulated in a normal fashion.This work was supported by Grant R01 GM-23367 from the National Institutes of Health. T. J. F. was supported by an NIH Predoctoral Traineeship in Developmental Biology; G. A. M. is supported by NIH Career Development Award GM-00052.     

The amino acid sequences of the cytochromes c-555 from two green sulphur bacteria of the genus Chlorobium.

Amino acid seauences are proposed for the cytochromes c-555 from Chlorobium thiosulphatophilum and from the Chlorobium limicola component of "Chloropseudomonas ethylica 2K". Each is a sincle polypeptide chain, the former of 86, the latter of 99 residues, and, when aligned so as to give the best match, 47 residues are common to the two sequences. The sequences show some resemblance to those of cytochromes c5 and f. The bacteriochlorophyll a-proteins were also isolated and purified, and their amino acid compositions compared (see the Appendix). There are significant differences in the compositions, but not as great as those found for the cytochromes c-555. The significance of these observations for the taxonomy of the Chlorobiaceae and for the further development of the comparative biochemistry of cytochrome c is discussed. Detailed evidence for the sequences of the cytochromes c-555 has been deposited as Supplementary Publication SUP 50073 (36 pages) at the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies may be obtained on the terms given in Biochem. J. (1976) 153, 5.     

Metabolic engineering of sugar catabolism in lactic acid bacteria

Lactic acid bacteria are characterized by a relatively simple sugar fermentation pathway that, by definition, results in the formation of lactic acid. The extensive knowledge of traditional pathways and the accumulating genetic information on these and novel ones, allows for the rerouting of metabolic processes in lactic acid bacteria by physiological approaches, genetic methods, or a combination of these two. This review will discuss past and present examples and future possibilities of metabolic engineering of lactic acid bacteria for the production of important compounds, including lactic and other acids, flavor compounds, and exopolysaccharides.     

Clofibric acid stimulates branched-chain amino acid catabolism by three mechanisms

Prolonged activation of the c-Jun N-terminal kinase (JNK) has been suggested as a signal for apoptosis in response to a wide variety of stimuli. Using three cytocidal RNA or protein synthesis inhibitors (actinomycin D, anisomycin, and emetine), the potential role of JNK in activation of the mitochondrial apoptotic cascade was investigated in A549-S cells. Protein synthesis inhibition per se was not the cause of cell death as cycloheximide induced only growth arrest. All the cytocidal inhibitors induced cytochrome c release and caspases 9 activation within hours, but only anisomycin caused persistent JNK activation. Although, the JNK inhibitor, SP600125, inhibited JNK-dependent anisomycin-induced c-Jun phosphorylation, it was ineffective in preventing anisomycin-induced caspase activation and cell death. Thus, all three lethal macromolecule synthesis inhibitors can activate the mitochondrial apoptotic machinery independent of JNK activation, demonstrating that the mitochondrial apoptotic pathway can be activated independently of the JNK pathway in the absence of protein synthesis.