Metabolic pathways for cytotoxic end product formation from glutamate- and aspartate-containing peptides by Porphyromonas gingivalis

Abstract

Metabolic pathways involved in the formation of cytotoxic end products by Porphyromonas gingivalis were studied. The washed cells of P. gingivalis ATCC 33277 utilized peptides but not single amino acids. Since glutamate and aspartate moieties in the peptides were consumed most intensively, a dipeptide of glutamate or aspartate was then tested as a metabolic substrate of P. gingivalis. P. gingivalis cells metabolized glutamylglutamate to butyrate, propionate, acetate, and ammonia, and they metabolized aspartylaspartate to butyrate, succinate, acetate, and ammonia. Based on the detection of metabolic enzymes in the cell extracts and stoichiometric calculations (carbon recovery and oxidation/reduction ratio) during dipeptide degradation, the following metabolic pathways were proposed. Incorporated glutamylglutamate and aspartylaspartate are hydrolyzed to glutamate and aspartate, respectively, by dipeptidase. Glutamate is deaminated and oxidized to succinyl-coenzyme A (CoA) by glutamate dehydrogenase and 2-oxoglutarate oxidoreductase. Aspartate is deaminated into fumarate by aspartate ammonia-lyase and then reduced to succinyl-CoA by fumarate reductase and acyl-CoA:acetate CoA-transferase or oxidized to acetyl-CoA by a sequential reaction of fumarase, malate dehydrogenase, oxaloacetate decarboxylase, and pyruvate oxidoreductase. The succinyl-CoA is reduced to butyryl-CoA by a series of enzymes, including succinate-semialdehyde dehydrogenase, 4-hydroxybutyrate dehydrogenase, and butyryl-CoA oxidoreductase. A part of succinyl-CoA could be converted to propionyl-CoA through the reactions initiated by methylmalonyl-CoA mutase. The butyryl- and propionyl-CoAs thus formed could then be converted into acetyl-CoA by acyl-CoA:acetate CoA-transferase with the formation of corresponding cytotoxic end products, butyrate and propionate. The formed acetyl-CoA could then be metabolized further to acetate.

FIG. 1

Utilization of single-form and peptide-form amino acids contained in tryptone by the washed cells of P. gingivalis ATCC 33277. Glx, glutamate plus glutamine; Asx, aspartate plus asparagine.

FIG. 2

Proposed metabolic pathways for glutamate and aspartate in P. gingivalis. 1, glutamate dehydrogenase; 2, 2-oxoglutarate oxidoreductase; 3, succinate-semialdehyde dehydrogenase; 4, 4-hydroxybutyrate dehydrogenase; 5, enzyme(s) for the conversion of 4-hydroxybutyrate into crotonoyl-CoA; 6, butyryl-CoA oxidoreductase; 7, acyl-CoA:acetate CoA-transferase; 8, methylmalonyl-CoA mutase; 9, aspartate aminotransferase; 10, fumarase; 11, malate dehydrogenase; 12, oxaloacetate decarboxylase; 13, pyruvate oxidoreductase; 14, phosphotransacetylase; 15, acetate kinase; 16, fumarate reductase. Broken lines, not measured but expected pathway.