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Chapter 9 : Introduction to Metabolic Pathways
The major pathway for assimilation of nitrogen is through glutamine. The step that links metabolism of carbon and nitrogen is conversion of 2-ketoglutarate to glutamate and glutamine. It is not surprising that in gram-negative bacteria the ultimate determinant of expression of nitrogen metabolism genes is the ratio of the intracellular concentrations of 2-ketoglutarate and glutamine. Some biosynthetic pathways in gram-positive bacteria have received so little attention that they could not be the subject of separate chapters. These include those for synthesis of L- and D-alanine and histidine and the intersecting pathways for glycine, serine, and cysteine. The his genes of Streptomyces coelicolor are organized in three unlinked clusters, one of which has been sequenced nearly in its entirety. The major route of serine biosynthesis in Bacillus subtilis and Micrococcus luteus is the 3-phosphoglycerate pathway characteristic of most bacteria. The first enzyme of this pathway, 3-phosphoglycerate dehydrogenase, is feedback inhibited by serine. In Clostridium acidi-urici, however, a different pathway for interconversion seems to function. In this case, glycine and formaldehyde condense to form serine. Mutations at two B. subtilis loci cause serine auxotrophy. SerA- mutants require either serine or glycine for growth. Growth is improved if threonine and serine are both provided. The first two enzymes of sulfate utilization, ATP sulfurylase and adenosine-5'-phosphosulfate kinase, and enzyme activities that convert activated sulfate to sulfite and sulfide and catalyze incorporation into cysteine are present in both B. subtilis and Escherichia coli. The only pathway to D-alanine is by racemization of L-alanine.