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Category: Bacterial Pathogenesis; Microbial Genetics and Molecular Biology
Biosynthesis and Function of Membrane Lipids, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555818388/9781555810535_Chap28-1.gif /docserver/preview/fulltext/10.1128/9781555818388/9781555810535_Chap28-2.gifAbstract:
Bacillus spp.especially Bacillus subtilis, have major advantages for the study of the function of membrane lipids in bacterial membrane physiology. Bacillus spp. offer two additional biological phenomena to membrane lipid research. The first is sporulation, during which membrane lipid phenomena accompanying differentiation could be studied. The second is temperature-induced control of the composition of the membrane lipids, in which detectable amounts of unsaturated fatty acids (UFAs) are synthesized by Bacillus spp. only at low growth temperatures. This chapter focuses on the aspects of lipid metabolism that differ from those of the better studied gram-negative bacteria. The primary focus is on B. subtilis, because of its advantages in genetic analysis. Fatty acid synthesis is the best-studied aspect of Bacillus lipid metabolism. The chapter first focuses on the aspects of the pathway utilized in other bacteria and then discusses the product diversification reactions that result in the distinctive fatty acid compositions of bacilli. Two efforts that used E. coli as a model organism have contributed substantially for understanding of membrane lipids in bacteria. The first was the isolation of a growing collection of well-characterized mutants affecting lipid metabolism. The second is the continuing progress in molecular cloning of the lipid metabolism genes of this organism.
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Acylation of sn-G3P by two successive transfers of acyl groups from acyl-ACP. This pathway has been demonstrated in E. coli and clostridia. sn-G3P is probably synthesized by reduction of a glycolytic intermediate such as dihydroxyacetone phosphate or glyceraldehyde 3-phosphate.
Acylation of sn-G3P by two successive transfers of acyl groups from acyl-ACP. This pathway has been demonstrated in E. coli and clostridia. sn-G3P is probably synthesized by reduction of a glycolytic intermediate such as dihydroxyacetone phosphate or glyceraldehyde 3-phosphate.
Initiation of fatty acid biosynthesis in E. coli. The initiation of new acyl chains is accomplished by the action of three enzymes: 1, acetyl-CoA carboxylase; 2, malonyl-CoA: ACP transacylase; 3, 3-ketoacyl-ACP synthase III.
Initiation of fatty acid biosynthesis in E. coli. The initiation of new acyl chains is accomplished by the action of three enzymes: 1, acetyl-CoA carboxylase; 2, malonyl-CoA: ACP transacylase; 3, 3-ketoacyl-ACP synthase III.
Elongation cycle of fatty acid biosynthesis. The elongation of a growing acyl chains is accomplished by the action of four enzymes: 1, 3-ketoacyl-ACP synthase; 2, 3-ketoacyl-ACP reductase; 3, 3-hydroxyacyl-ACP dehydrase; 4, enoyl reductase (frans-2-acyl-ACP reductase).
Elongation cycle of fatty acid biosynthesis. The elongation of a growing acyl chains is accomplished by the action of four enzymes: 1, 3-ketoacyl-ACP synthase; 2, 3-ketoacyl-ACP reductase; 3, 3-hydroxyacyl-ACP dehydrase; 4, enoyl reductase (frans-2-acyl-ACP reductase).
Proposed pathway for incorporation of branched-chain 2-keto acids into fatty acids in B. subtilis. Branched-chain amino acids are converted to branched-chain 2-keto acids by a branched-chain amino acid transaminase (reaction 1). The branched-chain 2-keto acid can then be converted into a CoA ester by a branched-chain 2-keto acid dehydrogenase (reaction 2) or an aldehyde derivative by a branched-chain 2-keto acid decarboxylase (reaction 3). It is assumed that the primers produced by reactions 2 and 3 could then be condensed with malonyl-ACP (see text and Fig. 6 ).
Proposed pathway for incorporation of branched-chain 2-keto acids into fatty acids in B. subtilis. Branched-chain amino acids are converted to branched-chain 2-keto acids by a branched-chain amino acid transaminase (reaction 1). The branched-chain 2-keto acid can then be converted into a CoA ester by a branched-chain 2-keto acid dehydrogenase (reaction 2) or an aldehyde derivative by a branched-chain 2-keto acid decarboxylase (reaction 3). It is assumed that the primers produced by reactions 2 and 3 could then be condensed with malonyl-ACP (see text and Fig. 6 ).
Synthesis of complex lipids. The three phospholipid species found in E. coli are synthesized from phosphatidic acid (top center structure) by a series of reactions catalyzed by six enzymes: 1, CDP-diglyceride synthase (phosphatidate cytidyltransferase); 2, phosphatidylserine synthase; 3, phosphatidylserine decarboxylase; 4, phosphatidylglycerol-phosphate synthase; 5, phosphatidylglycerol-phosphate phosphatase; 6, cardiolipin synthase. The same reactions are believed to occur in bacilli. In addition, diglyceride is synthesized by dephosphorylation of phosphatidic acid by phosphatidic acid phosphatase (reaction 7). A portion of the diglyceride is then glucosylated by one or two transfers of glucose from UDP-glucose (reaction 8 and 9). Reactions 7 through 9 have been detected in other organisms but not bacilli. Not shown is the amino acylation of phosphatidylglycerol (the product of reaction 5) by lysyl-tRNA to form lysylphosphatidylglycerol.
Synthesis of complex lipids. The three phospholipid species found in E. coli are synthesized from phosphatidic acid (top center structure) by a series of reactions catalyzed by six enzymes: 1, CDP-diglyceride synthase (phosphatidate cytidyltransferase); 2, phosphatidylserine synthase; 3, phosphatidylserine decarboxylase; 4, phosphatidylglycerol-phosphate synthase; 5, phosphatidylglycerol-phosphate phosphatase; 6, cardiolipin synthase. The same reactions are believed to occur in bacilli. In addition, diglyceride is synthesized by dephosphorylation of phosphatidic acid by phosphatidic acid phosphatase (reaction 7). A portion of the diglyceride is then glucosylated by one or two transfers of glucose from UDP-glucose (reaction 8 and 9). Reactions 7 through 9 have been detected in other organisms but not bacilli. Not shown is the amino acylation of phosphatidylglycerol (the product of reaction 5) by lysyl-tRNA to form lysylphosphatidylglycerol.
Pathways of branched-chain fatty acid and UFA syntheses in B. subtilis. (A) Pathway of synthesis from branched-chain acyl-CoA esters as primers. (B) The other proposed pathway ( 52 ) of synthesis from branched-chain 2-keto acids as primer sources. UFAs are formed by cold-induced desaturation of phospholipids ( 25 ) or acyl-CoAs ( 19 ).
Pathways of branched-chain fatty acid and UFA syntheses in B. subtilis. (A) Pathway of synthesis from branched-chain acyl-CoA esters as primers. (B) The other proposed pathway ( 52 ) of synthesis from branched-chain 2-keto acids as primer sources. UFAs are formed by cold-induced desaturation of phospholipids ( 25 ) or acyl-CoAs ( 19 ).
Fatty acid composition of total membrane lipid extracts from B. subtilis a
Fatty acid composition of total membrane lipid extracts from B. subtilis a