- characteristics
- Biological importance of the hepatoses
- In photosynthesis and the pentose phosphate pathway
- In lipo-polysaccharides (LPS)
- In the glycoproteins of bacteria
- Synthesis
- References
The heptoses are monosaccharides having seven carbons and with the empirical formula C 7 H 14 O 7. These sugars, such as other monosaccharides, are polyhydroxylated and can be: aldoheptoses, which have an aldehyde function at carbon one, or ketoheptoses, which have a ketone group at carbon 2.
Heptoses are synthesized in metabolic pathways, such as the Calvin cycle of photosynthesis and the non-oxidative phase of the pentose phosphate pathway. They are constituents of lipo-polysaccharides (LPS) in the cell wall of Gram-negative bacteria such as Escherichia coli, Klebsiella sp., Neisseria sp., Proteus sp., Pseudomonas sp., Salmonella sp., Shigella sp., And Vibrio sp.
Source: Fvasconcellos
characteristics
Heptoses, similar to hexoses, exist predominantly in their cyclic form. Aldoheptoses have five asymmetric carbons and cycle to form a pyranose. In contrast, ketoheptoses possess four asymmetric carbons, where they also form pyranoses.
A very common natural ketoheptose in living organisms is sedoheptulose. This sugar is important in the formation of hexose sugars in photosynthesis and carbohydrate metabolism in animals.
When sedoheptulose is heated in dilute mineral acid, it forms an equilibrium mineral mixture, where 80% is crystallized as 2,7-anhydro-β-D-altro-heptulopyranose and 20% is sedoheptulose.
The chemical determination of the heptoses is made with sulfuric acid and cysteine, diphenylamine and floroglucinol. Under certain conditions, it is possible to differentiate heptose from other sugars. It can even differentiate between aldoheptoses and ketoheptoses.
Many aldoheptoses have the glycero-D-mannoheptose configuration. Heptose, together with the eight-carbon keto-sugar acid (3-deoxy-D-manno-2-octulosonic acid, a Kdo sugar), are structural components of LPS, in the outer membrane of the lipid bilayer of bacteria.
LPS can be extracted using a 45% phenol in water mixture. Then, the heptoses and KDO sugars can be identified by colorimetric and chromatographic techniques.
Biological importance of the hepatoses
In photosynthesis and the pentose phosphate pathway
Enzymes that convert triose phosphate, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, produced by the assimilation of CO 2, into starch are found in the stroma of the chloroplast. The formation of triose phosphate and the recovery of the carbons, to begin the fixation of CO 2 again, constitute two stages of the Calvin cycle.
During the carbon recovery stage, the enzyme aldolase is responsible for converting erythrose 4-phosphate (a four-carbon metabolite (E4P)) and dihydroxyketone phosphate (a three-carbon metabolite) into sedoheptulose 1,7-bisphosphate.
This ketoheptosse is transformed by several steps, enzymatically catalyzed, into ribulose 1,5-bisphosphate.
Ribulose 1,5-bisphosphate is the initiating metabolite of the Calvin cycle. On the other hand, the biosynthesis of sedoheptulose 7-phosphate (S7P) takes place in the pentose phosphate pathway, which is a pathway present in all living organisms. In this case, the action of a transketolase transforms two phosphate pentose into S7P and glyceraldehyde-3-phosphate (GAP).
Then, through two steps catalyzed by a transaldolase and a transketolase, S7P and GAP are transformed into fructose-6-phosphate and GAP. Both are metabolites of glycolysis.
In lipo-polysaccharides (LPS)
Heptoses are present in lipopolysaccharides and polysaccharides of the capsule of bacteria. The structural motif of LPS in Enterobacteriaceae consists of lipid A, which consists of a dimer of 2-amino-2-deoxy-D-glucose linked by β - (1®6) bond. It has two phosphate esters and long chain fatty acid groups.
Lipid A is linked to a central region by a bridge of three sugars Kdo and ketodeoxyoctulosonic acid, linked by glycosidic bonds (2®7). This region is linked to L-glycero-D-mannoheptoses heptose, with alpha anomeric configuration. There is an O-antigenic region.
This structural motif is present in Gram negative bacteria, such as Escherichia coli, Klebsiella sp., Yersinia sp., Pseudomonas sp., Salmonella sp., As well as other pathogenic bacteria.
There are variants of heptose that include different configurations of the stereocenter of pyranoses in oligosaccharides, as well as of side chains in polysaccharides. D-glycero-D-manno-heptopyranosil is present in Yersinia enterocolitica, Coxiella burnetti, Mannheimia haemolitica, Aeromonas hydrophila, and Vibrio salmonicida.
Heptose D-glycero-D-manno-heptose are present as side chain units in the outer region of the LPS of Proteus and Haemophilus influenzae strains; and as short oligomeric side chains linked by α - (1®3) or α - (1®2), linked to the Klebsiella pneumonie LPS structural motif.
In Vibrio cholerae strains, the O-antigenic region possesses D-glycero-D-manno-heptose with both anomeric configurations (alpha and beta).
In the glycoproteins of bacteria
Its surface layers (S layers) are composed of identical protein subunits, which cover it in a two-dimensional organization. They are found in Gram-positive and Gram-negative bacteria and archaebacteria. The proteins in this layer have glycopeptides that are elongated by polysaccharide chains.
The glycoproteins of Aneurinibacillus thermoaerophilus, a gram-positive bacterium, possess repeating units of disaccharides ®3) -Dglycero- β -D-mano-Hepp- (1®4) - α -L-Rhap- (1® in the S layer.
One of the functions of glycoproteins is adhesion. For example, there is a glycoprotein that measured adhesion as an autotransporter protein (AIDA-I) in E. coli strains. Glycoprotein biosynthesis occurs by glycosyl transferases, such as heptosyl transferase, which requires ADP glycero-manno-heptose.
Synthesis
The chemical synthesis and the combination of chemical and enzymatic methods of activated heptose phosphate and heptose-nucleotide have made it possible to elucidate the metabolic pathways that microorganisms use to produce these substances.
Many synthesis methods prepare 6-epimeric manno-heptose to synthesize L-glycero-D-manno-heptose. These methods are based on elongation of the chain from the anomeric carbon, or aldehyde group, using Grignard reagents. Glycosylations are carried out in the presence of acyl protecting groups.
In this way, there is stereocontrol preserving the α -anomeric configuration. Anomeric thioglycosides and trichloroacetimidate derivatives serve as heptosyl group donors. The more recent procedures involve the selective formation of β -heptosides and 6-deoxy-heptoside derivatives.
Activated heptose-nucleotide biosynthesis begins from sedoheptulose 7-phosphate, which is converted to D-glycero-D-manno-heptose 7-phosphate. A phosphomutase has been proposed to form anomeric heptosyl phosphate. Then, a heptosyl transferase catalyzes the formation of ADP D-glycero-D-manno-heptose.
Finally, an epimerase changes the configuration of ADP D-glycero-D-manno-heptose to ADP L-glycero-D-manno-heptose.
Additionally, chemical studies have been carried out to find out the mechanisms by which these enzymes carry out catalysis. For example, they use benzylated benzyl mannopyranoside, which is oxidized to give the manouronic derivative.
Treatment with hydrochloric acid transforms the manouronic derivative into diazoketone. Treatment with diazobenzyl phosphate produces a mixture of L-glycero-7-phosphate and D-glycero-7-phosphate.
References
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- Cui, SW 2005. Food carbohydrates: chemistry, physical properties, and applications. CRC Press, Boca Raton.
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- Hofstad, T. 1974. The Distribution of heptose and 2-keto-3-deoxy-octonate in Bacteroidaceae. Journal of General Microbiology, 85, 314–320
- Kosma, P. 2008. Occurrence, synthesis and biosynthesis of bacterial heptoses. Current Organic Chemistry, 12, 1021-1039.
- Nelson, DL, Cox, MM 2017. Lehninger principles of biochemistry. WH Freeman, New York.
- Pigman, W. 1957. The carbohydrates: chemistry, biochemistry, physiology. Academic Press, New York.
- Pigman, W., Horton, D. 1970. The carbohydrates: chemistry and biochemistry. Academic Press, New York.
- Sinnott, ML 2007. Carbohydrate chemistry and biochemistry structure and mechanism. Royal Society of Chemistry, Cambridge.
- Stick, RV, Williams, SJ 2009. Carbohydrates: the essential molecules of life. Elsevier, Amsterdam.
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