Sulfoglycolysis is a catabolic process in primary metabolism in which sulfoquinovose is metabolized to produce energy and carbon-building blocks. Sulfoglycolysis pathways occur in a widevariety of organisms, and enable key steps in the degradation of sulfoquinovosyl diacylglycerol, a sulfolipid found in plants and cyanobacteria into sulfite and sulfate. Sulfoglycolysis converts sulfoquinovose into pyruvate CH3COCOO− + H+. The free energy is used to form the high-energy molecules ATP and NADH. Unlike glycolysis, all known sulfoglycolysis pathways convert only half the carbon content of sulfoquinovose into pyruvate; the remained is excreted as a C3-sulfonate: 2,3-dihydroxypropanesulfonate or sulfolactate. Two sulfoglycolytic processes are known. The sulfoglycolytic Embden-Meyerhof-Parnas pathway, first identified in Escherichiacoli, involves the degradation of sulfoquinovose to 2,3-dihydroxypropanesulfonate, and shares similarity with the Embden-Meyerhof-Parnas glycolysis pathway. The sulfoglycolytic Entner-Doudoroff pathway, first identified in Pseudomonas putida SQ1, involves the degradation of sulfoquinovose to sulfolactate, and shares similarity to the Entner-Doudoroff pathway of glycolysis. Both pathways lead to the production of dihydroxyacetone phosphate in processes that consume ATP. Energy is formed in later stages through the 'pay-off' phase of glycolysis through substrate-level phosphorylation to produce ATP and NADH.
Growth of bacteria on sulfoquinovose and its glycosides
A range of bacteria can grow on sulfoquinovose or its glycosides as sole carbon source. E. coli can grow on sulfoquinovose, methyl α-sulfoquinovoside and α-sulfoquinovosyl glycerol. Growth on sulfoquinovosyl glycerol is both faster and leads to higher cell density than for growth on sulfoquinovose. Pseudomonas aeruginosa strain SQ1, Klebsiella sp. strain ABR11, Klebsiella oxytoca TauN1, and Agrobacterium sp. strain ABR2 can grow on sulfoquinovose as sole carbon source. A strain of Flavobacterium was identified that could grow on methyl α-sulfoquinovoside.
Production of sulfoquinovose and its mutarotation
Sulfoquinovose is rarely found in its free form in nature; rather it occurs predominantly as a glycoside, SQDG. SQDG can be deacylated to form lyso-SQDG and sulfoquinovosylglycerol. Sulfoquinovose is obtained from SQ glycosides by the action of sulfoquinovosidases, which are glycoside hydrolases that can hydrolyse the glycosidic linkage in SQDG, or its deacylated form, sulfoquinovosyl glycerol. The first sulfoquinovosidase identified was YihQ from Escherichia coli. It exhibits a preference for the naturally occurring 2’R-SQGro. Sulfoquinovosidases cleave SQ glycosides with retention of configuration, initially forming α-sulfoquinovose. Sulfoglycolysis encoding operons contain gene sequences encoding aldose-1-epimerases that act as sulfoquinovose mutarotases, catalyzing the interconversion of the α and β anomers of sulfoquinovose.
Sulfo-EMP pathway
The major steps in the sulfo-EMP pathway are:
isomerization of sulfoquinovose to sulfofructose ;
phosphorylation of sulfofructose to sulfofructose-1-phosphate ;
retro-aldol cleavage of sulfofructose-1-phosphate to afford dihydroxyacetone phosphate and -sulfolactaldehyde ;
reduction of sulfolactaldehyde to -2,3-dihydroxypropane-1-sulfonate.
Expression of proteins within the sulfo-EMP operon of E. coli is regulated by a transcription factor termed CsqR. CsqR binds to DNA sites within the operon encoding the sulfo-EMP pathway, functioning as a repressor. SQ and SQGro function as derepressors of CsqR.
Sulfo-ED pathway
The major steps in the sulfo-ED pathway are:
oxidation of sulfoquinovose to sulfogluconolactone ;
hydrolysis of sulfogluconolactone to sulfogluconate acid ;
dehydration of sulfogluconic acid to 2-keto-3,6-dideoxy-6-sulfogluconate ;
retro-aldol cleavage of 2-keto-3,6-dideoxy-6-sulfogluconate to give pyruvate and -sulfolactaldehyde ;
oxidation of sulfolactaldehyde to -sulfolactate.
Degradation of DHPS and SL
The C3 sulfonates DHPS and SL are metabolized for their carbon content, as well as to mineralize their sulfur content. Metabolism of DHPS typically involves conversion to SL. Metabolism of SL can occur in several ways including:
elimination of sulfite to afford pyruvate;
oxidation to sulfopyruvate, transamination to cysteate, and elimination of sulfite to afford pyruvate and ammonia;
oxidation to sulfopyruvate, decarboxylation to sulfoacetaldehyde, and phosphorylation to afford acetylphosphate and sulfite.
