Sulfoglycolysis

Sulfoglycolysis is a catabolic process in primary metabolism in which sulfoquinovose (6-deoxy-6-sulfonato-glucose) is metabolized to produce energy and carbon-building blocks. Sulfoglycolysis pathways occur in a wide variety of organisms, and enable key steps in the degradation of sulfoquinovosyl diacylglycerol (SQDG), a sulfolipid found in plants and cyanobacteria into sulfite and sulfate. Sulfoglycolysis converts sulfoquinovose (C₆H₁₂O₈S⁻) into various smaller metabolizable carbon fragments such as pyruvate and dihydroxyacetone phosphate that enter central metabolism. The free energy is used to form the high-energy molecules ATP (adenosine triphosphate) and NADH (reduced nicotinamide adenine dinucleotide). Unlike glycolysis, which allows metabolism of all carbons in glucose, sulfoglycolysis pathways convert only a fraction of the carbon content of sulfoquinovose into smaller metabolizable fragments; the remainder is excreted as C₃-sulfonates 2,3-dihydroxypropanesulfonate (DHPS) or sulfolactate (SL); or C₂-sulfonates isethionate or sulfoacetate.

Several sulfoglycolytic pathways are known:

• The sulfoglycolytic Embden-Meyerhof-Parnas (sulfo-EMP) pathway, first identified in Escherichia coli, involves the degradation of sulfoquinovose to 2,3-dihydroxypropanesulfonate (DHPS), and shares similarity with the Embden-Meyerhof-Parnas glycolysis pathway. This pathway leads to the production of the C3 intermediate dihydroxyacetone phosphate.
• The sulfoglycolytic Entner-Doudoroff (sulfo-ED) pathway, first identified in Pseudomonas putida SQ1, involves the degradation of sulfoquinovose to sulfolactate, and shares similarity to the Entner-Doudoroff pathway of glycolysis. This pathway leads to the production of the C3 intermediate pyruvate.
• The sulfofructose transaldolase pathway, first identified in Bacillus aryabhattai and Bacillus megaterium, involves isomerization of SQ to sulfofructose, and then a transaldolase cleaves SF to 3-sulfolactaldehyde (SLA), while the non-sulfonated C3-(glycerone)-moiety is transferred to an acceptor molecule, glyceraldehyde phosphate (GAP), yielding fructose-6-phosphate (F6P). The SLA released can either be oxidized (to sulfolactate) or reduced (to dihydroxypropanesulfonate) and then excreted.
• The sulfoglycolytic transketolase (sulfo-TL) pathway was first identified in Clostridium sp. MSTE9. It involves isomerization of SQ to sulfofructose, and then a transketolase cleaves SF to 4-sulfoerythrose (SE), while the C2-moiety is transferred to an acceptor molecule, glyceraldehyde phosphate (GAP), yielding xylulose-5-phosphate (Xu5P). 4-Sulfoerythrose is isomerized to 4-sulfoerythrulose (SEu), whereupon a second round of transketolase catalyzed reaction cleaves SE to sulfoacetaldehyde, while the non-sulfonated C2-moiety is transferred to an acceptor molecule, glyceraldehyde phosphate (GAP), yielding a second molecule of xylulose-5-phosphate (Xu5P). Finally, the sulfoacetaldehyde is reduced to isethionate and excreted.

Additionally, there are sulfoquinovose 'sulfolytic' pathways that allow degradation of sulfoquinovose through cleavage of the C-S bond. These include:

• The sulfoglycolytic sulfoquinovose monooxygenase (sulfo-SMO) pathway, first identified in Agrobacterium tumerfaciens and Novosphingobium aromaticivorans, involves the degradation of sulfoquinovose to glucose and sulfite. Glucose formed in this pathway enters glycolysis.
• The sulfoglycolytic sulfoquinovose dioxygenase (sulfo-SMO) pathway.

In all pathways, energy is formed by breakdown of the carbon-rich fragments in later stages through the 'pay-off' phase of glycolysis through substrate-level phosphorylation to produce ATP and NADH.