Subsystem: Fermentations: Lactate
This subsystem's description is:
This subsystem has been initially encoded by master:NeilJ_UCSD and master:MaryP_UCSD, whose contributions we gratefully acknowledge, as "Homofermentative_lactate_fermentation" and Heterofermentative_lactate_fermentation. It is currently curated by SelkovEE and SvetaG.
Three major types of lactate fermentations have been described: homofermentation, heterofermentation, and bifidum pathway (see attached diagrams). Homofermentation is associated with the Embden-Meyerhof glycolytic pathway (see the corresponding SS for details); heterofermentation uses the Pentose Phosphate pathway (see the corresponding SS). Theoretical ATP yield of homofermentation is 2 ATP molecules per glucose, the yield of heterofermentative pathway is 1 ATP per glucose. Bifidobacteria perform specific bifidum pathway with net gain of 2.5 ATP molecules per 1 glucose.
Although the presence of Xylulose-5-phosphate phosphoketolase is generally considered diagnostic for lactate heterofermentation pathway, in a few organisms containing XPK, no lactate dehydrogenase could be identified (variant code [9] - see below), hence apparently XPK activity is not necessarily associated with production of lactate.
The presence of Fructose-6-phosphate phosphoketolase enzymatic activity is characteristic for bifidum pathway. However, since no distinction between Fructose-6-phosphate phosphoketolase and Xylulose-5-phosphate phosphoketolase activities can be done currently based on protein sequence alone, and a single enzyme has been shown to catalyze both reactions in several species, all clear XPK/FPK homologs have been annotated as potentially capable of catalyzing both reactions - “Xylulose-5-phosphate phosphoketolase (EC 4.1.2.9); Fructose-6-phosphate phosphoketolase (EC 4.1.2.22)” probably leading to severe over-annotation. In an attempt to minimize the damage of it, the XPK/FPK homologs where FPK activity was experimentally demonstrated (in Bifidum species) or could be inferred from genome context (see below) were annotated “Xylulose-5-phosphate phosphoketolase (EC 4.1.2.9) / Fructose-6-phosphate phosphoketolase (EC 4.1.2.22)” with a slash “/”. Also, such organisms were assigned variant codes [_4]. In addition to Bifidobacteria these include Clostridium acetobutylicum and Streptococcus agalactiae where the absence of Glucose-6-phosphate 1-dehydrogenase (EC 1.1.1.49) and 6-phosphogluconate dehydrogenase, decarboxylating (EC 1.1.1.44) may necessitate FPK activity in XPK/FPK homolog.
For more information, please check out the description and the additional notes tabs, below
| Diagram | Functional Roles | Subsystem Spreadsheet | Description | Additional Notes | Scenarios | |||||||||
Diagram 'd02' is not a new diagram.
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