Fermentation dihydroxyacetone

Experiments with glyceraldehyde and dihydroxyacetone showed them to be fermentable, but results with methyl glyoxal were conflicting. With Lebedev juice no lactic acid was formed, but with top yeast Neuberg reported that lactic acid was detectable. In 1913 he and Kerb therefore proposed sugar was converted to methyl glyoxal from which pyruvic acid and ethanol were derived ... 

In aU bacteria able to ferment glycerol to 1,3-PD the genes encoding for the four enzymes specific for glycerol fermentation are expressed in common in the so-caUed dha regulon dha from dihydroxyacetone, the first intermediate in the... 

Dihydroxyacetone (HOCH2COCH2OH, melting point 89°C) is made by the action of sorbose bacterium fermentation of glycerin (HOCH2CHOHCH2OH). 

Fig. 5.1. Fermentation pathways present in facultative anaerobic eukaryotes. Examples of fermentation pathways present in the cytosol and in subcellular compartments. Fermentation processes localized in hydrogenosomes (1-3) and mitochondria (4) are indicated by the shaded box. Examples of the anaerobic ATP-producing organelles shown can be found in trichomon-ads (1), chytridiomycete fungi (2), Nyctotherus ovalis (3), and parasitic helminths, bivalves and Euglena gracilis (4). CoA coenzyme A, DHAP dihydroxyacetone phosphate, Fd ferredoxin, Gly-3-P, glyceraldehyde-3-phosphate, PFO pyruvate ferredoxin oxidoreductase...

Fig. 8.9 Anaerobic fermentation of glycerol [61]. Compounds GLR, glycerol DHA, dihydroxyacetone.

The reason more glycerol is not produced under standard fermentation conditions is that acetaldehyde has a much greater affinity for DPNH than has dihydroxyacetone phosphate. The over-all relationships may be expressed as follows. 

The reactions that follow are analogous to those described for the second form of fermentation. That is, since the acetaldehyde is unavailable as an acceptor for DPNH, the latter can be used in reducing dihydroxyacetone phosphate to glycerol phosphate, which finally yields glycerol. The total equation can be expressed as follows. 

Fig. 3.8. Formation of glyceraldehyde-3-phosphate and dihydroxyacetone-1-phosphate from fructose-1,6-diphosphate during fermentation...

Early in fermentation when yeast is growing, removal of pyruvate for biosynthesis might be expected to lead to a build up of NADH and thus to a halt in catabolism. To avoid this, cells reduce dihydroxyacetone phosphate to glycerol phosphate. This, in turn, is dephosphorylated to produce glycerol which is excreted. 

As ketogenesis requires oxygen and is inhibited at alcohol levels >5% (v/v) (Aldercreutz, 1986 Yamada et al., 1979), the presence of substantial amounts of dihydroxyacetone in wine likely reflects carryover from infected musts rather than formation during alcoholic fermentation. In this... 

Yamada, S., K. Nabe, M. Izuo, and I. Chibata. 1979. Enzymic production of dihydroxyacetone by AcetobactersuboxydansATCC621. J. Ferment. Technol. 57 321-325. 

Each of the two triose phosphates participates in a different enzymatic oxidation-reduction system. Dihydroxyacetone phosphate may be reduced to glycerol phosphate and phosphoglyceraldehyde may be oxidized to phosphoglyceric acid. Both of these are enzyme-catalyzed reactions in which the pyridine nucleotide, DPN, participates. The two reactions may thus be linked by the coenzyme, and in some crude extracts glycerol phosphate and phosphoglyceric acid may accumulate as end-products of fermentation. 

The biochemistry of this redirection of the ethanol fermentation is quite simple. The sulphite forms a condensation product with acetaldehyde, which is normally the immediate precursor of ethanol. This cannot be reduced by alcohol dehydrogenase, and the cell must find an alternative route to reoxidize NADH. It does this by reducing dihydroxyacetone phosphate, and after hydrolysis of the phosphate ester, glycerol is excreted (Figure 6.2). This manipulation of the yeast s metabolism, the details of which were not then understood, allowed a minor fermentation product to become the major one. 

Under some conditions, dihydroxyacetone formed during glycolysis can be reduced to glycerol through glycerol-3-phosphate (Fig. 1.8), resulting in the oxidation of an additional molecule of NADH to NAD. This reaction is important under fermentation conditions when NAD is in short supply within the cell. The formation of glycerol is favored if acetaldehyde is not available to be reduced to ethanol as would be the case in formation of bound SO2 by reaction of acetaldehyde and bisulfite (Section 5.2.1). 

Glycerol (propane-l,2,3-triol) mainly occurs in foods in theform of triacylglycerols, and many other lipids (glyceroKpids). Glycerol is also produced as a byproduct of alcoholic fermentation (also in alkaline media together with acetic acid salts) by reduction of the intermediate 1,3-dihydroxyacetone phosphate via glycerol 3-phosphate. 

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