Physiology and Biochemistry of Sourdough Yeasts - Sourdough Biotechnology - page 157

Fructose

Fructose

Fru-6-P

Fru-1,6-P

Fba1

Dihydroxyacetonephosphate Glyceraldheyde-3-phosphate

NADH

Gut2 Gdp1 , Gdp2

NAD

Dak1, Dak2

Glycerol-3-phospate

Gpp1, Gpp2

Gut1

Fps1

NAD(P) + NAD(P)H

Glycerol Dihydroxyacetone

Ypr1, Gcy1

Glycerol

Gup1, Gup2

Fig. 6.3 Part of the glycolytic pathway and pathways for production of glycerol. Expression of the
genes encoding the proteins that are underlined is stimulated after osmotic shock

decarboxylated to acetyl CoA by means the pyruvate dehydrogenase multi-enzyme
complex. This reaction links glycolysis to the citric acid cycle, where acetyl CoA is
completely oxidised to give two moles of CO 2 and reductive equivalents in the form
of NADH and FADH 2 . The citric acid cycle is an amphibolic pathway, as it combines
both catabolic and anabolic functions. This latter function is the consequence of the
production of intermediates for the synthesis of amino acids and nucleotides.
During alcoholic fermentation, yeasts re-oxidise NADH to NAD in a two-step
reaction starting from pyruvate. The first reaction is via decarboxylation through
pyruvate decarboxylase, followed by the reduction of the acetaldehyde to ethanol,
catalysed by the alcohol dehydrogenase (ADH). Simultaneously, glycerol is generated
from dihydroxyacetone phosphate to ensure, with an alternative pathway, the regen-
eration of NAD according to the mechanism described in Fig. 6.3 .