Wine Chemistry and Biochemistry

8D Yeasts and Wine Flavour 345

sulfate

sulfite

sulfide

L-cysteine homocysteine

glutathione

L-methionine

S-adenosyl-

L-methionine

O-acetyl-L-homoserine

homoserine

L-aspartate

L-serine

3-phospho-

glycerate

Met5p

Met10p

Cys3p Cys4pMet17p

sulfate sulfide

Sul1p,Sul2p

Met6p

glutathione cysteinemethionine

3 NAD(P)H

acetyl-CoA

(pantothenate)

ATP

glucose

glucose

Str2p Str3p

sulfide

+ pyruvate

+ ammonia

cysteine

desulfhyrase

Mup1p

Mup3p

Gap1p

Mup1p

Yct1p

Hgt1,Opt1p Gap1p

cysta-

thionine

α-keto-

γ-(methylthio)-

butyrate

methional

methanethiol methionol

methanethioacetate

demethiolase

transaminase

CO 2

Adh1p

Ydr380wp

threonine

NH 3 NADH

acetyl-CoA Atf1p?

acetaldehyde

or ethanol

ethanethiol

ethanethio-

acetate

acetyl-CoA Atf1p?

sulfide?

ATP

ATP

a thio-

redoxin

sulfite

Ssu1p

Fig. 8D.8Sulfur metabolism inSaccharomyces cerevisiaeyeast
Cysteine and methionine are accumulated by general and specific permeases (Mup1p, Mup3p,
Gap1p, Yct1p) according to nutrient composition of must, but typically low concentrations
cause early induction of the sulfate assimilation pathway. Sulfate is actively accumulated (Sul1p,
Sul2p) and reduced to sulfite, which is then reduced to sulfide by sulfite reductase (Met5p
and Met10p). Extracellular sulfite can also be reduced to sulfide. Sulfide is sequestered by
O-acetyl-L-homoserine to form methionine and also by serine to form cysteine. Relative to
the rate of sulfide formation, a shortage of precursor compounds (O-acetyl-L-homoserine and
serine) or cofactors (pantothenate or acetyl-CoA) can lead to surplus H 2 S. Under stress condi-
tions caused by nutrient starvation, glutathione can be used as a source of nitrogen with infor-
mation of H 2 S. Methionine can be deaminated and reductively decarboxylated to methionol.
Methanethiol can be formed from methionine by demethiolase and esterified to methanethioac-
etate. Sulfide can also react with ethanol/acetaldehyde to form ethanethiol, and esterified to
ethanethioacetate

can lead to abundant H 2 S production because its uptake is essentially unregulated.

H 2 S is sequestered by the organic nitrogen precursor,O-acetyl-L-homoserine, to

form homocysteine. This latter compound is condensed with serine to produce cys-

teine, which can be incorporated into the sulfur reserve and antioxidant compound

glutathione. Homocysteine is also converted to methionine from which Met-tRNA

andS-adenosylmethionine are produced. Together with cysteine these compounds

regulate the sulfate assimilation pathway.

Degradation of the sulfur amino acids cysteine and methionine to H 2 S and other

volatile sulfur compounds has been observed under laboratory conditions but their

roles under wine fermentation conditions are less clear (Eschenbruch 1974; Jiranek

et al. 1995a; Moreira et al. 2002; Perp`ete et al. 2006; Rankine 1963; Vos and