trans

186 PROTEIN METABOLISM
CO 2
Dec-SAM
(11)
SAM
5-methyl thioadenosine
(12)
(10)
2-K-4-M-thiobutyrate
(2)
(14)
(15)
5-methylthioribose
1-phosphate
(16)
1-P-2,3-diK-5-M-thiopentane
5-M-thioribulose-1-P
Metanethiol NH 4
α-ketobutyrate
‘CH
SAM
3 ’
(1) (2) (3)
METHIONINE
Betaine
mTHF
α-KG NH 4 SH 2
(9)
(5)
S-adenosyl-homocysteine
Homocysteine
(6)
Cystathionine
(7)
(4)
SERINE
α-KG NH 4
5-methylthioribose
(13)
SERINE SH 2
(8)
CYSTEINE
FIGURE 8.3 Methionine salvage pathways and catabolism. SAM, S-adenosylmethionine; dec-SAM, decarboxylated
S-adenosylmethionine; -KG, -ketoglurate; ‘CH 3 ’, methyl group transferred to several substrates; 5-M-thioribulose-
1-P, 5-methylthioribulose-1-phosphate; 1-P-2,3-diK-5-M-thiopentane, 1-phospho-2,3-diketo-5-methylthiopentane;
2-K-4-M-thiobutyrate, 2-keto-4-methylthiobutyrate; mTHF, methyltetrahydrofolate. (1) methionine -lyase;
(2) S-adenosylmethionine synthetase; (3) S-adenosylmethionine-linked methyl transferases; (4) S-adenosylhomocysteine
hydrolase; (5) methionine synthase; (6) cystathionine -synthetase; (7) -cystathionase; (8) L-serine
desulfhydrase; (9) homocysteine desulfurase; (10) S-adenosylmethionine decarboxylase; (11) spermidine synthase;
(12) 5-methylthioadenosine nucleosidase; (13) 5-methylthioribose kinase; (14) 5-methylthioadenosine phosphorylase;
(15) 5-methylthioribose 1-phosphate isomerase; (16) methionine aminotransferase.
Cysteine can be obtained from methionine,
through the reactions of cystathionine-synthetase
and -cystathionase (Figure 8.3).
Cystathionine--synthetase has been studied
in T. vaginalis and in nematodes; like the
mammalian enzyme, both parasite enzymes
have L-serine sulfhydrase activity which catalyzes
the reversible interconversion of cysteine
and serine, with the exchange of SH 2 .
In contrast to the mammalian enzyme, the
parasite enzymes have ‘activated L-serine
sulfhydrase’ activity catalyzing the reaction of
cysteine with a number of R-SH compounds,
resulting in the formation of a cysteine thioether
and the liberation of SH 2 . Both the nematode
and the T. vaginalis enzymes are inhibited by a
number of compounds, such as dichlorophene
and hexachlorophene, which have both antitrichomonal
and anthelmintic effects.
Threonine and serine
Two pathways of threonine catabolism have
been described in some protozoa (Figure 8.4).
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA