Methylthioribose kinase

Fig. 1. Ethylene biosynthesis. The numbered enzymes are (1) methionine adenosyltransferase, (2) ACC (l-aminocyclopropane-l-carboxylic acid) synthase, (3) ethylene forming enzyme (EFE), (4) 5 -methylthio-adenosine nucleosidase, (5) 5 -methylthioribose kinase. Regulation of the synthesis of ACC synthase and EFE are important steps in the control of ethylene production. ACC synthase requires pyridoxal phosphate and is inhibited by aminoethoxy vinyl glycine EFE requires 02 and is inhibited under anaerobic conditions. Synthesis of both ACC synthase and EFE is stimulated during ripening, senescence, abscission, following mechanical wounding, and treatment with auxins.

S ATP + 5-methylthioribose (<1> enzyme may be involved in an alternative pathway of methionine synthesis in plant tissues [1] <2> may be a primary enzyme involved in the recycling of the methylthio group of 5-methylthioribose back into methionine [2,3] <3> key step in recycling of methionine from 5 -methylthioadenosine a co-product of polyamine biosynthesis, expression of methylthioribose kinase may be under control of the methionine regulon [4]) (Reversibility [1-4, 6]) [1-4, 6, 7]... 

Guranowski, A. Plant 5-methylthioribose kinase. Plant Physiol., 71, 932-935 (1983)... 

Ferro, A.J. Barrett, A. Shapiro, S.K. 5-Methylthioribose kinase. A new enzyme involved in the formation of methionine from 5-methylthioribose. J. Biol. Chem., 253, 6021-6025 (1978)... 

Ferro, A.J. Marchitto, K.S. 5-Methylthioribose kinase (Enterobacter aero-genes). Methods EnzymoL, 94, 361-364 (1983)... 

Kushad, M.M. Richardson, D.G. Ferro, A.J. 5 -Methylthioadenosine nucleosidase and 5-methylthioribose kinase activities and ethylene production during tomato fruit development and ripening. Plant Physiol., 79, 525-529 (1985)... 

Cornell, K.A. Winter, R.W. Tower, P.A. Riscoe, M.K. Affinity purification of 5-methylthioribose kinase and 5-methylthioadenosine/S-adenosylhomocys-teine nucleosidase from Klebsiella pneumoniae [corrected]. Biochem. J., 317, 285-290 (1996)... 

Fig, 7. Pathways for the metabolism of methionine to 5 -methylthioadenosine (MTA) and recycling of MTA to methionine. Methionine can serve as a carbon source for the synthesis of polyamines and, in some tissues, ethylene. 5 -Methylthioadenosine is a product of both processes. Only the methylthio group of methionine is recycled, the C4 moiety for the resynthesis of methionine being derived from the ribosyl moiety of ATP. The enzymes involved are (1) SAM synthetase, (2) SAM decarboxylase, (3) various C3 transfer enzymes of polyamine biosynthesis, (4) MTA nucleosidase, (5) methylthioribose kinase, (6) three( ) uncharacterized enzymes, (7) aminotransferase, and (8) aminocyciopropane carboxylate synthase. 

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