Cysteine synthesis free pathway

Fig. 2. Summary of the free and bound pathways of sulfate assimilation in plants. Some related reactions and points of entry of several forms of inorganic sulfur are also shown. The reaction sequence catalyzed by (1) ATP sulfurylase, (2) APS sulfotransferase, (3) thiosulfonate reductase, and (4) cysteine synthase constitutes the bound sulfate assimilation pathway. The synthesis of OAS is catalyzed by (5) serine transacetylase. The reaction sequence (I), (6)-(9)or (1), (2), (10), (8), (9) constitutes the free pathway reactims (7) and (10) are nonenzymatic, (6) is catalyzed by APS sulfotransferase, (8) by sulfite reductase, and (9) by cysteine synthase. APS and PAPS are interrelated via (11) APS kinase and (12) NDP phophohydrolase. APS can be hydrolyzed via (13) APS sulfohydrolase or (14) APS cyclase.

Plants and bacteria produce the reduced sulfur required for the synthesis of cysteine (and methionine, described later) from environmental sulfates the pathway is shown on the right side of Figure 22-13. Sulfate is activated in two steps to produce 3-phosphoadeno-sine 5 -phosphosulfate (PAPS), which undergoes an eight-electron reduction to sulfide. The sulfide is then used in formation of cysteine from serine in a two-step pathway. Mammals synthesize cysteine from two amino acids methionine furnishes the sulfur atom and serine furnishes the carbon skeleton. Methionine is first converted to 5-adenosylmethionine (see Fig. 18-18), which can lose its methyl group to any of a number of acceptors to form A-adenosylhomocysteine (adoHcy). This demethylated product is hydrolyzed to free homocys-... 

Sulfate. As for the production of hydrogen sulfide from sulfur dioxide/sulfite at least three possible pathways for the light-dependent synthesis of hydrogen sulfide in response to sulfate can be assumed, i.e. first the light-dependent reduction of sulfate to carrier-bound sulfide followed by a release of the sulfide moiety from its carrier second the light-dependent reduction of sulfate to carrier-bound sulfide followed by an incorporation of the sulfide moiety into cysteine and subsequent degradation of cysteine third the release of sulfite from carrier-bound sulfite followed by reduction of free sulfite to sulfide (see Figure 1). 

Taurine is a dietary essential in the cat, which is an obligate carnivore with a limited capacity for taurine synthesis from cysteine. On a taurine-free diet, neither supplementary methionine nor cysteine will maintain normal plasma concentrations of taurine, because cats have an alternative pathway of cysteine metabolism reaction with mevalonic acid to yield felinine (3-hydroxy-1,1-dimethylpropyl-cysteine), which is excreted in the urine. The activity of cysteine sulfinic acid decarboxylase in cat liver is very low. 

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