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Amino acids can be broken down to yield pyruvate, which in turn can be used to construct glucose molecule in the process of gluconeogenesis. What happens to the sulfur atom in the amino acid cysteine when it is processed into pyruvate?

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Image source: NCBI Bookshelf

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Cysteine is a dispensable amino acid required for synthesis of protein and non-protein compounds. These non-protein compounds include pyruate (hence acetyl coenzyme A), taurine, sulfate, and glutatione (GSH). The fate of the cycteine is hence sketched in following diagram (Ref.1):

Cysteine Metabolism

As evidence from the diagram, the sulfur atom in cysteine has been converted to sulfate ion ($\ce{SO4^2-}$) through $\ce{HS-}$ ion. This fact has been previously proved experimentally (Ref.2). Accordingly, the first major step in cysteine catabolism in mammalian tissues is the oxidation of cysteine to cysteine sulfinic acid catalyzes by cysteine dioxygenase (CDO). The reaction mechanism of this non-heme iron dioxygenase has been recently reviewed (Ref.3).

References:

  1. G. Courtney-Martin, P. B. Pencharz, “Chapter 19: Sulfur Amino Acids Metabolism From Protein Synthesis to Glutathione,“ In The Molecular Nutrition of Amino Acids and Proteins; Dominique Dardevet, Ed.; Academic Press, Elsevier Inc.: London, United Kingdom, 2016, pp. 265-286 (https://doi.org/10.1016/C2014-0-02227-7).
  2. Catherine L. Weinstein, Rudy H. Haschemeyer, Owen W. Griffith, “In vivo studies of cysteine metabolism. Use of D-cysteinesulfinate, a novel cysteinesulfinate decarboxylase inhibitor, to probe taurine and pyruvate synthesis,” The Journal of Biological Chemistry 1988, 263(32), 16568-16579 (http://www.jbc.org/content/263/32/16568.full.pdf+html).
  3. Crisjoe A. Josepha, Michael J. Maroney, “Cysteine dioxygenase: structure and mechanism,” Chem. Commun. 2007, (32), 3338-3349 (DOI: 10.1039/B702158E).
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According to https://www.uniprot.org/uniprot/P25325, 3-mercaptopyruvate sulfurtransferase is also involved in the catabolism of cysteine:

Transfer of a sulfur ion to cyanide or to other thiol compounds. Also has weak rhodanese activity. Detoxifies cyanide and is required for thiosulfate biosynthesis. Acts as an antioxidant. In combination with cysteine aminotransferase (CAT), contributes to the catabolism of cysteine and is an important producer of hydrogen sulfide in the brain, retina and vascular endothelial cells. Hydrogen sulfide H2S is an important synaptic modulator, signaling molecule, smooth muscle contractor and neuroprotectant. Its production by the 3MST/CAT pathway is regulated by calcium ions.

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