FIGURE 1.

H₂S production via cysteine degradation in the human gut microbiome. (A) Pathways of H₂S production via cysteine degradation in the human gut microbiome. Pathways with labels ending in “activity” refer to a set of genes that convert cysteine to the indicated products. Cysteine desulfurase activity (CYD) = (dcyD, mgl, metC, cysM, cysK, malY, yhaM, tnaA, iscS). AMetR = AdoMet recycling present in Bacillus subtilis (metK, various methylases, mtnN, luxS) (Hullo et al., 2007). (B) Visualization of H₂S production across human tissues (image obtained from Papatheodorou et al., 2020). H₂S is produced endogenously in the brain, liver, and heart via cystathionine gamma-lyase and cystathionine beta-synthase and is tightly regulated to avoid toxic effects of H₂S overproduction. Color coded organs refer to the type of H₂S production active in those organs. (C) Physiological effects of H₂S on the healthy, IBD and CRC gut. H₂S contributes to the degradation of the protective mucosal barrier which could cause or exacerbate inflammation and infection by opportunistic species for patients diagnosed with IBD. In CRC, Fusobacterium nucleatum is closely associated with colonic tumors and are well known H₂S producers (Castellarin et al., 2012).