Figure 1.

Sulfur-containing small molecules. Many biocofactors such as thiamine, iron-sulfur (Fe/S) clusters, and molybdenum cofactor (MoCo) contain sulfur atoms in their structure (upper column). Various sulfur-containing nucleosides are also identified. For example, the 2-thiocytidine derivatives are 2-thiocytidine (s²C) found in bacteria and 2-thio-2′-O-methyluridine (s²Um) found in humans. 4-thiouridine (s⁴U) is found in bacteria. Various types of the hyper-modified uridine, such as 5-carboxymethylaminomethyl-2-thiouridine (cmnm⁵s²U), 5-methylaminomethyl-2-thiouridine (mnm⁵s²U), 5-taurinomethyl-2-thiouridine (τm⁵s²U), and 5-methoxycarbonylmethyl-2-thiouridine (mcm⁵s²U) are found at U₃₄. Various methylthio-adenosine derivatives, such as 2-methylthio- N6-isopentenyladenosine (ms²i⁶A) and 2-methylthio-N6-threonylcarbamoyladenosine (ms²t⁶A), are also found in both bacteria and eukaryotes. Sulfur atoms incorporated into the molecule in the biosynthetic pathway are shown with circles.