Protein thiocarboxylates are members of sulfur transfer protein family and have been shown to be involved in a variety of important biosynthetic pathways like vitamin B1, molybdopterin, cysteine, thionucleosides among many others. Despite their importance, there has been lack of systematic efforts towards identifying new thiocarboxylate-forming proteins. In this work, we have taken efforts in developing two strategies to label them in bacterial cell-free extracts using fluorescent tags. In addition, bioinformatics search for new thiocarboxylate-forming proteins using a genomic database, theseed.uchicago.edu and a protein database, Pfam, yielded a new methionine biosynthetic pathway that involves a protein thiocarboxylate as the sulfur donor to make the precursor, homocysteine. This discovery further validated the need for developing methods to identify, in cell-free extracts, proteins carrying this important post-translational modification. The sulfur source for the protein thiocarboxylate involved in the methionine biosynthetic pathway has also been identified as sulfate or sulfite.