Interestingly, the same thiol group of the cysteine side-chain can be
involved in both a chalcogen and a hydrogen bond (Table 2 ).
About 65% of the chalcogen bonds are associated with hydrogen bonds. An
example is shown in Figure 2 : cysteine 31 (chain A) of human
thymidylate kinase forms a chalcogen bond with the main-chain oxygen
atom of alanine 37 and forms a hydrogen bond with the main-chain oxygen
atom of valine 27 (PDB file 1e9a, [17]).