Riboflavine metal-binding

The metal complexes of riboflavin-5 -phosphate (flavin mononucleotide, FMN) have been studied. Zn(FMN)-2H20 shows some perturbation of the IR bands of the phosphate group, suggesting that metal binding occurs at the phosphate group.579 Reviews are available.9-15 468c h 1470... 

Apart from amino acids, peptides and proteins, many other metal-binding substances play essential roles in all living cells. Firstly, there are the pteridines (including folic acid) and purines, whose stability constants have been determined (Albert, 1953 Albert and Serjeant, 1960). Riboflavine is most avid in the partly reduced state (Hemmerich, Veeger and Wood, 1965). Spermine 11 A), and the diamines spermidine 11.5) and putrescine 11.6) also compete. 

As for reduction processes, C02 free radicals were shown to react specifically with disulfide bonds (122). They were extensively used to study the redox properties of disulfide bonds, thiyl and disulfide free radicals in proteins. This is discussed in paragraph 5. However, they do react with thiol functions also (37). For proteins containing a prosthetic group, the reduction concerns also oxidized valencies of metals and flavins. Flavin adenine dinucleotide (FAD) or Flavin Mononucleotide (FMN). The proportion of reduced disulfide/reduced prosthetic group varies considerably with the protein. For instance, lipoamide dehydrogenase contains one disulfide bond close to a flavin (FAD). Free radicals can reduce only the flavin, although both are in the active site (123). In chicken egg white riboflavin binding protein, competitive formation of both disulfide and semireduced flavin is observed (124). 

Lead appears to be able to interact with complex small biomolecules as well, such as flavins for example, bis(lO-methylisoalloxazine) perchlorate tetrahy-drate (223). IsoaUoxazine is a planar three-ringed heterocychc amino cofactor associated with riboflavin and is active in oxidation-reduction reactions with metals such as Mo and Fe. Lead binds to bis(lO-methylisoalloxazine) in a 1 1 metal-ligand complex, with two additional waters bound resulting in a four coordinate molecule with a total of four oxygen donors. An active lone pair results in a distorted square-pyramidal structure. As is the case for citrate, extensive hydrogen bonding was observed in the crystal lattice. 

---