What’s in a covalent bond?

Abstract

Many enzymes use one or more cofactors, such as biotin, heme, or flavin. These cofactors may be bound to the enzyme in a noncovalent or covalent manner. Although most flavoproteins contain a noncovalently bound flavin cofactor (FMN or FAD), a large number have these cofactors covalently linked to the polypeptide chain. Most covalent flavin–protein linkages involve a single cofactor attachment via a histidyl, tyrosyl, cysteinyl or threonyl linkage. However, some flavoproteins contain a flavin that is tethered to two amino acids. In the last decade, many studies have focused on elucidating the mechanism(s) of covalent flavin incorporation (flavinylation) and the possible role(s) of covalent protein–flavin bonds. These endeavors have revealed that covalent flavinylation is a post‐translational and self‐catalytic process. This review presents an overview of the known types of covalent flavin bonds and the proposed mechanisms and roles of covalent flavinylation.

Formate Oxidase, an Enzyme of the Glucose-Methanol-Choline Oxidoreductase Family, Has a His-Arg Pair and 8-Formyl-FAD at the Catalytic Site

“…38) Flavinylation of the protein in the choline oxidase has been reported to be favorable to its reductive half reaction.…”

“…38) In choline oxidase, the His 466 with protonated N "2 atom and Asn 510 have been found to contribute to stabilization of the intermediate. 39,41) During the modification of FAD in FOD AO , the intermediate might be transiently formed and stabilized by the protonated side chains of His 511 and Arg 554 , which are situated at $4:5 Å from the N1 and O4 atoms of the isoalloxazine ring (Fig.…”

Bioorthogonal Catalytic Activation of Platinum and Ruthenium Anticancer Complexes by FAD and Flavoproteins

“…Three concentrations (0.2, 2, and 20 m m ) of MES (as buffer, pH 6) or NADH in phosphate buffer (PB, pH 7, 100 m m ) were employed for this purpose. MES was selected as an electron donor to follow up our previous work on riboflavin11 while NADH was chosen for its relevance as a biological cofactor in numerous reactions catalyzed by flavoenzymes 19. Moreover, metal‐based catalytic drugs have been recently shown to kill cancer cells by interfering with cellular NAD + /NADH homeostasis 20, 21, 22…”

“…In cells, flavins are bound to proteins through covalent and non‐covalent interactions,19 which control their (photo)redox properties 24. Exploiting flavoproteins as selective catalysts is therefore an exciting prospect for the design of bioorthogonal activation strategies for metal‐based prodrugs.…”

The Crystal Structure and Mechanism of an Unusual Oxidoreductase, GilR, Involved in Gilvocarcin V Biosynthesis

“…This observation classifies GilR into a small group of a relatively new type of the vanillyl-alcohol-oxidase flavoprotein family characterized by bicovalently tethered cofactors (67). It is still not entirely clear what role the covalent bonds serve, but it has been shown in related enzymes that the bicovalent attachment increases the redox potential and also may affect the accessibility of the FAD molecule (67)(68)(69)(70).…”