Stability and structure of Cd2+ and Pb2+ complexes with biotin, lipoic acid and some of their derivatives in solution

Micronutrient cofactor research with extensions to applications

Following identification of essential micronutrients, there has been a continuum of research aimed at revealing their absorption, transport, utilization as cofactors, and excretion and secretion. Among those cases that have received our attention are vitamin B6, riboflavin, biotin, lipoate, ascorbate, and certain metal ions. Circulatory transport and cellular uptake of the water-soluble vitamins exhibit relative specificity and facilitated mechanisms at physiological concentrations. Isolation of enzymes and metabolites from micro–organisms and mammals has provided information on pathways involved in cofactor formation and metabolism. Kinases catalysing phosphorylation of B6and riboflavin have a preference for Zn2+in stereospecific chelates with adenosine triphosphate. The synthetase for flavin adenine dinucleotide prefers Mg2+. The flavin mononucleotide-dependent oxidase that converts the 5′–phosphates of pyridoxine and of pyridoxamine to pyridoxal phosphate is a connection between B6and riboflavin and is a primary control point for conversion of B6to its coenzyme. Sequencing and cloning of a side–chain oxidase for riboflavin was achieved. Details on binding and function have been delineated for some cofactor systems, especially in several flavoproteins. There is both photochemical oxidation and oxidative catabolism of B6and riboflavin. Both biotin and lipoate undergo oxidation of their acid side chains with redox cleavage of the rings. Applications from our findings include the development of affinity absorbents, enhanced drug delivery, delineation of residues in biopolymer modification, pathogen photoinactivation in blood components, and input into human dietary recommendations. Ongoing and future research in the cofactor arena can be expected to add to this panoply. At the molecular level, the way in which the same cofactor can participate in diverse catalytic reactions resides in interactions with surrounding enzyme structures that must be determined case by case. At the level of human intake, more knowledge is desirable for making micronutrient recommendations based on biochemical indicators, especially for the span between infancy and adulthood.

A trail of research on cofactors: an odyssey with friends

Over the span of 40 y and with the participation of over 60 students and postdoctoral colleagues, my laboratory has been able to elucidate numerous aspects of cofactor metabolism and function. Findings have been on the absorption, transport, utilization and excretion of vitamin B-6, riboflavin, biotin, lipoate and ascorbate. Specificity studies on those trace but essential enzymes that catalyze conversion of such vitamins as B-6 and riboflavin to their functional coenzymes led to our development of "biochemically specific absorbents" that prototypically exemplified what later was called "affinity chromatography." Characterization of the purified kinases for B-6 and riboflavin revealed preference for Zn2+ with the eucaryotic enzymes and delimited effects of inhibitors that relate to drug action. Flavin adenine dinucleotide synthetase, separable from flavokinase in mammals, prefers Mg2+. Specifics for binding and function of flavocoenzymes were delineated for several flavoproteins. The flavin mononucleotide-dependent oxidase that converts the 5'-phosphates of pyridoxine and of pyridoxamine to pyridoxal phosphate is a connection between riboflavin and B-6 that we characterized in mechanistic detail and found to be the primary control point for conversion of B-6 to its coenzyme. Sequencing and cloning of a side-chain oxidase for riboflavin was achieved. Isolation and identification of metabolites of biotin and of lipoic acid, first from bacteria obtained by enrichment culture and then from mammals, provided seminal information on catabolic pathways involved, as have our other studies with flavin catabolites isolated from milk and urine.

The coordinating properties of d-biotin

The coenzyme d-biotin offers in its anionic form to metal ions 3 possible binding sites: the carboxylate group of the valerate side chain, the ureido residue of the 2-imidazolidone ring, and the thioether sulfur of the tetrahydrothiophene ring; the coordinating properties of these groups are summarized and compared. Hydrogen bond formation of the ureido group has also been observed, and hydrogen bonding may possibly be important in biotin-bicarbonate recognition. The aliphatic part of the valeric acid side chain can undergo hydrophobic interactions. Such interactions and/or the stereoselective sulfur-metal ion coordination could be the means for a correct 'fixation' of the biotinyl moiety at the surface of a protein, thus creating the active enzyme-substrate complex.