Cofactors Articles 2

© 2011

Cofactor diversity in biological oxidations: implications and applications

            (Duine 2001) Download

Until recently, it was generally believed that enzymatic oxidation and reduction requires the participation of either a nicotinamide (NAD(P)+) or a flavin (FAD, FMN), in agreement with the existence of NAD(P)/H-dependent dehydrogenases/reductases and flavoprotein dehydrogenases/reductases/oxidases. However, during the past 20 years, the unraveling of the enzymology of the oxidation and reduction of C1-compounds by bacteria has led to the discovery of many new redox cofactors, some of them discussed here as they have a wider physiological significance than just enabling enzymatic C1-conversions to occur. A good example is the quinone cofactors, encompassing PQQ (2,7,9-tricarboxy-1H-pyrrolo[2,3-f]-quinoline-4,5-dione), TTQ (tryptophyl tryptophanquinone), TPQ (topaquinone), LTQ (lysyl topaquinone), and several others whose structures have still to be elucidated. Another example is mycothiol (1-O-(2'-[N-acetyl-L-cysteinyl]amido-2'-deoxy-alpha-D-glucopyranosyl)-D-my o-inosoitol), the counterpart of glutathione, once thought to be a universal coenzyme. Because these novel cofactors assist in reactions that can also be catalyzed by already known enzyme "classic cofactor" combinations, and first indications suggest that the chemistry of the reactions is not unique, one may wonder about the evolutionary background for this cofactor diversity. However, as will be illustrated by examples, from a practical point of view the diversity is beneficial, as it has increased the arsenal of enzymes suitable for application.

Structure, mechanism and catalytic duality of thiamine-dependent enzymes

            (Frank, Leeper et al. 2007) Download

Thiamine is an essential cofactor that is required for processes of general metabolism amongst all organisms, and it is likely to have played a role in the earliest stages of the evolution of life. Here, we review from a structural perspective the enzymatic mechanisms that involve this cofactor. We explore asymmetry within homodimeric thiamine diphosphate (ThDP)-dependent enzyme structures and discuss how this may be correlated with the kinetic properties of half-of-the-sites reactivity, and negative cooperativity. It is likely these structural and kinetic hallmarks may arise through reciprocal coupling of active sites. This mode of communication between distant active sites is not unique to ThDP-dependent enzymes, but is widespread in other classes of oligomeric enzyme. Thus, it appears likely to be a general phenomenon reflecting a powerful mechanism of accelerating the rate of a chemical pathway. Finally, we speculate on the early evolutionary history of the cofactor and its ancient association with protein and RNA.

Off-pathway, oxygen-dependent thiamine radical in the Krebs cycle

            (Frank, Kay et al. 2008) Download

The catalytic cofactor thiamine diphosphate is found in many enzymes of central metabolism and is essential in all extant forms of life. We demonstrate the presence of an oxygen-dependent free radical in the thiamine diphosphate-dependent Escherichia coli 2-oxoglutarate dehydrogenase, which is a key component of the tricarboxylic acid (Krebs) cycle. The radical was sufficiently long-lived to be trapped by freezing in liquid nitrogen, and its electronic structure was investigated by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR). Taken together, the spectroscopic results revealed a delocalized pi radical on the enamine-thiazolium intermediate within the enzyme active site. The radical is generated as an intermediate during substrate turnover by a side reaction with molecular oxygen, resulting in the continuous production of reactive oxygen species under aerobic conditions. This off-pathway reaction may account for metabolic dysfunction associated with several neurodegenerative diseases. The possibility that the on-pathway reaction may proceed via a radical mechanism is discussed.

Emerging roles for riboflavin in functional rescue of mitochondrial beta-oxidation flavoenzymes

            (Henriques, Olsen et al. 2010) Download

Riboflavin, commonly known as vitamin B2, is the precursor of flavin cofactors. It is present in our typical diet, and inside the cells it is metabolized to FMN and FAD. As a result of their rather unique and flexible chemical properties these flavins are among the most important redox cofactors present in a large series of different enzymes. A problem in riboflavin metabolism or a low intake of this vitamin will have consequences on the level of FAD and FMN in the cell, resulting in disorders associated with riboflavin deficiency. In a few number of cases, riboflavin deficiency is associated with impaired oxidative folding, cell damage and impaired heme biosynthesis. More relevant are several studies referring reduced activity of enzymes such as dehydrogenases involved in oxidative reactions, respiratory complexes and enzymes from the fatty acid beta-oxidation pathway. The role of this vitamin in mitochondrial metabolism, and in particular in fatty acid oxidation, will be discussed in this review. The basic aspects concerning riboflavin and flavin metabolism and deficiency will be addressed, as well as an overview of the role of the different flavoenzymes and flavin chemistry in fatty acid beta-oxidation, merging clinical, cellular and biochemical perspectives. A number of recent studies shedding new light on the cellular processes and biological effects of riboflavin supplementation in metabolic disease will also be overviewed. Overall, a deeper understanding of these emerging roles of riboflavin intake is essential to design better therapies.

Flavoenzymes

            (Joosten and van Berkel 2007) Download

Flavoenzymes are colourful oxidoreductases that catalyze a large variety of different types of reactions. Flavoenzymes have been extensively studied for their structural and mechanistic properties and are gaining momentum in industrial biocatalytic applications. Some of these enzymes catalyze the oxidative modification of protein substrates. New insights in oxidative flavoenzymes and in particular in novel family members point towards their potential application in the pharmaceutical, fine-chemical and food industries.

Clinical aspects of coenzyme Q10: an update

            (Littarru and Tiano 2010) Download

The fundamental role of coenzyme Q(10) (CoQ(10)) in mitochondrial bioenergetics and its well-acknowledged antioxidant properties constitute the basis for its clinical applications, although some of its effects may be related to a gene induction mechanism. Cardiovascular disease is still the main field of study and the latest findings confirm a role of CoQ(10) in improving endothelial function. The possible relation between CoQ(10) deficiency and statin side effects is highly debated, particularly the key issue of whether CoQ(10) supplementation counteracts statin myalgias. Furthermore, in cardiac patients, plasma CoQ(10) was found to be an independent predictor of mortality. Studies on CoQ(10) and physical exercise have confirmed its effect in improving subjective fatigue sensation and physical performance and in opposing exercise-related damage. In the field of mitochondrial myopathies, primary CoQ(10) deficiencies have been identified, involving different genes of the CoQ(10) biosynthetic pathway; some of these conditions were found to be highly responsive to CoQ(10) administration. The initial observations of CoQ(10) effects in Parkinson's and Huntington's diseases have been extended to Friedreich's ataxia, where CoQ(10) and other quinones have been tested. CoQ(10) is presently being used in a large phase III trial in Parkinson's disease. CoQ(10) has been found to improve sperm count and motility on asthenozoospermia. Moreover, for the first time CoQ(10) was found to decrease the incidence of preeclampsia in pregnancy. The ability of CoQ(10) to mitigate headache symptoms in adults was also verified in pediatric and adolescent populations.

Structural biology of enzymes involved in NAD and molybdenum cofactor biosynthesis

            (Rizzi and Schindelin 2002) Download

The structural analysis of all enzymes in a metabolic pathway is a prerequisite to answering fascinating questions, such as those relating to the evolutionary relationships between enzymes within the same and related pathways. Furthermore, the observed impressive diversity of catalytic functions displayed by these enzymes can lead to the synthesis of highly complex or unstable molecules, frequently involving unusual chemical reactions. Moreover, a detailed description of the active site of each enzyme in a pathway is of immense importance for the rational design of new drugs. The recent progress made in the structural biology of enzymes involved in NAD and molybdenum cofactor biosynthesis presents a significant step toward these goals.

S-Adenosyl-L-methionine: beyond the universal methyl group donor

            (Roje 2006) Download

S-Adenosyl-l-methionine (AdoMet or SAM) is a substrate in numerous enzyme-catalyzed reactions. It not only provides methyl groups in many biological methylations, but also acts as the precursor in the biosynthesis of the polyamines spermidine and spermine, of the metal ion chelating compounds nicotianamine and phytosiderophores, and of the gaseous plant hormone ethylene. AdoMet is also the source of catalytic 5'-deoxyadenosyl radicals, produced as reaction intermediates by the superfamily of radical AdoMet enzymes. This review aims to summarize the present knowledge of catalytic roles of AdoMet in plant metabolism.

Vitamin K, osteoporosis and degenerative diseases of ageing

         (Vermeer and Theuwissen 2011) Download

The function of vitamin K is to serve as a co-factor during the post-translational carboxylation of glutamate (Glu) residues into gamma-carboxyglutamate (Gla) residues. The vital importance of the Gla-proteins essential for normal haemostasis is well recognized. During recent years, new Gla-containing proteins have been discovered and the vitamin K-dependent carboxylation is also essential for their function. It seems, however, that our dietary vitamin K intake is too low to support the carboxylation of at least some of these Gla-proteins. According to the triage theory, long-term vitamin K inadequacy is an independent, but modifiable risk factor for the development of degenerative diseases of ageing including osteoporosis and atherosclerosis.


References

Duine, J. A. (2001). "Cofactor diversity in biological oxidations: implications and applications." Chem Rec 1(1): 74-83.

Frank, R. A., C. W. Kay, et al. (2008). "Off-pathway, oxygen-dependent thiamine radical in the Krebs cycle." J Am Chem Soc 130(5): 1662-8.

Frank, R. A., F. J. Leeper, et al. (2007). "Structure, mechanism and catalytic duality of thiamine-dependent enzymes." Cell Mol Life Sci 64(7-8): 892-905.

Henriques, B. J., R. K. Olsen, et al. (2010). "Emerging roles for riboflavin in functional rescue of mitochondrial beta-oxidation flavoenzymes." Curr Med Chem 17(32): 3842-54.

Joosten, V. and W. J. van Berkel (2007). "Flavoenzymes." Curr Opin Chem Biol 11(2): 195-202.

Littarru, G. P. and L. Tiano (2010). "Clinical aspects of coenzyme Q10: an update." Nutrition 26(3): 250-4.

Rizzi, M. and H. Schindelin (2002). "Structural biology of enzymes involved in NAD and molybdenum cofactor biosynthesis." Curr Opin Struct Biol 12(6): 709-20.

Roje, S. (2006). "S-Adenosyl-L-methionine: beyond the universal methyl group donor." Phytochemistry 67(15): 1686-98.

Vermeer, C. and E. Theuwissen (2011). "Vitamin K, osteoporosis and degenerative diseases of ageing." Menopause Int 17(1): 19-23.