Coenzyme biochemical function

Coenzyme A has two distinctly different biochemical functions, which have already been considered briefly in Chapters 12 and 13 and can be summarized as follows ... 

For several years after the discovery of cobalamin its biochemical function remained a mystery, a major reason being the extreme sensitivity of the coenzymes to decomposition by light. Progress came after Barker and associates discovered that the initial step in the anaerobic fermentation of glutamate by Clostridium... 

In this chapter we are concerned, not primarily with vitamins per se, but with coenzymes. Many coenzymes are modified forms of vitamins. The modifications take place in the organism after ingestion of the vitamins. Coenzymes act in concert with enzymes to catalyze biochemical reactions. Tightly bound coenzymes are sometimes referred to as prosthetic groups. A coenzyme usually functions as a major component of the active site on the enzyme, which means that understanding the mechanism of coenzyme action usually requires a complete understanding of the catalytic process. 

In this chapter, mitochondrial dysfunctions will be briefly reviewed, but the neuroprotective effects of coenzyme Q10 in neurodegenerative disease will be discussed in detail. For additional documentations, the readers should consult the two volumes on mitochondrial ubiquinone (coenzyme Q10) biochemical, functional, medical, and therapeutic aspects in human health and disease (Ebadi et al., 2001). 

Ebadi, M., Marwah, J., and Chopra, R., Mitochondrial Ubiquinone (Coenzyme Q10) Biochemical, Functional, Medical, and Therapeutic Aspects in Human Health and Disease, Vol. 1, 2, Prominent Press, Scottsdale, AZ, 2001. 

Many of the coenzymes are derivatives of vitamins. A coenzyme function is known for each of the water-soluble vitamins except vitamin C. A detailed discussion of the water-soluble vitamins and their conversion to coenzymes is found in Chapter 6. The biochemical functions of fat-soluble vitamins (A, D, E, K) are, with exception, less clearly understood. They are also further considered in Chapter 6. 

Crane FL (2001) Biochemical functions of coenzyme QIO. Journal of the American College of Nutrition 20, 591-8. 

As with the other B vitamins that act as coenzymes biochemical assessment of vitamin Bg can, be made by direct chemical analysis of the vitamer or its metabolites, or by functional means. Measurements that have been used are PLP in plasma or red cells, its metabolite 4 PA in urine or plasma, the activity and activation coefficient of the red cell aminotransferases (aspartate and alanine), and the tryptophan load metabolite excretion test. As no single marker adequately reflects status, a combination of these markers olfers the best approach. 

Most vitamins function either as a hormone/ chemical messenger (cholecalciferol), structural component in some metabolic process (pantothenic acid), or a coenzyme (phytonadi-one, thiamine, riboflavin, niacin, pyridoxine, biotin, folic acid, cyanocobalamin). At least one vitamin has more than one biochemical role. Vitamin A as an aldehyde (retinal) is a structural component of the visual pigment rhodopsin and, in its acid form (retinoic acid), is a regulator of cell differentiation. The precise biochemical functions of ascorbic acid and a-tocopherol still are not well defined. 

Biochemical Function. The best way to describe tocopherol s role is that of a lipid-soluble antioxidant. It protects unsaturated lipids from o gen-induced peroxide formation. There Is evidence for both free-radical one-electron chemistry (Fig. 8.20) and two-electron qui-none-hydroquinone chemistry (Fig. 8.21) (35). The oxidized/reduced glutathione system may he part of the system that regenerates reduced a-tocopherol. At one time it was thought that the preference for the 2-R stereoisomers indicated that the vitamin was part of a biochemical oaddation/reduction system, possibly as a coenzyme. So far that role for a-tocopherol has not been found. The current evidence points to the... 

The cardiomyopathy is directly related to a reduction in the normal biochemical function of the vitamin thiamine in heart muscle. Inhibition of the a-keto acid dehydrogenase complexes causes accumulation of a-keto acids in heart muscle (and in blood), resulting in a chemically-induced cardiomyopathy. Impairment of two other functions of thiamine may also contribute to the cardiomyopathy. Thiamine pyrophosphate serves as the coenzyme for transketolase in the pentose phosphate pathway, and pentose phosphates accumulate in thiamine deficiency. In addition, thiamine triphosphate (a different coenzyme form) may function in Na conductance channels. 

Biochemical function in human metabolism. Activation of metabolites by coenzyme A while a thioester as a high-energy compound is generated. Examples acetyl CoA, succinyl-CoA, acyl-CoA-derivates. The acyl-carrier protein is a component of the fatty acid-synthetase complex. Both coenzymes transfer acyl groups. 

Biochemical functions pantothenate is a component of coenzyme A (CoASFI) which has numerous functions in carbohydrate, lipid and amino acid metabolism. (NB The SH refers to the terminal sulphydryl group in coenzyme A, Chapter 9)... 

Rubio-Gozalbo, M.E., Smeitink, J.A.M. and Sengers, R.C.A. Mitochondriocytopathies in pediatric patients. In Mitochondrial Ubiquinone (Coenzyme QIO) Biochemical, functional, medical and therapeutic aspects in human health and diseases (eds M. Ebadi, J. Marwah, R.K. Chopra). Prominent Press, Scottsdale, in press. 

The biochemical function of nicotinic acid and nicotinamide actually comes down to that of NAD and NADP as coenzymes of dehydrogenases. The latter func-... 

The sulfur content in the human body is about 140 g. Foods contain a large number of covalent sulfur compounds. Many sulfur compounds perform important biochemical functions as biocatalysts (e.g. thiamine, pantothenic acid bound in coenzyme A and biotin), and sulfur-containing amino acids, cysteine and methionine, are protein constituents. Many sulfur compounds are important precursors of flavour-active compounds. 

---