Soluble coenzymes

Figure 8.27 Reduction of aldehyde in SCCO2 by an isolated enzyme, horse liver alcohol dehydrogenase (HLADH) [20c] (a) Reaction scheme (b) fluorinated coenzyme soluble in CO2 and (c) effect of coenzyme on the reaction.

Chemists and biochemists And it convenient to divide the principal organic substances present m cells into four mam groups carbohydrates proteins nucleic acids and lipids Structural differences separate carbo hydrates from proteins and both of these are structurally distinct from nucleic acids Lipids on the other hand are characterized by a physical property their solubility m nonpolar solvents rather than by their structure In this chapter we have examined lipid molecules that share a common biosynthetic origin m that all their carbons are derived from acetic acid (acetate) The form m which acetate occurs m many of these processes is a thioester called acetyl coenzyme A... 

Squalene monooxygenase, an enzyme bound to the endoplasmic reticulum, converts squalene to squalene-2,3-epoxide (Figure 25.35). This reaction employs FAD and NADPH as coenzymes and requires Og as well as a cytosolic protein called soluble protein activator. A second ER membrane enzyme, 2,3-oxidosqualene lanosterol cyclase, catalyzes the second reaction, which involves a succession of 1,2 shifts of hydride ions and methyl groups. 

Many procedures have been suggested to achieve efficient cofactor recycling, including enzymatic and non-enzymatic methods. However, the practical problems associated with the commercial application of coenzyme dependent biocatalysts have not yet been generally solved. Figure A8.18 illustrates the continuous production of L-amino adds in a multi-enzyme-membrane-reactor, where the enzymes together with NAD covalently bound to water soluble polyethylene glycol 20,000 (PEG-20,000-NAD) are retained by means of an ultrafiltration membrane. 

The water-soluble B vitamins supply important components of numerous coenzymes. Many coenzymes contain, in addition, the adenine, ribose, and phosphoryl moieties of AMP or ADP (Figure 7-2). Nicotinamide and riboflavin are components of the redox coenzymes... 

Ubiquinones (coenzymes Q) Q9 and Qi0 are essential cofactors (electron carriers) in the mitochondrial electron transport chain. They play a key role shuttling electrons from NADH and succinate dehydrogenases to the cytochrome b-c1 complex in the inner mitochondrial membrane. Ubiquinones are lipid-soluble compounds containing a redox active quinoid ring and a tail of 50 (Qio) or 45 (Q9) carbon atoms (Figure 29.10). The predominant ubiquinone in humans is Qio while in rodents it is Q9. Ubiquinones are especially abundant in the mitochondrial respiratory chain where their concentration is about 100 times higher than that of other electron carriers. Ubihydroquinone Q10 is also found in LDL where it supposedly exhibits the antioxidant activity (see Chapter 23). 

Vitamins, especially the water-soluble ones, provide the coenzymes for the most fundamental cellular reactions. Thus, their presence is required by vertebrates and invertebrates as well as by monocellular organisms, such as bacteria and other fungi and by protists. One finds in nature many microorganisms which need those vitamins that they are unable to produce at all or in sufficient quantities. Thus, one has a wide choice of... 

B39. Brown, G. M., Biosynthesis of water soluble vitamins and derived coenzymes. Physiol. Revs. 40, 331-368 (1960). 

NADH-coenzyme Q (CoQ) oxidoreductase, transfers electrons stepwise from NADH, through a flavoprotein (containing FMN as cofactor) to a series of iron-sulfur clusters (which will be discussed in Chapter 13) and ultimately to CoQ, a lipid-soluble quinone, which transfers its electrons to Complex III. A If, for the couple NADH/CoQ is 0.36 V, corresponding to a AG° of —69.5 kJ/mol and in the process of electron transfer, protons are exported into the intermembrane space (between the mitochondrial inner and outer membranes). 

Many enzymes require the participation of dissociable coenzymes such as NAD+, NADP+ or ATP for their catalytic activities. The use of coenzymes to activate immobilized enzymes on a large scale is hampered by their relatively low stability and high cost. Attempts are therefore being made to stabilize the coenzymes and to find suitable means for their continuous regeneration. The principal approach has been to covalently attach a co-enzyme to a polymeric water-soluble matrix, thus making the co-enzyme, like the enzyme, potentially reusable (9,10). 

Coenzyme An organic nonprotein molecule, frequently a phosphorylated derivative of a water-soluble vitamin, that binds with the protein molecule (apoenzyme) to form the active enzyme (holoenzyme). [EU]... 

The first of these new, electron transferring components was coenzyme Q (CoQ). Festenstein in R.A. Morton s laboratory in Liverpool had isolated crude preparations from intestinal mucosa in 1955. Purer material was obtained the next year from rat liver by Morton. The material was lipid soluble, widely distributed, and had the properties of a quinone and so was initially called ubiquinone. Its function was unclear. At the same time Crane, Hatefi and Lester in Wisconsin were trying to identify the substances in the electron transport chain acting between NADH and cytochrome b. Using lipid extractants they isolated a new quininoid coenzyme which showed redox changes in respiration. They called it coenzyme Q (CoQ). CoQ was later shown to be identical to ubiquinone. 

Vitamins have historically been classified as either water soluble or lipid soluble. Water-soluble vitamins are precursors for coenzymes and are reviewed in the context of the reactions for which they are important A summary of these vitamins is shown in Table I-lO-l. 

Ferredoxins (Fds) are widespread in the three domains of life and an abundance of sequence data and structural information are available for Fds isolated from several sources. In particular, the bacterial type Fds are small electron-transfer proteins that posses cubane xFe-yS clusters attached to the protein matrix by Fe ligation of Cys via a conserved consensus ligating sequence. The archaeal type ferredoxins are water-soluble electron acceptors for the acyl-coenzyme A forming 2-oxoacid/ferredoxin oxidoreductase, a key enzyme involved in the central archaeal metabolic pathways. Fds have been distinguished according to the number of iron and inorganic sulphur atoms, 2Fe-2S, 4Fe-4S/3Fe-4S (Fig. Ib-d) and Zn-containing Fds. 

The complexity of the environment surrounding the coenzyme has prevented most simple model systems from dramatically enhancing thiamine reactivity or specificity [46-48]. Peptide- or protein-based models have the advantage of presenting a reasonably complex environment to the coenzyme functionality within a water soluble, yet synthetically accessible, scaffold. 

Finally, we come to the last of the vitamins that appear on the contents list of my multivitamin pill—pantothenic acid. This water-soluble vitamin serves a single purpose in physiology and biochemistry it is a precursor to a far more complex molecule known as coenzyme A or, simply, CoASH. 

However, a more favourable pathway is used, employing a more reactive nucleophile. Rather than using the enolate anion derived from acetyl-CoA, nature uses the enolate anion derived from malonyl-CoA. Malonyl-CoA is obtained from acetyl-CoA by means of an enzymic carboxylation reaction, incorporating CO2 (usually from the soluble form bicarbonate). Now CO2 is a particularly unreactive material, so this reaction requires the input of energy (from ATP) and the presence of a suitable coenzyme, biotin, as the carrier of CO2 (see Section 15.9). The... 

Soluble cytoplasmic sulfotrans-ferases conjugate activated sulfate (3 -phosphoadenine-5 -phosphosulfate) with alcohols and phenols. The conjugates are acids, as in the case of glucuronides. In this respect, they differ from conjugates formed by acetyltransfe-rases from activated acetate (acetyl-coenzyme A) and an alcohol or a phenol... 

All oxidoreductases (see p. 88) require coenzymes. The most important of these redox coenzymes are shown here. They can act in soluble form (S) or prosthetically (P). Their normal potentials E° are shown in addition to the type of reducing equivalent that they transfer (see p. 18). 

The pyridine nucleotides NAD"" and NADP" (1) are widely distributed as coenzymes of dehydrogenases. They transport hydride ions (2e and 1 see p. 32) and always act in soluble form. NAD" transfers reducing equivalents from catabolic pathways to the respiratory chain and thus contributes to energy... 

The five different coenzymes involved are associated with the enzyme components in different ways. Thiamine diphosphate is non-covalently bound to El, whereas lipoamide is covalently bound to a lysine residue of E2 and FAD is bound as a prosthetic group to E3. NAD" and coenzyme A, being soluble coenzymes, are only temporarily associated with the complex. 

The B group of vitamins covers water-soluble vitamins, all of which serve as precursors for coenzymes. Their numbering sequence is not continuous, as many substances that were originally regarded as vitamins were not later confirmed as having vitamin characteristics. 

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