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The Example of Coenzyme

Coenzyme Vitamin Additional Chemical group(s) component transferred Distribution [Pg.81]

NAD+ and NADP+ Niacin (B3) ADP Electrons Bacteria, archaea and eukaryotes [Pg.81]

Coenzyme A Pantothenic acid (B5) ADP Acetyl group and other acyl groups Bacteria, archaea and eukaryotes [Pg.81]

Tetrahydrofolic acid Folic acid (B9) Glutamate Methyl, formyl, residues methylene and formimino groups Bacteria, archaea and eukaryotes [Pg.81]

Menaquinone Vitamin K None Carbonyl group and electrons Bacteria, archaea and eukaryotes [Pg.81]


To give an example of how a coenzyme functions we will look at the example of coenzyme B]2 which has proved a popular target for model studies over many years. Coenzyme B12 and its derivatives such as vitamin B12 are also based on tetrapyrrole macrocycles, Figure 2.10. The structures of these... [Pg.114]

We have stressed the direct relationship of specific nutritional needs to enzyme building, but this is only one possibility. In the case of nicotinamide, for example, which in the form of coenzymes I and II functions in oxidation-reduction reactions, an individual s need may be great because of the genetic ineffectiveness of the mechanism for building nicotinamide into enzyme systems, but the difficulty may lie at another site. Possibly there is difficulty in digestion (of the combined forms) or more likely absorption, which precludes the individual from getting a substantial portion of the nicotinamide out of his food to the cells that need it. Even the mechanism for transport may be at fault. We wish to emphasize that the effectiveness or ineffectiveness of the structures and mechanisms which may be... [Pg.204]

Many biomolecules are made up of smaller units in a modular fashion, and they can be broken down into these units again. The construction of these molecules usually takes place through condensation reactions involving the removal of water. Conversely, their breakdown functions in a hydrolytic fashion—i.e., as a result of water uptake. The page opposite illustrates this modular principle using the example of an important coenzyme. [Pg.12]

Proton transfers are particularly common. This acid-base catalysis by enzymes is much more effective than the exchange of protons between acids and bases in solution. In many cases, chemical groups are temporarily bound covalently to the amino acid residues of the enzyme or to coenzymes during the catalytic cycle. This effect is referred to as covalent catalysis (see the transaminases, for example p. 178). The principles of enzyme catalysis sketched out here are discussed in greater detail on p. 100 using the example of lactate dehydrogenase. [Pg.90]

We present here the mechanisms for four enzymes chymotrypsin, hexoldnase, enolase, and lysozyme. These examples are not intended to cover all possible classes of enzyme chemistry. They are chosen in part because they are among the best understood enzymes, and in part because they clearly illustrate some general principles outlined in this chapter. The discussion concentrates on selected principles, along with some key experiments that have helped to bring these principles into focus. We use the chymotrypsin example to review some of the conventions used to depict enzyme mechanisms. Much mechanistic detail and experimental evidence is necessarily omitted no one book could completely document the rich experimental history of these enzymes. Also absent from these discussions is the special contribution of coenzymes to the catalytic activity of many enzymes. The function of coenzymes is chemically varied, and we describe each as it is encountered in Part II. [Pg.213]

This has been one of the most controversial areas of bioenergetics and is concerned with the role of coenzyme Q. The simplest view of the role of this coenzyme is that it acts as a mobile (2H+ + 2e ) carrier, linking complexes I and II with complex III. However, coenzyme Q may be involved in (H+ + e ) transfer within complex III. One model for this is the proton-motive Q cycle (Fig. 14-6), developed by Mitchell in 1975. This model satisfies prediction (2) of Example 14.10, in that coenzyme Q acts as an (H+ +e ) carrier in two loops. In this model, reduced coenzyme Q (QH2) is linked to oxidized coenzyme Q (Q) via the free-radical semiquinone (QH-) This model provides an explanation for the H+/e stoichiometry. [Pg.410]

Acetylation can also affect Lys5, Lys8 and Lys12. The e-amino group of Lys20 is also modified, but by mono- or di-methylation. The biochemical methylating agent is S-adenosylmethionine (Section 8.5). The biosyntheses of coenzyme A and. S -adcno-sylmethionine provide additional examples of the biochemical utilisation of cysteine and methionine, respectively. [Pg.181]

Some enzymes require participation of coenzymes or metal ions for catalytic activity. Flavin derivatives (20) can be considered as an example of coenzyme analogues attached to artificial enzymes. The reactivity of flavin derivatives attached to a PEI derivative or a cationic polystyrene derivative was considerably greater than that of monomeric flavin analogues. ... [Pg.253]

A substantial number of pharmaceutically and clinically related problems require the detection and determination of small amounts of metal ions and other inorganic constituents of biological and xenobiotic substances (1-3). Some obvious examples are the detections of heavy metals and lithium in biological fluids and tissue samples in cases of suspected intoxication and the determination of potassium for purposes of quality control in intravenous solutions to be given to cardiac patients. Trace amounts of nonmetals such as selenium and iodine, which are associated with the functions of coenzymes or hormones, also must be analyzed in order to determine their roles in metabolic pathways. [Pg.401]

There are reciprocal relationships between the parameters summarized above. On the one hand enzyme stability measurements strongly depend on the concentrations of substrates, coenzymes, buffers etc. in the assay. On the other hand the choice of an appropriate concentration level is a consequence of the enzyme kinetics investigated afterwards. A compromise has to be found between different optimization criteria e. g. a lower temperature leads to a reduced enzyme activity but results in a higher enzyme stability. In the example of the oxynitrilase reaction (Eq. (12)) a low pH value is a prerequisite for high enantiomeric purity of the product but lowers enzyme activity. As a consequence, only a rough optimization can be carried out at this level. [Pg.189]

Many of the water-soluble vitamins are the precursors of coenzymes. Niacin (nicotinic acid) is a precursor of NAD, for example. Pantothenic acid is a precursor of coenzyme A. [Pg.1091]

Several j3-amino acids exist. A unit of j3-alanine, for example, is contained within the structure of coenzyme A (Section 22.1 D). Write the structural formula of j3-alanine. [Pg.645]

Thus the biological importance of the phosphopantetheine group as a catalytic centre is widespread. Numerous examples of the role of coenzyme A are known and the list of phosphopantetheine enzyme centres is growing. The principal reactive element is the thiol, although other attributes of the unique peptide will undoubtedly prove important. The thiol serves as the site of thioester formation and its particular chemical attributes facihtate acyl transfer, carbon chain modification and condensation reactions. The phosphopantetheine thiol represents the most... [Pg.86]

There are a number of simple model complexes of cobalt that mimic various aspects of the chemistry of coenzyme B,2< Examples, such as Co(DMGH)2, Co(C2(DO)(DOH))p " and Co(salen), are shown in Figure 8.2. These complexes have derivatives with cobalt in the (III), (II) and (I) oxidation states, analogous to coenzyme B,2- They form a... [Pg.343]

The example of the reversible oxidation of ethanol (CH3CH2OH) to ethanal (acetaldehyde, CH3CHO) by yeast alcohol dehydrogenase (alcohol NAD " oxidore-ductase [EC 1.1.1.1]) is one such (now classical) process and is discussed below. The oxidoreductase uses nicotinamide adenine dinucleotide (NAD") (oxidized form) as its coenzyme, and it is in the pyridine (azabenzene, C5H5N) ring that the reduction is clearly seen. [Pg.594]

Another example in which the intervention of a coenzyme is explained, is the case of the conversion of G—1—into G—6—P. Here, G—1,6—PP plays the part of coenzyme. Unlike the case of DPN cited previously, this coenzyme does not function as a second substrate, but on the contrary it is constantly reformed from the substrate. In effect a cycle is operating in which G—6—P accumulates. [Pg.175]


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