Coenzymes availability

Coenzyme availability can also often have a limiting effect (5). If the coenzyme is regenerated by a second, independent metabolic pathway, the speed of the second pathway can limit that of the first one. For example, glycolysis and the tricarboxylic acid cycle are mainly regulated by the availability of NAD" (see p. 146). Since NAD is regenerated by the respiratory chain, the latter indirectly controls the breakdown of glucose and fatty acids (respiratory control, see p. 144). 

Although a variety of oxidizing agents are available for this transformation it occurs so readily that thiols are slowly converted to disulfides by the oxygen m the air Dithiols give cyclic disulfides by intramolecular sulfur-sulfur bond formation An example of a cyclic disulfide is the coenzyme a lipoic acid The last step m the laboratory synthesis of a lipoic acid IS an iron(III) catalyzed oxidation of the dithiol shown... 

Section 27 21 Often the catalytically active functions of an enzyme are nothing more than proton donors and proton acceptors In many cases a protein acts m cooperation with a coenzyme, a small molecule having the proper func tionahty to carry out a chemical change not otherwise available to the protein itself... 

As its name implies, the citric acid cycle is a closed loop of reactions in which the product of the hnal step (oxaloacetate) is a reactant in the first step. The intermediates are constantly regenerated and flow continuously through the cycle, which operates as long as the oxidizing coenzymes NAD+ and FAD are available. To meet this condition, the reduced coenzymes NADH and FADH2 must be reoxidized via the electron-transport chain, which in turn relies on oxygen as the ultimate electron acceptor. Thus, the cycle is dependent on the availability of oxygen and on the operation of the electron-transport chain. 

The citric acid cycle is an integral part of the process by which much of the free energy liberated during the oxidation of fuels is made available. During oxidation of acetyl-CoA, coenzymes are reduced and subsequendy reoxidized in the respiratory chain, hnked to the formation of ATP (oxicktive phosphorylation see Figure 16-2 and also Chapter 12). This process is aerobic, requiring oxygen as the final oxidant of the reduced coenzymes. The enzymes of the citric acid cycle are lo-... 

Six compounds have vitamin Bg activity (Figure 45-12) pyridoxine, pyridoxal, pyridoxamine, and their b -phosphates. The active coenzyme is pyridoxal 5 -phos-phate. Approximately 80% of the body s total vitamin Bg is present as pyridoxal phosphate in muscle, mostly associated with glycogen phosphorylase. This is not available in Bg deficiency but is released in starvation, when glycogen reserves become depleted, and is then available, especially in liver and kidney, to meet increased requirement for gluconeogenesis from amino acids. 

Although the reduction potentials of DNA bases and UV induced DNA lesions inside a DNA double strand or inside the active site of a DNA photolyase, together with the reduction potential of the photoexcited FADH- in the photolyases, are not known, currently available redox potentials indicate that the single electron reduction of a nucleobase or a UV induced dimer lesion by a reduced and deprotonated flavin coenzyme is a weakly exothermic process. The reduced and deprotonated FADH- in its photoexcited state is... 

In addition to the aspects mentioned above, the extent of PHA accumulation might depend on quite different features. The only physiological inhibitor of PHA synthase that has been identified is coenzyme A as pointed out in an earlier section. Since the concentration of coenzyme A in the cytoplasma will probably never rise to very high levels, it may be questionable whether this inhibition is physiologically relevant at all. In our opinion, the availability of a hy-droxyacyl coenzyme A thioester provided by the biosynthesis pathway is most important to initiate PHA biosynthesis (compare also [8] - this book). 

It may seem strange that we have left the transfer of sulfur out of this description, but it was available initially as H2S, which diffuses easily, compare H20, and is reactive with metal ions and some organic centres. Sulfur from intermediate states of oxidation of this element, e.g. S2Of, is transferred by molybdenum enzymes. Later, when sulfur became sulfate, a coenzyme (PAPS) was required for its transfer (see aerobes and eukaryotes).)... 

The necessary communication network in all chemotypes was based originally on exchangeable reduced available internal ions such as Mg2+ and Fe2+ and reduced organic chemicals, substrates and coenzymes, often linked to phosphate, and feedback and forth of information about cell conditions to and from the genetic central control. 

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