Reaction under physiological

AG° is predictive only under standard conditions Under standard conditions, AG° can be used to predict the direction a reaction proceeds because, under these conditions, AG° is equal to AG. However, AG° cannot predict the direction of a reaction under physiologic conditions, because it is composed solely of constants (R, T, and Keq) and is, therefore, not altered by changes in product or substrate concentrations. 

The accumulation problem is most severe for enzymes such as the digestive enzymes, which have to cope with pulses of high substrate concentrations. If the concentration of the substrate is below the KM for the reaction under physiological concentrations, no intermediate accumulates in vivo in any case, since the enzyme is unbound. But in a test tube experiment in which the experimenter can use artificially high concentrations of substrate, an intermediate can sometimes be made to accumulate. An example occurs with glyceraldehyde 3-phosphate dehydrogenase. As Table 12.4 shows, the concentration of the aldehyde is below the Km in vivo. But in the laboratory, the acylenzyme accumulates at saturating substrate concentrations. 

The heme-thiolate cytochromes P450 (P450) catalyze regio- and stereospecific reactions under physiological conditions [18-20], including the hydrox-ylation of hydrocarbons (Equation Eq. 1), alkene epoxidation, heteroatom (N,S) oxidation, dealkylation, and (anaerobic) dehalogenation [21]. 

Enzyme-catalyzed attachment of amino acids to proteins represents an attractive and interesting way for improving the nutritional value of food proteins. The enzymes that participate in the gastrointestinal digestion of food proteins catalyze exclusively hydrolytic reactions under physiological conditions. However the synthetic activity of proteolytic enzymes was reported first by Danilewski in 1886, and more recently a number of studies have been devoted to plastein formation from con-... 

Unfortunately, turnover control of PSII is more complicated than the above description would indicate. Because turnover of the S states is achieved via a photochemical reaction, the yield of the reaction depends on both the electron donors and the electron acceptors. The overall picture of electron transfer in PSII is shown in Figure 2 (II). Light induces a series of electron-transfer reactions that lead to the formation of progressively more stable charge-separated states. The dominant reaction under physiological conditions leads to a one-step advancement of the S state and reduction of the secondary quinone electron acceptor (Qb). In purified PSII preparations, however, the quinones are depleted and the QB site will mostly be unoccupied unless exogenous quinones are added. 

The AG of a reaction under physiological standard-state conditions (all substrates and products at 1 M except H ) is given by ... 

In contrast to the relatively stable C-H, C-O, C-S, C-N and C-Hal bonds, the analogous silicon-element bonds (Si-H, Si-O, Si-S, Si-N, Si—Hal) normally undergo hydrolytic cleavage reactions under physiological conditions (-> Si-OH). This reactivity pattern causes a stability-related limitation of the sila-substitution approach. A further restriction results from the inability of the silicon atom to form stable silicon-element multiple bonds of the (p-p)7i type. For instance, in contrast to the C=0, C=C and C=C bonds, the analogous Si=0, Si=C and Si=C bonds are not stable under ordinary conditions. 

The reactions catalyzed by the epimerase, isomerase, transketolase, and transaldolase are all reversible reactions under physiologic conditions. Thus, ribose 5-phosphate required for purine and pyrimidine synthesis can be generated from intermediates of the glycolytic pathway, as well as from the oxidative phase of the pentose phosphate pathway. The sequence of reactions that generate ribose 5-phos-phate from intermediates of glycolysis is indicated below. 

Pictet-Spengler ring closure Reactions under physiological conditions... 

Until the mid-1940 s the metabolic fate of a compound could be followed only with great difficulty. Biosynthetic studies often were limited to model experiments in unbiological systems, which served to demonstrate the chemical feasibility of certain reactions under physiological conditions . This situation changed drastically in the 1950 s when radioactive isotopes became readily available. The use of the so-caUed tracer technique, i.e.,... 

Ttetrahydroisoquinolines from amines—Reactions under physiological conditions s. 17, 843... 

Njoroge, F.G. Monnier, V.M. The chemistry of the Maillard reaction under physiological conditions A Review. In The Maillard Reaction in Aging Diabetes, and Nutrition, Alan R. Liss Inc., 1989 pp. 85-107. 

Njorge, F. G., Sayre, L. M., and Monnier, V. M., 1987, Detection of D-glucose-derived pyrrole compounds during Maillard reaction under physiological conditions, Carbohydr. Res. 167 211-220. 

The Zn ion, among the series of transition metals, is a cofactor which is not involved in redox reactions under physiological conditions. As a Lewis acid similar in strength to Mg , Zn participates in similar reactions. Hence, substituting the Zn ion for the Mg ion in some enzymes is possible without loss of enzyme activity. Both metal ions can function as stabilizers of enzyme conformation and their direct participation in catalysis is readily revealed in the case of alcohol dehydrogenase. This enzyme isolated from horse liver consists of two identical polypeptide chains, each with one active site. Two of the four Zn ions in the enzyme readily dissociate. Although this dissociation has no effect on the quaternary structure, the enzyme activity is lost. As described under section 2.3.1.1, both of these Zn ions are involved in the formation of the active site. In catalysis they polarize the substrate s C—O linkage and, thus, facilitate the transfer of hydride ions from or to the cosubstrate. Unlike the dissociable ions, removal of the two residual Zn ions is possible only under drastic conditions, namely disruption of the enzyme s quaternary structure which is maintained by these two ions. 

Especially transition metal icMis play an important role in the induction of oxidative DNA damage. While neither superoxide anions nor hydrogen peroxide are able to react with DNA directly, in the presence of transition metals like iron, copper, cobalt, or nickel they are converted into highly reactive hydroxyl radicals by Fenton-type reactions. In contrast, cadmium ions are not able to participate in redox reactions under physiological conditions, yet, oxidative stress and the interference with cellular redox regulation may be of high relevance in cadmium-induced carcinogenicity. Increased levels of ROS due to cadmium exposure have been observed both in vitro and in vivo [31]. Different cadmium compounds have been shown to induce DNA strand breaks and oxidative DNA base modifications in... 

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