Amphibolic reactions

A summary of the major catabolic and amphibolic reactions for the anaerobic metabolism of glucose is presented in Fig. 17.11. 

Amphibolic reaction chains. The degradation products of Cm. enter general metabolism as pyruvate and acetyl-CoA (Fig. 2). 

On the other hand, both chrysotile and the amphiboles exhibit a high degree of chemical inertia towards strong alkaHes over extended periods. At high temperatures, reactions with alkaHes (NaOH, KOH, Ca(OH)2) become significant over relatively short periods for example, crocidoHte was reported to be attacked by potassium hydroxide above 100°C (22). 

Removal of a-amino nitrogen by transamination (see Figure 28-3) is the first catabolic reaction of amino acids except in the case of proline, hydroxyproline, threonine, and lysine. The residual hydrocarbon skeleton is then degraded to amphibolic intermediates as outhned in Figure 30-1. 

Similarly to Mn(IV)- and Fe(III)-oxides, some primary minerals were shown to promote polymerization of hydroquinone (19). Olivines, pyroxenes, and amphiboles accelerated the polymerization reaction to a greater extent than micas and feldspars. Microcline and quartz were ineffective- The effect was greatest for tephroite, a manganese-bearing silicate with the ideal chemical formula M SiO. Fayalite, the corresponding Fe(II) analog (Fe2Si0 ), was effective, but to a lesser extent. 

Ghose S. (1982). Subsolidus reactions and microstructures in amphiboles. In Reviews in Mineralogy, vol. 9A, P. H. Ribbe (series ed.), Mineralogical Society of America. 

Veblen, D. R. (1981). Non-classical pyriboles and polysomatic reactions in biopyriboles, pp. 189-236. In D. R. Veblen, ed. Amphiboles and Other Hydrous Pyriboles—Mineralogy. Reviews in Mineralogy, 9A. Min. Soc. America, Washington, D. C. 

The reaction rate constant and the diffusivity may depend weakly on pressure (see previous section). Because the temperature dependence is much more pronounced and temperature and pressure often co-vary, the temperature effect usually overwhelms the pressure effect. Therefore, there are various cooling rate indicators, but few direct decompression rate indicators have been developed based on geochemical kinetics. Rutherford and Hill (1993) developed a method to estimate the decompression (ascent) rate based on the width of the break-dovm rim of amphibole phenocryst due to dehydration. Indirectly, decompres-... 

Some other intracrystalline exchange reactions have also been investigated to some extent, such as Fe, Ni, and Mg exchange between Ml and M2 sites in olivine (Ottonello et al., 1990 Henderson et al., 1996 Redfem et al., 1996 Heinemann et al., 1999 Merli et al., 2001), Fe and Mg exchange between Ml + M2 + M3 and M4 sites in amphibole (Ghiorso et al., 1995), order-disorder reaction for Mg and Al, or for Mg and Fe +, between the tetrahedral and octahedral sites (O Neill, 1994 Harrison and Putnis, 1999 Andreozzi and Princivalle, 2002), and... 

Since the initial observation of flavin radical species by Michaelis and coworkers the involvement of flavins in one-electron oxidation-reduction processes in biological systems has occupied the attention of workers in the field of redox enzymology up to the present time. Flavin coenzymes occupy a unique role in biological oxidations in that they are capable of functioning in either one-electron or two-electron transfer reactions. Due to this amphibolic reactivity, they have been termed in a recent review to be at the crossroads of biological redox processes. 

Nonlinear Precipitation of Secondary Minerals from Solution. Most of the studies on dissolution of feldspars, pyroxenes, and amphiboles have employed batch techniques. In these systems the concentration of reaction products increases during an experiment. This can cause formation of secondary aluminosilicate precipitates and affect the stoichiometry of the reaction. A buildup of reaction products alters the ion activity product (IAP) of the solution vis-a-vis the parent material (Holdren and Speyer, 1986). It is not clear how secondary precipitates affect dissolution rates however, they should depress the rate (Aagaard and Helgeson, 1982) and could cause parabolic kinetics. Holdren and Speyer (1986) used a stirred-flow technique to prevent buildup of reaction products. 

Apart from the production of NADH and FADH2, which are the high-energy fuels of electron transport, the citric acid cycle has two other major functions. Several of its intermediate compounds are used to synthesize other cell constituents. This, the provision of molecules for other metabolic or biosynthetic pathways, is the anabolic function of the cycle (Table 12.1). Alternatively, certain other processes occurring within the cell may produce intermediates of the citric acid cycle. These compounds enter the reactions of the cycle, and their degradation involves the catabolic role of the cycle. These two major capabilities classify the citric acid cycle as an amphibolic pathway (Greek amphi meaning both sides ). 

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