Shunt pathway

In the M. capsulatus (Bath) system, all three components are necessary to obtain turnover with NADH as the reductant (57). With the M. trichosporium OB3b system, protein B is apparently not required (27). Instead, in this latter system, protein B increases the initial rates of the catalytic hydroxylation reaction (27). Catalysis can be achieved by means of a shunt pathway with hydrogen peroxide and Hox alone from both organisms (58-60). The efficiency of the shunt pathway, however, varies significantly. With M. trichosporium OB3b, alcohol yields greater than those obtained with the completely reconstituted system have been observed (58). Furthermore, upon addition of protein... 

B, the initial rate constants are diminished, in contrast to those observed for the catalytic system (61). The reasons for the different effects of protein B on the two reactions with Hox are unknown. With Hox from M. capsulatus (Bath), activities of only —10% of the values observed under optimal catalytic conditions were found with the H202 shunt pathway, assuming specific activities to be greater than 200 mU/mg (59). As a consequence of the poor yields observed, the effect of protein B on the system was not investigated further. 

The intercept defines ion flux across the epithelia (J0) while the slope provides a measure of the magnitude of the shunt pathway (Js) under short-circuit OF = 0) conditions. However, as discussed in the following section, mediated solute transport may also be voltage-dependent. 

Rh-H / -elimination step in step E gives the vinyl-borane which then dissociates to generate the first intermediate. This cycle explains the production of equimolar quantities of alkane and -vinylborane with unreactive R2BH derived from ephedrines with catecholborane there is an additional shunt pathway — step C in competition with step E leading to the primary regio-isomer of alkylborane. 

Certain microorganisms have a modification of this cycle in which isocitric acid is cleaved to succinic acid and glyoxylic acid. The latter acid is condensed with acetyl-CoA to form malic acid. In this modification (the glvoxvlic acid cvcle), oxalsuccinic acid and alpha-ketoglularic acid are not involved. This is sometimes referred to as the glyoxylate shunt pathway. 

Transketolase reactions leading via the pentose or hexose monophosphate shunt pathway of glucose oxidation to the eventual production of pentoses for RNA/DNA synthesis and NADPH for the biosynthesis of fatty acids... 

Except for a few examples [103], see below, the focus of synthetic P450 chemistry has been on the meso-tetraphenylporphyrin system mainly because free meso-positions can easily be oxidized in the presence of O donors employed in the shunt pathway . 

Frizzell RA, Schultz SG. Ionic conductances of extracellular shunt pathway in rabbit ileum. Influence of shunt on transmural sodium transport and electrical potential differences. J Gen Physiol 1972 59 318-346. 

It is instructive to contrast this model for desorption through multilaminates with the desorption properties of naturally occurring biological laminates, in particular, stratum corneum. While desorption from model multilaminates should be linear with time, desorption from the stratum corneum in spite of its microscopic laminate appearance, is linear with the square root of time (18). This result forms some of the best evidence that shunt pathways dominate skin transport (18). 

Figure 18.10 The hexose monophosphate shunt pathway. A, glucose-6-phosphate dehydrogenase B, 6-phosphogluconate dehydrogenase C, pentose-5-phosphate iso-merase D, pentose phosphate epimerase E, transaldolase F, transketolase G, phospho-hexoseisomerase. (Reproduced by permission from Williams JF. A critical examination of the evidence for the reactions of the pentose pathway in animal tissues. Trends Biochem Sri December 316, 1980.)...

Tencer J, Frick IM, Oquist BW, Aim P, Rippe B. Size-selectivity of the glomerular barrier to high molecular weight proteins upper size limitations of shunt pathways. Kidney Int 1998 53(3) 709—15. 

Fig. 10.2. Catalytic cycle of P450 including the peroxide shunt pathway. RH is substrate, and ROH is product. The porphyrin molecule is represented as a parallelogram. The overall charge on the structures is shown to the left of each bracket. Intermediates 1, 2, 7, and 8 are neutral. Refer to text for a full description.

P450s are capable of utilizing an oxygen atom from peroxide to catalyze oxygen insertion without electron transport proteins or the NAD(P)H cofactor, through the peroxide shunt pathway ... 

The P450 shunt pathway and peroxygenase activity of peroxidases share identical overall reaction equations. P450s generally have high Km values for H 202 values of 15 mM... 

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