The Reaction Cycle of

FIGURE 10.22 The reaction cycle of bacteriorhodopsin. The intermediate states are indicated by letters, with subscripts to indicate the absorption maxima of the states. Also indicated for each state is the configuration of the retinal chromophore (all-tram or 13-cas) and the protonation state of the Schiff base (C=N or C=N H). 

In the late 1960s, Wilkinson postulated the reaction cycle of the ligand-modified rhodium catalyzed hydroformylation (Fig. 6). 

The self-peroxidation reactions of Cu,Zn-SOD provide a particularly novel mechanism for the formation of protein radicals. Hydrogen peroxide is generated during the reaction cycle of the enzyme ... 

The reaction cycle of these enzymes begins with reduction of both coppers from Cu(II) to Cu(I) (Eq. 18-54, step a). Both 02 and substrate bind (steps b and c, but not necessarily in this order). The 02 bound to CuB is reduced to a peroxide anion that remains bound to CuB. Both CuA and CuB donate one electron, both being oxidized to Cu(II). These changes are also included in step c of Eq. 18-54. One proposal is that the resulting peroxide is cleaved homolytically while removing the pro-S hydrogen of the glycyl residue. 

Figure 3. Schematic representation of the reaction cycle of SERCA pumps. The SERCA pumps exist in two conformational state El binds Ca2+ with high affinity at the cytoplasmic site of the SER membrane, while E2 has low affinity for Ca2+ and thus releases it on the opposite site of the membrane. ATP phosphorylates a highly conserved aspartic acid residue allowing for the translocation of Ca2+ in the SER lumen...

Tanford, C. (1984). Twenty questions concerning the reaction cycle of the sarcoplasmic reticulum calcium pump. Crit. Rev. Biochem. 17, 123-151. 

Here, we consider the reaction cycle of the H2 production process in the oxidized system based on the reverse reaction of that given in Eq. (9-2), as shown in Figure 9-15,... 

In this mechanism X+ represents an oxidant—a dangerously reactive peroxide perhaps, or even Fe(III) which must be reduced to Fe(TI) as part of the reaction cycle of many iron-dependent enzymes. 

Figure 8 shows a scheme of the reaction cycle of copper ATPases, assuming that they work by a mechanism analogous to that of Ca - or Na+, K+-ATPases. To pump ions, the enzyme must cycle between a state with a high-affinity copper-binding site accessible from only one side of the membrane and a low-affinity state in which the copper cavity is accessible from the other side of the membrane. The high- and low-affinity forms of P-type ATPases were initially named Ei and E2 by Racker (1980) and for many years these ATPases were called E]E2-ATPases, until they were renamed P-type by Pedersen and Carafoli (1987a). 

On reaction with a stoichiometric amount of hydroperoxide, catalase and horseradish peroxidase are converted to a green colored intermediate. Compound I (5). The chemical nature of Compound I has been extensively debated since its discovery by Theorell 59). Recently, Dolphin et al. 60) have demonstrated that upon one-equivalent oxidation several metalloporphyrins are converted to stable porphyrin jr-cation radicals, the absorption spectra of which possess the spectral characteristics of Compound I, namely, a decreased Soret w-n transition and an appearance of the 620-670-nm absorption bands. Since Moss et al. 61) proposed the presence of Fe(IV) in Compound I of horseradish peroxidase from Mossbauer spectroscopic measurements, it is attractive to describe Compound I as Fe(IV)-P, where P is a porphyrin w-cation radical. Then, Compound I and Compound ES become isoelectronic. Both contain Fe(IV) and a radical the former as a porphyrin radical (P ) and the latter as a protein radical (R ). Then the reaction cycles of horseradish and cytochrome c peroxidases may be compared as shown in Fig. 4. 

FIGURE 13. Species identified in the reaction cycle of copper amine oxidases. Oxidised, resting state enzyme (1) reacts with substrate to form a substrate Schiff base (2). Proton abstraction by the active site base (Asp383 in ECAO) leads, via a carbanion intermediate (3) to the product Schiff base (4). Hydrolysis releases the product aldehyde, leaving reduced cofactor in equilibrium between aminoquinol/Cu (S) and semiquinone/Cu (6). The reduced cofactor is reoxidised by molecular oxygen, releasing ammonium ions and hydrogen peroxide. (Modified from Wilmot et al., 1999 with permission). 

Table 14 summarizes the product yields from the combination of substrate (PhsP, Ph2SO, PhCH20H, or 1,4-cyclohexadiene) with O2, PhNHNHPh, and Fe (MeCN)4 + in DMF. The system acts as an effective monooxyge-nase/dehydrogenase, and as such mimics the reaction cycle of the cytochrome P-450 monooxygenase/reductase system ... 

Figure 13.3. Phosphoaspartate. Phosphoaspartate (also referred to as P-aspartyl phosphate) is a key intermediate in the reaction cycles of P-type ATPases.

From the reaction cycle of cytochrome P-450, three main steps can be defined (1) the formation of an enzyme-substrate complex that can be monitored by the spin variation of the ferric enzyme (2) the reduction of Fe3+—S, either enzymically or chemically (Fe3+—S - Fe2+—S and (3) the fixation of oxygen to give a ternary oxycompound by addition of oxygen to the ferrous form Fe2+—S + 02 — Fe2s—02. 

FIGURE 11.5 The reaction cycle of Ca -ATPase pumps. In the Ei conformation of the pump, is bound with high affinity at the cytoplasmic side of the plasma membrane. In the E2 configuration, the binding site exposes Ca to the external site of the plasma membrane, where its lower affinity for Ca favours its release. (Adapted fmm Di Leva et at, 2008. Copyright 2008, with permission fmm Elsevier.)... 

The reaction cycle of isopenicillin N synthase observed by X-ray diffraction,... 

The reaction cycle of the heterotrimeric G proteins involves the formation and breaking of numerous protein-protein contacts. In a dynamic way, protein-protein interactions are formed and resolved during the cycle, defining distinct states of the G protein and leading to new functions and reactions. A wealth of structural information is now available for most of the distinct functional states of the G proteins. Structures are available for... 

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