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). 

Scheme 10.17 Reaction cycle of the flavin-dependent squalene monooxygenase. Dashed arrows indicate electron transport.

Scheme 10.31 Reaction cycle of KG-dependent (KG = a-keto-glutarate) enzymes. Metal ligands from protein side chains and water are omitted for clarity. One of the oxygens of O2 is incorporated into succinate. The other oxygen is either incorporated into the product or reduced to water depending on the nature of the reaction.

AT-acetyltryptamines could be obtained via microwave-assisted transition-metal-catalyzed reactions on resin bound 3-[2-(acetylamino)ethyl]-2-iodo-lH-indole-5-carboxamide. While acceptable reaction conditions for the application of microwave irradiation have been identified for Stille heteroaryla-tion reactions, the related Suzuki protocol on the same substrate gave poor results, since at a constant power of 60 W, no full conversion (50-60%) of resin-bound 3-[2-(acetylamino)ethyl]-2-iodo-lH-indole-5-carboxamide could be obtained even when two consecutive cross-coupling reaction cycles (involving complete removal of reagents and by-products by washing off the resin) were used (Scheme 36). Also under conventional heating at 110 °C, and otherwise identical conditions, the Suzuki reactions proved to be difficult since two cross-coupling reaction cycles of 24 h had to be used to achieve full conversion. 

A catalytic reaction is composed of several reaction steps. Molecules have to adsorb to the catalyst and become activated, and product molecules have to desorb. The catalytic reaction is a reaction cycle of elementary reaction steps. The catalytic center is regenerated after reaction. This is the basis of the key molecular principle of catalysis the Sabatier principle. According to this principle, the rate of a catalytic reaction has a maximum when the rate of activation and the rate of product desorption balance. 

Fig. 4. E1-E2 reaction cycle of the Na,K-pump with four major occluded conformations and ping-pong sequential cation translocation. The phosphoforms can occlude Na" and dephosphoforms can occlude or Rb. Na and K without brackets are cations bound to an open form such that they can exchange with medium cations [Na ] or [K ] within brackets are occluded and prevented from exchanging with medium cations. It is proposed that release of Na cxt accompanies transition from EiP[3Na] to E2P[2Na], since the capacity for occlusion of Na in the ouabain-stabilized E2P form is lower than in the EjP form prepared by incubation with CrATP [29] or oligomycin [89].

Fig. 2. E]-E2 reaction cycle of H,K-ATPase, accounting for the transport of two H and two K ions per molecule of hydrolysed ATP. The ATPase reaction proceeds from 2H E, ATP through 2H E -P and 2H E2-P to 2K E. Details of the reaction cycle are described in the text.

When a multiproduct plant is constructed, the amount of each product to be made each year must be included in the scope. This is called the product mix. It is important because the product mix determines the size of much of the equipment. One resin may require a reaction cycle of three hours, while another takes six hours. If the majority of the product is the former resin, a smaller reactor is required than if the majority were the latter. 

Nickel complexes of this group are of interest in biomimetic work. By means of ligand (320) the complete reaction cycle of acteyl CoA synthase could be executed (Scheme 2). Ligand (320) can also be synthesized by a template reaction. Upon reduction of the Ni11 complex (321) with Na/Hg, the ligand backbone is cleaved, resulting in a thermally stable trinuclear Ni11 alkyl thiolato complex (322). 

Figure 17.5 Proposed reaction cycle of sulfite oxidase. (From Enemark et al., 2004. Copyright (2004) American Chemical Society.)...

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

Figure 8.3 Outline reaction cycle of NiFe hydrogenase.The minimal hydrogenase is depicted, consisting of the [NiFe] centre in the large subunit, and the proximal [4Fe-4S] cluster (C) in the small subunit.The reaction is written in the direction of the oxidation of H2. Electrons are transferred out through the other iron-sulfur clusters to an acceptor protein (not shown).The equivalent states of the NiFe centre B, SR, R and C are indicated. Reduced centres are shaded. Electron transfers are accompanied by transfers of hydrons (not shown).

Figure 2. The catalytic reaction cycle of the Tetrahymena ribozyme (E), showing the binding and docking reactions (leading to the formation of E-S complex), followed by a bond cleavage breaking step (the rate constant for which is kchem) and release of the 5 -fragment in the multiturnover steps (rate constant equals k t).

Are there other types of self-replication in nature, possibly based on a quite different mechanism There are not many, but there is a famous case the formose reaction, described in 1861, and based on a reaction cycle of formaldehyde. This reaction has already been mentioned in Chapter 3, on the subject of prebiotic... 

When controlled nitridation of surface layers is required, as for example in the modification of the chemical properties of the surface of a support, the atomic layer deposition (ALD) technique can be applied." This technique is based upon repeated separate saturating reactions of at least two different reactants with the surface, which leads to the controlled build-up of thin films via reaction of the second component with the chemisorbed residues of the first reactant. Aluminium nitride surfaces have been prepared on both alumina and silica supports by this method wherein reaction cycles of trimethylaluminium and ammonia have been performed with the respective supports, retaining their high surface areas." This method has been applied to the modification of the support composition for chromium catalysts supported on alumina." ... 

Since the reduction potential of MV2+/MV is low enough (—0.44 V at pH 7) to reduce protons, the presence of platinum as a catalyst in the solution containing MV 7 brings about hydrogen formation. Scheme 1 is a typical model of photo-induced charge separation and electron relay to yield H2. It also represents the half reaction cycles of the reduction site for the photochemical conversion shown in Fig. 3. 

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 ... 

Szabo A, Langer T, Schroder H, Flanagan J, Bukau B, Harti FU (1994) The ATP hydrolysis-dependent reaction cycle of the Escherichia coli Hsp70 system DnaK, DnaJ, and GrpE. Proc Natl Acad Sci USA 91 10345-10349... 

Fig. 3. Photochemical reaction cycle of [Run(bpy)3]2+ initiated by excitation of the MLCT transition.

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. 

By replacing the natural substrate homoprotocatechuate (3,4-dihydroxyphenyl acetate) with the slower reacting substrate analog 4-nitrocatechol, six intermediates of the catalytic reaction cycle of 2,3-HPCD could be observed experimentally, four of which could even be characterized by X-ray crystal structure analysis [22], Scheme 2.4 gives an overview of the proposed reaction mechanism. 

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...

Figure 11. Proposed reaction cycles of the Na+-K+-ATPase (A) and the SR Ca2+-AT-Pase (B) involving transitions between different conformational states of the enzymes (see text for further explanation). The cytoplasmic side of the membrane is upward and the extracytoplasmic side downward. Brackets indicate that all the cation binding sites reside in an occluded state. A tentative H+-countertransport limb is shown for the SR Ca2+-ATPase (most likely n = 2). s indicates a relatively slow reaction step. ATP boxes indicate steps accelerated by ATP not being hydrolyzed. Mg2+ serving as a cofactor in phosphorylation and dephosphorylation is not shown.

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

---