Reaction homodimeric

Firefly. Firefly luciferase (EC 1.13.12.7) is a homodimeric enzyme (62 kDa subunit) that has binding sites for firefly luciferin and Mg ATP . Amino acid sequence analysis has iadicated that beetle luciferases evolved from coen2yme A synthetase (206). Firefly bioluminescence is the most efficient bioluminescent reaction known, with Qc reported to be 88% (5), and at 562 nm (56). At low pH and ia the presence of certain metal ions (eg, Pb ", ... 

Cluster Fx was also identified via its EPR spectral features in the RCI photosystem from green sulfur bacteria 31, 32) and the cluster binding motif was subsequently found in the gene sequence 34 ) of the (single) subunit of the homodimeric reaction center core (for a review, see 54, 55)). Whereas the same sequence motif is present in the RCI from heliobacteria (50), no EPR evidence for the presence of an iron-sulfur cluster related to Fx has been obtained. There are, however, indications from time-resolved optical spectroscopy for the involvement of an Fx-type center in electron transfer through the heliobacterial RC 56). 

TKase is a homodimeric protein with a subunit of about 70kDa. The X-ray structures of TKase of E. colif S. cerevisiaeX Leishmania mexicana and mize have been solved. In addition, the crystal structures of a number of site-directed mutants have been determined. Schneider and co-workers have reported a series of studies in which they have mutated important residues of active site of TKase to elucidate the reaction mechanism and explain the origin of the stereospecificity of the C—C bond-forming process (Table The conserved... 

Xanthine oxidoreductase (XOR) is a molybdenum-containing complex homodimeric 300-kDa cytosolic enzyme. Each subunit contains a molybdopterin cofactor, two nonidentical iron-sulfur centers, and FAD (89). The enzyme has an important physiologic role in the oxidative metabolism of purines, e.g., it catalyzes the sequence of reactions that convert hypoxanthine to xanthine then to uric acid (Fig. 4.36). 

In 1943, more than a century after the initial report, Ukai et al. showed that thia-zolium salts such as 7 and 8 catalyze the homodimerization of aldehydes in the presence of base [2], This discovery was paramount because, while cyanide ions are inherently achiral, thiazolium salts can be modified to act as a source of chirality to render the reaction enantioselective. 

In an effort to circumvent a homodimerization event acyl silanes have been used to promote a cross-benzoin reaction. Initial reports by Johnson and co-workers employed potassium cyanide to catalyze the regiospecific cross silyl benzoin reaction to afford a single regioisomer in good yield (Eq. 2) [45 7]. 

The preparation of a trisulfide bridge between two different linear peptides is outlined in Scheme 3. In a first step, the free thiol of one peptide forms an activated mixed disulfide by reaction with TV,A1 -1hiobisphthalimide, and upon the subsequent addition of the second cysteine peptide, nucleophilic displacement of the phthalimide moiety by the free thiol yields the desired interchain trisulfide bridged peptide 3.1 3 Homodimeric analogues, e.g. 4, are prepared similarly.[13]... 

Tropf, S. et al., Reaction mechanisms of homodimeric plant polyketide synthases (stilbenes and chalcone synthase) a single active site for the condensing reaction is sufficient for synthesis of stilbenes, chalcones, and 6 -deoxychalcones. J. Biol. Chem., 270, 7922, 1995. 

Understanding the relative rates of both productive heterocoupling and homodimerization reactions allows for the judicious selection of cross-partners that can participate in a highly selective CM reaction, even when equal stoichiometries of reactants are employed. There are five relevant equilibria and 10 rate constants in CM (Scheme 4 the rate constants for olefin E/Z isomerization have been excluded for simplicity). In the simple scenario where all the rates are similar, and the reaction can achieve equilibrium, the expected statistical cross-product yield is 50%. If, however, one olefin (e.g., R CH=CH2), as a consequence of either steric or electronic factors, reacts at a slower rate k- ) than the other reactions, such that k, k/ k-, and it is assumed that the productive cross-... 

As illustrated above, various possible alkylidene intermediates and numerous primary and secondary pathways are involved in olefin CM. To simplify selective reaction design, an empirical product selectivity model was recently developed by Grubbs and co-workers, in which some degree of orthogonality amongst olefin cross-partners was established by categorizing the relative capacity of olefins to homodimerize in the presence of a given metathesis catalyst. ... 

Olefins can be divided into four categories on the basis of their propensity to homodimerize (Figure 2). Type I olefins are able to undergo rapid homodimerization and whose homodimers can equally participate in CM. A CM reaction between two olefins of this type will generally result in a statistical product mixture. Type II olefins homodimerize slowly, and, unlike type I olefins, their homodimers can only be consumed with difficulty in subsequent metathesis reactions. Type III olefins are unable to undergo homodimerization, but have the capacity to undergo CM with either type I or II olefins. As with type I olefins, the reaction between either two type II or type III olefins should result in non-selective CM. Type IV olefins are inert to olefin CM, but do not inhibit the reaction therefore, they can be regarded as spectators to CM. 

Despite the use of protecting group strategies, compounds containing amino moieties are often plagued with poor reactivities in CM. For instance, the CM reactions of unsubstituted allylic carbamates typically proceed in low yields (Scheme The predominant side-products obtained in these reactions are alkene homodimerization and... 

The practical value of this type of reaction is restricted to symmetrical allenes with vastly different reactivities. Since the less reactive allene has to be present in excess or otherwise homodimerization competes, this allene has to be readily available in large quantities. An example of such a mixed dimerization is the cycloaddition of allene (2) with cyclonona-l,2-di-ene (1). ... 

Allene ketene cycloadditions are of greater synthetic utility than cither mixed allene dimerization or mixed ketene dimerization. In this class of reaction the ketene is the more reactive species and homodimerization of ketene can be minimized by use of excess allene. Such cycloadditions always result in 2-alkylidenecyclobutanones with the sp carbons of both moieties forming the initial bond. In substituted allenes and ketenes, mixtures of stereoisomers of 2-alkylidenecyclobutanones are obtained with very little stereoselectivity, the stereoisomers arise from cisUrcins isomerism in the cyclobutane ring and EjZ isomerism of the exocyclic double bond. In unsymmetrically substituted allenes some regiochemical preference for ketene cycloaddition is observed. Examples of dimethylketene allene cycloadditions are summarized in Table 1,2... 

The cyclodimerization of ketenes would in principle give cyclobutane-1,3-diones. The problem with this class of reactions is the formation of homodimers in instances when a particular ketene is too reactive or when two ketenes possess similar stabilities. The cycloaddition of mcthylphenylketene and isopropylphenylkctenc is a case in point of cyclodimcrization of similarly reactive ketenes giving a statistical distribution of cross-dimerization and homodimerization products.14... 

As discussed in Section 6.9 1, 3-dienes and dienophiles in which multiple bonds are not activated by electron-withdrawing or electron-releasing substituents fail to undergo cycloaddition except under the most severe conditions. Particular difficulty is encountered in the cycloaddition of two unactivated species since homodimerization can be a competitive and dominant reaction pathway. The use of transition-metal catalysts, however, has proved to be a valuable solution. Complexation of unactivated substrates to such catalysts promotes both inter- and intramolecular cycloadditions. Consequently, the cycloaddition of such unactivated compounds, that is, simple unsubstituted dienes and alkenes, catalyzed by transition metals is a major, important area of study.655 In addition, theoretical problems of the transformation have frequently been addressed in the more recent literature. 

The polyketide synthases responsible for chain extension of cinnamoyl-CoA starter units leading to flavonoids and stilbenes, and of anthraniloyl-CoA leading to quinoline and acridine alkaloids (see page 377) do not fall into either of the above categories and have now been termed Type TTT PKSs. These enzymes differ from the other examples in that they are homodimeric proteins, they utilize coenzyme A esters rather than acyl carrier proteins, and they employ a single active site to perform a series of decarboxylation, condensation, cyclization, and aromatization reactions. 

Tropf S, Karcher B, Schroder G, Schroder J (1995) Reaction mechanisms of homodimeric plant polyketide syntahses (stilbene synthase and chalcone synthase). J Biol Chem 270 7922-7928... 

This reaction is accomplished by several types of iron ribonucleoside reductase (RNR) [1,5,7,13,118], One of the best-characterized RNRs (from E. coli) contains two homodimeric protein components, R1 and R2. The R2 protein comprises an oxygen bridged dinuclear Fe(III) in its oxidized form [119], All RNRs promote the formation of a stable organic radical, which, eventually, leads to the abstraction of a hydrogen atom from the ribose. In the case of E. coli RNR, the latter is accomplished by the R1 protein, specifically by a cysteinyl residue a redox active cystine, also part of Rl, provides the required reducing equivalents (Figure 25). 

ADMET is a step growth polymerization in which all double bonds present can react in secondary metathesis events. However, olefin metathesis can be performed in a very selective manner by correct choice of the olefinic partner, and thus, the ADMET of a,co-dienes containing two different olefins (one of which has low homodimerization tendency) can lead to a head-to-tail ADMET polymerization. In this regard, terminal double bonds have been classified as Type I olefins (fast homodimerization) and acrylates as Type II (unlikely homodimerization), and it has been shown that CM reactions between Types I and II olefins take place with high CM selectivity [142], This has been applied in the ADMET of a monomer derived from 10-undecenol containing an acrylate and a terminal double bond (undec-10-en-l-yl acrylate) [143]. Thus, the ADMET of undec-10-en-l-yl acrylate in the presence of 0.5 mol% of C5 at 40°C provided a polymer with 97% of CM selectivity. The high selectivity of this reaction was used for the synthesis of block copolymers and star-shaped polymers using mono- and multifunctional acrylates as selective chain stoppers. 

A possible side reaction that can take place is the homodimerization of ketyl 58 to form benzopinacol 63. However, due to the relatively big difference in reaction rates of hydrogen abstraction and recombination, normally only a very small amount of this is formed. 

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