Feature 29-1 Enzymes

Some toxic effects of heavy metals pertain to a change of amino acid contents in urine. It is established that accumulation of metals in hair and nails and secretions through urine and saliva are definitely correlated. It was obviously due to enzyme features as well as the level of environmental pollution. 

Compared with other kinds of catalysts, for example homogeneous catalysts, in which ligands are responsible for specificity, and heterogeneous catalysts, in which catalyti-cally active centers are attached to solid carriers such as zeolites or metal oxides, enzymes feature the advantages and disadvantages listed in Table 1.1. 

Benzoylformate decarboxylase (BFD), a TPP-dependent enzyme, features a high degree of sequence similarity with a whole family of other TPP-dependent decarboxylases, among them pyruvate decarboxylase (PDC), one of the most important enzymes in metabolism. 

The PPIase domains of cyclophilin and FKBPs do not display any similarities whereas members within each subfamily show a high degree of homology in sequence and in three-dimensional structure, suggesting that conservation of the overall shape of the active site and of certain residues is essential for PPIase activity. Moreover, both enzymes feature structural differences from parvulins, in particular Pinl (Fig. 12.2) [1,9,12]. 

Table 8.1 Summary of kinetic constants for the enzymes featured in Section 8.1 of Chapter 8. ALL the data are obtained from the enzyme database - BRENDA (http //www.brenda-enzymes.info/) except where indicated (a) vaLue of k n/Kn estimated assuming aLL substrate added in buffer is avaiLabLe to enzyme i.e., has not been sequestered as acetaL or ketaL derivatives, as appropriate) (b) kcat data from FieLds et aL, 2006 (c) /feat data from rat source (d) kinetic data from Masaki et ai, 2001 (e) /Ccat data from Day and Shaw, 1992 (f) kinetic data from bovine source (g) kinetic data from human source (h) aLL kinetic data from Wright et ai, 2006. Abbreviations GAP 3-gLyceraLdehyde 3-phosphate DHAP dihydroxyacetone phosphate OAA oxaLoacetate NADH reduced nicotinamide adenine dinucLeotide chloramp chLoramphenicoL thio-ACh thio acetyLchoLine. Roman numeraL denote compounds iLLustrated in the set of structures accompanying this tabLe. 

The N-terminal sequence of ArrA suggests that Arr is a member of the DMSO reductase family. These enzymes feature molybdenum coordinated by two bidentate dithiolene ligands contributed by two equivalents of the molybdopt-erin cofactor (16,18). 

There are two possible bisubstrate systems that combine the enzyme feature of the Ping Pong sequence with the hit-and-mn feature of the Theorell-Chance mechanism. These are in fact the hmiting cases of the common Ping Pong Bi Bi system, in which one of two central complexes has extremely short life. The reaction sequences are shown below ... 

Enzymes form products from substrates with high selectivity and efficiency at ambient temperatures. Enzymes feature well-defined binding pockets that bind substrates during reactions using non-covalent interactions. The behavior is analogous to a lock and key systan in which the substrate (key) has a complementary size and shape to the binding site (lock). Enzymes selectively promote the formation of the transition state and intermediates of a particular reaction. 

Supramolecular catalysts that mimic the ability of processive enzymes feature macrocycles with metal porphyrins as the catalyticaUy active sites. Warmnark and coworkers reported a dynamic snpramolecular combination of a Mn (III) salen complex and a Zn(II) porphyrin through hydrogen bonding. The supramolecular complex has a binding cavity halfway between the Mn (III) salen and the Zn(II) porphyrin. The Mn (III) salen unit functions as the catalytic center for the epoxidation of olefin derivatives. However, reaction is not limited to substrates included within the cavity. Excluded substrates are also epoxidized by the supramolecular complex (Figure 12.11).5758... 

The cyclodextrin cage holds the ester while the metal ion positions other groups for attack. If cyclohexanol is added in the solution, it competes with the substrate and the efficiency of the system falls to 60%. Most of the catalytic power of the system is attributed to the binding of the substrate by the cyclodextrin moiety of the complex. In the absence of cyclodextrin, the rate enhancement is 350-fold. Thus, by combining the properties of cyclodextrin with those of a metal ion, a much more efficient catalytic system can be obtained that mimics enzyme features. 

Uses Defoamer for the mfg. of citric acid, amino acids, enzymes Features Increased yield exc. foam control Reguiatory 21 CFR 173.340(3) kosher pareve Properties Cl. liq. sp.gr. 0.998 vise. 400 cps... 

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