Metal enzymes, thermostability

Kim and Park subsequently reported that ruthenium pre-catalyst 2 racemizes alcohols within 30 min at room temperature [53]. However, when combined with an enzyme (lipase) in DKR at room temperature, very long reaction times (1.3 to 7 days) were required, in spite of the fact that the enzymatic KR takes only a few hours (Scheme 5.24). Despite these compatibility problems, their results constituted an important improvement, since chemoenzymatic DKR could now be performed at ambient temperature to give high yields, which enables non-thermostable enzymes to be used. More recently, we communicated a highly efficient metal- and enzyme-catalyzed DKR of alcohols at room temperature (Scheme 5.24) [40, 54]. This is the fastest DKR of alcohols hitherto reported by the combination of transition metal and enzyme catalysts. Racemization was effected by a new class of very... 

Naturally occurring Upases are (R)-selective for alcohols according to Kazlauskas rule [58, 59]. Thus, DKR of alcohols employing lipases can only be used to transform the racemic alcohol into the (R)-acetate. Serine proteases, a sub-class of hydrolases, are known to catalyze transesterifications similar to those catalyzed by lipases, but, interestingly, often with reversed enantioselectivity. Proteases are less thermostable enzymes, and for this reason only metal complexes that racemize secondary alcohols at ambient temperature can be employed for efficient (S)-selective DKR of sec-alcohols. Ruthenium complexes 2 and 3 have been combined with subtilisin Carlsberg, affording a method for the synthesis of... 

The earlier investigations employed several different types of plexiglas constructions containing the immobilized enzyme column. These devices were thermostated in a water bath, and the temperature at the point of exit from the column was monitored with a thermistor connected to a commercial Wheatstone bridge. The latter was constructed for general temperature measurements and osmometry. Later, we developed more sensitive instruments for temperature monitoring indigenously the water bath was replaced by a carefully temperature-controlled metal block, which contained the enzyme column. The enzyme thermistor concept has been patented in several major countries. 

Analysis of NROR by plasma emission spectroscopy reveals that the enzyme does not contain any metals in significant amounts (>0.1 g-atom/mol) and flavin appears to be its only prosthetic group. The enzyme is quite thermostable, with the time required for a 50% loss in catalytic activity when it is incubated at 80° and 95° of 12 and 1.6 hr, respectively. These values are determined by maintaining the purified enzyme (0.4 mg/ml) in 60 mM EPPS, pH 8.0) in serum-stoppered glass vilas at the desired temperature and periodically removing samples to determine residual BVNOR activity at 80°. 

A widely-reported method for the DKR of secondary alcohols and a- and p-hydroxy acid esters involves ruthenium catalysed hydrogenation. No additional base is required as a cocatalyst (and consequently base-catalysed transesterification can be avoided) because one of the ligand s oxygen atoms can act as a basic centre. A robust ruthenium complex (named Shvo s catalyst) along with a p-chlorophenylacetate was developed by the BackvaU group. The metal catalyst must be used in combination with thermostable enzymes because it is activated by heat (Scheme 4.26). This system (with CALB) has been successfully used for the DKR of many secondary alcohols and diols (Scheme 4.27) [52, 63, 64]. 

The a-amylase (mol. wt. 5.0 x 10 pi 9.2) from a thermophilic bacterium (found in a hot spring in Japan) is more thermostable than a-amylases from mesophilic sources. It contains two cysteine residues, but no disulphide linkages, and 20% of a-helix. The effects of pH, metal ions, and denaturing agents on the thermal stability of the enzyme were examined. It is most stable at its isoelectric point and is stabilized by calcium ions one of the bound Ca ions could not be removed by electrodialysis unless the enzyme was first denatured. 

Isocitrate dehydrogenases have been studied in extreme thermophiles mainly from the point of view of elucidation of mechanisms of thermostability. This has included the studies of Hibino et al. [32] on the enzyme from a Bacillus stearothermophilus strain grown at 65 °C. In the presence of substrate the enzyme was markedly stabilized as judged by retention of tertiary conformation, reflected by the dramatically improved resistance to denaturation by urea. The thermostability of the partially purified isodtrate dehydrogenase from Thermus aquaticus was shown to be dependent upon the buffer used and on the addition of isodtrate [135]. This enzyme has also been purified from Thermus thermophilus HB8 [136]. The enzyme had a dimeric structure with identical 57.5 kDa subunits as judged by SDS-polyacrylamide gel electrophoresis, whilst gel filtration of the native enzyme on two different matrices gave values of 95 kDa and 120 kDa. The enzyme had a pH optimum of 7.8, required a divalent metal ion for activity (Mn " > Mg " ) and was NADP dependent, although NAD" was shown to replace NADP" with low efficiency. 

On the other hand, the racemization of amines is more difficult compared to that of alcohols. Several metal systems based on palladium (Pd), Ru, nickel (Ni), cobalt (Co), and Ir have been employed as the racemization catalysts. Pd-based catalysts include Pd/C, Pd/BaSO, and Pd/A10(0H). They are readily available but require higher temperatures for satisfactory racemization. So they should be coupled with thermostable enzymes such as Novozym 435 for the successful DKR. A possible mechanism for the Pd-catalyzed racemization of amine is described in Scheme 5.5. The racemization occurs via reversible dehydrogenation/hydrogena-tion steps including an imine intermediate. The imine intermediate can react with starting material to afford a secondary amine as the byproduct. The deamination of substrate and byproduct are also possible at elevated temperature. In case the... 

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