Photobiocatalyzed asymmetric reduction of ketones using Chlorella sp. MK201.

Aromatic ketones were reduced using suspension culture of Chlorella sp. MK201 under fluorescent light illumination producing the corresponding chiral alcohols in high yields with excellent enantiomeric excess (ee). For example, 2',3',4',5',6'-pentafluoroacetophenone at 0.25 mg/ml was converted to the corresponding (S)-alcohol in 80 % yield with >99 % ee by 1 mg dry wt of Chlorella/ml in 12 h illumination (2,000 lux).

Mutagenic potential of DNA glycation: miscoding by (R)- and (S)-N2-(1-carboxyethyl)-2'-deoxyguanosine.

Elevated circulating glucose resulting from complications of obesity and metabolic disease can result in the accumulation of advanced glycation end products (AGEs) of proteins, lipids, and DNA. The formation of DNA-AGEs assumes particular importance as these adducts may contribute to genetic instability and elevated cancer risk associated with metabolic disease. The principal DNA-AGE, N(2)-(1-carboxyethyl)-2'-deoxyguanosine (CEdG), is formed as a mixture of R and S isomers at both the polymer and monomer levels. In order to examine the miscoding potential of this adduct, oligonucleotides substituted with (R)- and (S)-CEdG and the corresponding triphosphates (R)- and (S)-CEdGTP were synthesized, and base-pairing preferences for each stereoisomer were examined using steady-state kinetic approaches. Purine dNTPs were preferentially incorporated opposite template CEdG when either the Klenow (Kf(-)) or Thermus aquaticus (Taq) polymerases were used. The Kf(-) polymerase preferentially incorporated dGTP, whereas Taq demonstrated a bias for dATP. Kf(-) incorporated purines opposite the R isomer with greater efficiency, but Taq favored the S isomer. Incorporation of (R)- and (S)-CEdGTP only occurred opposite dC and was catalyzed by Kf(-) with equal efficiencies. Primer extension from a 3'-terminal CEdG was observed only for the R isomer. These data suggest CEdG is the likely adduct responsible for the observed pattern of G transversions induced by exposure to elevated glucose or its alpha-oxoaldehyde decomposition product methylglyoxal. The results imply that CEdG within template DNA and the corresponding triphosphate possess different syn/anti conformations during replication which influence base-pairing preferences. The implications for CEdG-induced mutagenesis in vivo are discussed.

Light mediated cofactor recycling system in biocatalytic asymmetric reduction of ketone.

Reduction of an artificial ketone by Synechococcus elongatus PCC 7942 proceeds smoothly by the aid of light. The efficiency of the reaction is very high since the coenzyme NADPH is regenerated by using light energy.

Reduction of exogenous ketones depends upon NADPH generated photosynthetically in cells of the cyanobacterium Synechococcus PCC 7942.

Effective utilization of photosynthetic microorganisms as potential biocatalysts is favorable for the production of useful biomaterials and the reduction of atmospheric CO2. For example, biocatalytic transformations are used in the synthesis of optically active alcohols. We previously found that ketone reduction in cells of the cyanobacterium Synechococcus PCC 7942 is highly enantioselective and remarkably enhanced under light illumination. In this study, the mechanism of light-enhanced ketone reduction was investigated in detail using several inhibitors of photosynthetic electron transport and of enzymes of the Calvin cycle. It is demonstrated that light intensity and photosynthesis inhibitors significantly affect the ketone reduction activity in Synechococcus. This indicates that the reduction correlates well with photosynthetic activity. Moreover, ketone reduction in Synechococcus specifically depends upon NADPH and not NADH. These results also suggest that cyanobacteria have the potential to be utilized as biocatalytic systems for direct usage of light energy in various applications such as syntheses of useful compounds and remediation of environmental pollutants.

Nucleotide sequence context influences HIV replication fidelity by modulating reverse transcriptase binding and product release.

An RNA template/DNA primer (T/P) complex derived from the env gene of HIV-1 was used to examine the kinetic effects of specific basepair substitutions on dNTP incorporation and RNase H cleavage by HIV-1 reverse transcriptase (RT). Single basepair substitutions 2 or 6 nucleotides upstream from a defined polymerization site (denoted -2 and -6) were engineered by oligonucleotide synthesis to provide 7 T/P substrates for kinetic analysis. A -6 A/T substitution in the wild type sequence resulted in 14- and 7-fold increases in the apparent second order rate constants (k(2app)) for U/A and U/G basepair formation. The k(2app) for U/A formation was relatively unchanged for all other T/P basepair changes. The -6A/T substitution also uniquely lowered the RNase H cleavage rate by 3-fold. Combined kinetic and thermodynamic analyses indicated that these effects were due almost exclusively to increases in the KD (k(off)/k(on)) of initial binding of RT to the T/P and the rate of product release. The data suggest that certain sequence contexts may influence RT fidelity by modulating enzyme binding/dissociation rather than by altering dNTP binding affinity or the rate of the bond forming step.