Directed evolution of an ultrastable carbonic anhydrase for highly efficient carbon capture from flue gas.

Carbonic anhydrase (CA) is one of nature's fastest enzymes and can dramatically improve the economics of carbon capture under demanding environments such as coal-fired power plants. The use of CA to accelerate carbon capture is limited by the enzyme's sensitivity to the harsh process conditions. Using directed evolution, the properties of a ß-class CA from Desulfovibrio vulgaris were dramatically enhanced. Iterative rounds of library design, library generation, and high-throughput screening identified highly stable CA variants that tolerate temperatures of up to 107 °C in the presence of 4.2 M alkaline amine solvent at pH >10.0. This increase in thermostability and alkali tolerance translates to a 4,000,000-fold improvement over the natural enzyme. At pilot scale, the evolved catalyst enhanced the rate of CO2 absorption 25-fold compared with the noncatalyzed reaction.

Biotechnology for the acceleration of carbon dioxide capture and sequestration.

The potential for enzymatic acceleration of carbon dioxide capture from combustion products of fossil fuels has been demonstrated. Carbonic anhydrase (CA) accelerates post combustion CO(2) capture, but available CAs are woefully inadequate for the harsh conditions employed in most of these processes. In this review, we summarize recent approaches to improve CA, and processes employing this enzyme, to maximize the benefit from this extremely fast biocatalyst. Approaches to overcoming limitations include sourcing CAs from thermophilic organisms, using protein engineering to evolve thermo-tolerant enzymes, immobilizing the enzyme for stabilization and confinement to cooler regions and process modifications that minimize the (thermo-, solvent) stress on the enzyme.

Biocatalytic asymmetric synthesis of chiral amines from ketones applied to sitagliptin manufacture.

Pharmaceutical synthesis can benefit greatly from the selectivity gains associated with enzymatic catalysis. Here, we report an efficient biocatalytic process to replace a recently implemented rhodium-catalyzed asymmetric enamine hydrogenation for the large-scale manufacture of the antidiabetic compound sitagliptin. Starting from an enzyme that had the catalytic machinery to perform the desired chemistry but lacked any activity toward the prositagliptin ketone, we applied a substrate walking, modeling, and mutation approach to create a transaminase with marginal activity for the synthesis of the chiral amine; this variant was then further engineered via directed evolution for practical application in a manufacturing setting. The resultant biocatalysts showed broad applicability toward the synthesis of chiral amines that previously were accessible only via resolution. This work underscores the maturation of biocatalysis to enable efficient, economical, and environmentally benign processes for the manufacture of pharmaceuticals.

Remote interactions explain the unusual regioselectivity of lipase from Pseudomonas cepacia toward the secondary hydroxyl of 2'-deoxynucleosides.

Lipase from Pseudomonas cepacia (PCL) surprisingly favors acylation of the secondary hydroxyl at the 3'-position over the primary hydroxyl at the 5'-position in 2'-deoxynucleosides by up to >98:1. Catalytically productive tetrahedral intermediate analogues for both orientations were found by molecular modeling. However, acylation of the 3'-hydroxyl places the thymine base in the alternate hydrophobic pocket of PCL's substrate-binding site where it can hydrogen bond to the side-chain hydroxyls of Tyr23 and Tyr29 and the main chain carbonyl of Leu17. Conversely, acylation of the 5'-hydroxyl leaves the thymine base in the solvent where there is no favorable binding to the enzyme. We propose that these remote stabilizing interactions between the thymine base and PCL's substrate-binding site stabilize the 3'-acylation transition state and thus account for the unusual regioselectivity.

How substrate solvation contributes to the enantioselectivity of subtilisin toward secondary alcohols.

The current rule to predict the enantiopreference of subtilisin toward secondary alcohols is based on the size of the substituents at the stereocenter and implies that the active site contains two differently sized pockets for these substituents. Several experiments are inconsistent with the current rule. First, the X-ray structures of subtilisin show there is only one pocket (the S1' pocket) approximately the size of a phenyl group to bind secondary alcohols. Second, the rule often predicts the incorrect enantiomer for reactions in water. To resolve these contradictions, we refine the current rule to show that subtilisin binds only one substituent of a secondary alcohol and leaves the other in solvent. To test this refined empirical rule, we show that the enantioselectivity of a series of secondary alcohols in water varied linearly with the difference in hydrophobicity (log P/P0) of the substituents. This hydrophobicity difference accounts for the solvation of one substituent in water.

Subtilisin-catalyzed resolution of N-acyl arylsulfinamides.

We report the first biocatalytic route to sulfinamides (R-S(O)-NH2), whose sulfur stereocenter makes them important chiral auxiliaries for the asymmetric synthesis of amines. Subtilisin E did not catalyze hydrolysis of N-acetyl or N-butanoyl arylsulfinamides, but did catalyze a highly enantioselective (E > 150 favoring the (R)-enantiomer) hydrolysis of N-chloroacetyl and N-dihydrocinnamoyl arylsulfinamides. Gram-scale resolutions using subtilisin E overexpressed in Bacillus subtilis yielded, after recrystallization, three synthetically useful auxiliaries: (R)-p-toluenesulfinamide (42% yield, 95% ee), (R)-p-chlorobenzenesulfinamide (30% yield, 97% ee), and (R)-2,4,6-trimethylbenzenesulfinamide (30% yield, 99% ee). Molecular modeling suggests that the N-chloroacetyl and N-dihydrocinnamoyl groups mimic a phenylalanine moiety and thus bind the sulfinamide to the active site. Molecular modeling further suggests that enantioselectivity stems from a favorable hydrophobic interaction between the aryl group of the fast-reacting (R)-arylsulfinamide and the S1' leaving group pocket in subtilisin E.

Mechanistic study of an improbable reaction: alkene dehydrogenation by the delta12 acetylenase of Crepis alpina.

The mechanism by which the fatty acid acetylenase of Crepis alpina catalyzes crepenynic acid ((9Z)-octadeca-9-en-12-ynoic acid) production from linoleic acid has been probed through the use of kinetic isotope effect (KIE) measurements. This was accomplished by incubating appropriate mixtures of regiospecifically deuterated isotopomers with a strain of Saccharomyces cerevisiae expressing a functional acetylenase. LC/MS analysis of crepenynic acid obtained in these experiments showed that the oxidation of linoleate occurs in two discrete steps, since the cleavage of the C12-H bond is very sensitive to isotopic substitution (k(H)/k(D) = 14.6 +/- 3.0) while a minimal isotope effect (k(H)/k(D) = 1.25 +/- 0.08) was observed for the C13-H bond breaking step. These data suggest that crepenynic acid is produced via initial H-atom abstraction at C12 of a linoleoyl substrate. The relationship between the mechanism of enzymatic acetylenation and epoxidation is discussed.

Mechanistic characterization of omega-3 desaturation in the green alga Chlorella vulgaris.

alpha-Linolenic acid (ALA, 9(Z),12(Z),15(Z)-octadecatrienoic acid) derivatives are important plant lipids which play a critical key role in cold tolerance. The final steps of ALA biosynthesis feature a series of regio- and stereoselective dehydrogenation reactions which are catalyzed by a set of enzymes known as fatty acid desaturases. In conjunction with ongoing research into the structural biology of these remarkable catalysts, we have examined the mechanism of double bond introduction at C15,16 as it occurs in a model photosynthetic organism, Chlorella vulgaris. The individual deuterium kinetic isotope effects associated with the C-H bond cleavages at C-15 and C-16 of a thialinoleoyl analogue were measured via competition experiments using appropriately deuterium-labelled 7-thia substrates. A large kinetic isotope effect (KIE) (k(H)/k(D)=10.2+/-2.8) was observed for the C-H bond-breaking step at C-15 while the C-H bond cleavage at C-16 was found to be relatively insensitive to deuterium substitution (k(H)/k(D)=0.8+/-0.2). These results point to C-15 as the site of initial oxidation in omega-3 desaturation and imply that the Chlorella and corresponding plant systems share a common active site architecture.

Mechanism of 1,4-dehydrogenation catalyzed by a fatty acid (1,4)-desaturase of Calendula officinalis.

The mechanism by which the fatty acid (1,4)-desaturase of Calendula officinalis produces calendic acid from linoleic acid has been probed through the use of kinetic isotope effect (KIE) measurements. This was accomplished by incubating appropriate mixtures of linoleate and regiospecifically dideuterated isotopomers with a strain of Saccharomyces cerevisiae expressing a functional (1,4)-desaturase. GC-MS analysis of methyl calendate obtained in these experiments showed that the oxidation of linoleate occurs in two discrete steps since the cleavage of the C11-H bond is very sensitive to isotopic substitution (kH/kD = 5.7 +/- 1.0) while no isotope effect (kH/kD = 1.0 +/- 0.1) was observed for the C8-H bond breaking step. These data indicate that calendic acid is produced via initial H-atom abstraction at C11 of a linoleoyl substrate and supports the hypothesis that this transformation represents a regiochemical variation of the more common C12-initiated Delta12 desaturation process.

Exploring the hydroxylation-dehydrogenation connection: novel catalytic activity of castor stearoyl-ACP Delta(9) desaturase.

The novel product profile obtained by incubating chiral fluorinated substrate analogues with castor stearoyl-ACP Delta(9) desaturase has been rationalized through a series of labeling studies. It was found that the introduction of the Z-double bond between C-9 and C-10 of the parent substrate occurs with pro-R enantioselectivity--a result that accounts for the observed stereochemistry of oxidation products derived from (9R)- and (9S)-9-fluorostearoyl-ACP. Oxidation of (9R)-9-fluorostearoyl-ACP occurs via at least two rapidly interchanging substrate conformations in the active site as detected by reaction pathway branching induced by deuteration at C-10 and C-11. Hydroxylation and desaturation of this substrate share the same site of initial oxidative attack.