The whole-cell biocatalyst displaying Candida antarctica lipase B (CALB) on the yeast cell surface with alpha-agglutinin as the anchor protein was easy to handle and possessed high stability. The lyophilized CALB-displaying yeasts showed their original hydrolytic activity and were applied to an ester synthesis using ethanol and L: -lactic acid as substrates. In water-saturated heptane, CALB-displaying yeasts catalyzed ethyl lactate synthesis. The synthesis efficiency increased depending on temperature and reached approximately 74% at 50 degrees C. The amount of L: -ethyl lactate increased gradually. L: -Ethyl lactate synthesis stopped at 200 h and restarted after adding of L: -lactic acid at 253 h. It indicated that CALB-displaying yeasts retained their synthetic activity under such reaction conditions. In addition, CALB-displaying yeasts were able to recognize L: -lactic acid and D: -lactic acid as substrates. L: -Ethyl lactate was prepared from L: -lactic acid and D: -ethyl lactate was prepared from D: -lactic acid using the same CALB-displaying whole-cell biocatalyst. These findings suggest that CALB-displaying yeasts can supply the enantiomeric lactic esters for preparation of useful and improved biopolymers of lactic acid.
        
Title: Enzymatic synthesis of galactosyl lactic ethyl ester and its polymer for use as biomaterials Jia H, Wang P Ref: J Biotechnol, 132:314, 2007 : PubMed
Lactate-based chemicals and polymers including poly(lactic acid) (PLA) are highly valuable materials for biomedical, food and general-purpose applications. Chemical synthesis, albeit the high reaction velocities achieved with it, often leaves chemical residues that are subject to health and safety concerns. Alternative biosynthesis is preferred in order to overcome these problems. Herein we report a novel enzymatic synthesis for the preparation of beta-d-galactosyl-l-lactic acid ethyl ester (GLAEE). Such a product, which may find applications in food and personal care products, is generally difficult to synthesize via traditional chemical routes because the reactions have to be highly selective due to the multiple hydroxyl groups of sugars. We further explore the enzymatic polymerization of GLAEE to form a unique biopolymer, poly(beta-d-galactoside-co-l-lactic acid) (PGLA). Novozyme 435 was found efficient in catalyzing the polymerization reaction in acetone with a conversion yield of 60% within 100 h. The molecular weight of the polymer product ranged from about 800-2000 as analyzed by using ESI-MS. It is expected that a variety of sugar-hydroxyl acids copolymers can be prepared through the same approach and a new class of biomaterials can thus be developed.
        
Title: Inhibition of pig liver esterase by trifluoromethyl ketones: modulators of the catalytic reaction alter inhibition kinetics Allen KN, Abeles RH Ref: Biochemistry, 28:135, 1989 : PubMed
The kinetics of substrate hydrolysis by pig liver esterase show activation by various substrates as well as activation by organic solvents (both Vmax and Km increase) [Barker, D.L., & Jencks, W.P. (1969) Biochemistry 8, 3890]. The trifluoromethyl ketones 1,1,1-trifluoro-4-phenylbutan-2-one (TPB) and 1,1,1-trifluoro-4-(p-hydroxyphenyl)butan-2-one (OH-TPB) are slow, tight binding inhibitors of pig liver esterase with Ki values of 6.8 X 10(-9) M and 6.0 X 10(-9) M, respectively. Acetonitrile, TPB, and OH-TPB as well as the substrates pNPA and ethyl lactate caused a 15-130-fold increase in the rate of association (kon), and dissociation (koff), of the enzyme--TPB complex. The value of Ki (koff/kon) did not change. The effect cannot be attributed to half-sites reactivity since an increase in koff of OH-TPB is also observed with enzyme monomers. The results are consistent with a model proposed for the catalytic reaction (Barker & Jencks, 1969) which invokes two binding sites on each esterase subunit, a catalytic site and an effector site. Occupation of the effector site can increase koff and kon for the inhibitors TPB and OH-TPB. Not all compounds which bind at the effector site increase koff. Butanol binds at the effector site but does not effect koff of TPB. The results also indicate that an aromatic or a hydrophobic structure and a carbonyl group are required for optimal interaction with the effector site.
Esterases receive special attention because their wide distribution in biological systems and environments and their importance for physiology and chemical synthesis. The prediction of esterases substrate promiscuity level from sequence data and the molecular reasons why certain such enzymes are more promiscuous than others, remain to be elucidated. This limits the surveillance of the sequence space for esterases potentially leading to new versatile biocatalysts and new insights into their role in cellular function. Here we performed an extensive analysis of the substrate spectra of 145 phylogenetically and environmentally diverse microbial esterases, when tested with 96 diverse esters. We determined the primary factors shaping their substrate range by analyzing substrate range patterns in combination with structural analysis and protein-ligand simulations. We found a structural parameter that helps ranking (classifying) promiscuity level of esterases from sequence data at 94% accuracy. This parameter, the active site effective volume, exemplifies the topology of the catalytic environment by measuring the active site cavity volume corrected by the relative solvent accessible surface area (SASA) of the catalytic triad. Sequences encoding esterases with active site effective volumes (cavity volume/SASA) above a threshold show greater substrate spectra, which can be further extended in combination with phylogenetic data. This measure provides also a valuable tool for interrogating substrates capable of being converted. This measure, found to be transferred to phosphatases of the haloalkanoic acid dehalogenase superfamily and possibly other enzymatic systems, represents a powerful tool for low-cost bioprospecting for esterases with broad substrate ranges, in large scale sequence datasets.
        
Title: Efficient synthesis of enantiomeric ethyl lactate by Candida antarctica lipase B (CALB)-displaying yeasts Inaba C, Maekawa K, Morisaka H, Kuroda K, Ueda M Ref: Applied Microbiology & Biotechnology, 83:859, 2009 : PubMed
The whole-cell biocatalyst displaying Candida antarctica lipase B (CALB) on the yeast cell surface with alpha-agglutinin as the anchor protein was easy to handle and possessed high stability. The lyophilized CALB-displaying yeasts showed their original hydrolytic activity and were applied to an ester synthesis using ethanol and L: -lactic acid as substrates. In water-saturated heptane, CALB-displaying yeasts catalyzed ethyl lactate synthesis. The synthesis efficiency increased depending on temperature and reached approximately 74% at 50 degrees C. The amount of L: -ethyl lactate increased gradually. L: -Ethyl lactate synthesis stopped at 200 h and restarted after adding of L: -lactic acid at 253 h. It indicated that CALB-displaying yeasts retained their synthetic activity under such reaction conditions. In addition, CALB-displaying yeasts were able to recognize L: -lactic acid and D: -lactic acid as substrates. L: -Ethyl lactate was prepared from L: -lactic acid and D: -ethyl lactate was prepared from D: -lactic acid using the same CALB-displaying whole-cell biocatalyst. These findings suggest that CALB-displaying yeasts can supply the enantiomeric lactic esters for preparation of useful and improved biopolymers of lactic acid.
        
Title: Enzymatic synthesis of galactosyl lactic ethyl ester and its polymer for use as biomaterials Jia H, Wang P Ref: J Biotechnol, 132:314, 2007 : PubMed
Lactate-based chemicals and polymers including poly(lactic acid) (PLA) are highly valuable materials for biomedical, food and general-purpose applications. Chemical synthesis, albeit the high reaction velocities achieved with it, often leaves chemical residues that are subject to health and safety concerns. Alternative biosynthesis is preferred in order to overcome these problems. Herein we report a novel enzymatic synthesis for the preparation of beta-d-galactosyl-l-lactic acid ethyl ester (GLAEE). Such a product, which may find applications in food and personal care products, is generally difficult to synthesize via traditional chemical routes because the reactions have to be highly selective due to the multiple hydroxyl groups of sugars. We further explore the enzymatic polymerization of GLAEE to form a unique biopolymer, poly(beta-d-galactoside-co-l-lactic acid) (PGLA). Novozyme 435 was found efficient in catalyzing the polymerization reaction in acetone with a conversion yield of 60% within 100 h. The molecular weight of the polymer product ranged from about 800-2000 as analyzed by using ESI-MS. It is expected that a variety of sugar-hydroxyl acids copolymers can be prepared through the same approach and a new class of biomaterials can thus be developed.
        
Title: Inhibition of pig liver esterase by trifluoromethyl ketones: modulators of the catalytic reaction alter inhibition kinetics Allen KN, Abeles RH Ref: Biochemistry, 28:135, 1989 : PubMed
The kinetics of substrate hydrolysis by pig liver esterase show activation by various substrates as well as activation by organic solvents (both Vmax and Km increase) [Barker, D.L., & Jencks, W.P. (1969) Biochemistry 8, 3890]. The trifluoromethyl ketones 1,1,1-trifluoro-4-phenylbutan-2-one (TPB) and 1,1,1-trifluoro-4-(p-hydroxyphenyl)butan-2-one (OH-TPB) are slow, tight binding inhibitors of pig liver esterase with Ki values of 6.8 X 10(-9) M and 6.0 X 10(-9) M, respectively. Acetonitrile, TPB, and OH-TPB as well as the substrates pNPA and ethyl lactate caused a 15-130-fold increase in the rate of association (kon), and dissociation (koff), of the enzyme--TPB complex. The value of Ki (koff/kon) did not change. The effect cannot be attributed to half-sites reactivity since an increase in koff of OH-TPB is also observed with enzyme monomers. The results are consistent with a model proposed for the catalytic reaction (Barker & Jencks, 1969) which invokes two binding sites on each esterase subunit, a catalytic site and an effector site. Occupation of the effector site can increase koff and kon for the inhibitors TPB and OH-TPB. Not all compounds which bind at the effector site increase koff. Butanol binds at the effector site but does not effect koff of TPB. The results also indicate that an aromatic or a hydrophobic structure and a carbonyl group are required for optimal interaction with the effector site.