Enzymes In Organic Chemistry
Enzymes in OrganicChemistry!March 25th , 2013!Enzymes Enzymes are biological catalysts. They increasethe rate at which equilibrium is reached, butthey do not affect the equilibrium. Enzymes differ from ordinary chemical catalysts: Enzyme catalyzed reaction rates are typically 106 to1012 greater than those of the uncatalyzed reactions. Enzyme catalyzed reactions occur under relatively mildconditions (physiological conditions). Enzymes often have tremendous specificity withregards to both their substrates and products. The activity of enzymes can be regulated bycompounds other than their substrates and products.!
Classes of EnzymesHydrolases and oxidoreductases are of particular interest. !Enzymes in Organic Solvents Enzymes have evolved tofunction in an aqueousenvironment. This has hampered their usein organic synthesis. Over the last 15 years it hasbeen found that enzymescan also function innonaqueous environments. Enzyme properties areaffected by nonaqueousconditions and may differsignificantly from thosenormally associated withthe enzyme in an nicPhaseInsoluble EnzymeSoluble EnzymeSurfactant
Enzymes in Organic Solvents Many enzymes demonstrateincreased thermal stability inorganic media. Lipase in nonaqueousconditions has beendetermined to be stable at100 C for several hours. Thisis not true for the enzyme inaqueous solution It also demonstrated higheractivity at 20 C than lipasein aqueous solution. This enhanced stabilitydecreases as the watercontent nsoluble EnzymeSoluble EnzymeSurfactantEnzymes in Organic SolventsVarious approaches for usingenzymes in organic solvents:A. Enzyme solvated in the normalaqueous environment.B. Enzymes in a low-water solventsystem. Enzyme is suspended inan organic solvent.C. A microemulsion where theenzyme is encapsulated withina reverse micelle.D. Monophasic cosolvent systemwhere the enzyme is dissolvedin a mixture of water and watermiscible organic solvents.E. A biphasic organic-aqueoussystem where the enzymeresides in the aqueous uble EnzymeSoluble EnzymeSurfactant
Enzymes in a Low-Water Solvent System In organic media, enzymes requirewater for catalytic activity. In general, enzymatic catalysisimproves with increased hydration(refering to water associated withprotein, not water content of systemas a whole). The degree of hydration requireddepends on the enzyme and theorganic solvent. Lipases appear to require very fewwater molecules in order to be active. Some enzymes (such as chymotrypsin)are active when associated with 50 orfewer water molecules. Others (such as alcohol dehydrogenase)require sufficient water molecules for amonolayer to form.AqueousPhaseOrganicPhaseInsoluble EnzymeEnzymes in a Low-Water Solvent System Under anhydrous conditions, thecharged groups on the proteinsurface may interact tightly witheach other, resulting in an inactivelocked conformation. The presence of water moleculesmay increase enzyme flexibilityallowing the enzyme to becatalytically active. The nature of the organic solventalso influences the behavior ofsuspended enzymes. Enzyme activity Substrate specificity Regioselectivity Enantioselectivity Prochiral selectivityAqueousPhaseOrganicPhaseInsoluble Enzyme
Enzymes in a Low-Water Solvent System Polar organic solvents are capableof stripping the hydrating watermolecules from the protein surface,decreasing enzyme activity. Hydrophobic solvents are less likelyto disrupt or distort the aqueouslayer surrounding the enzyme,promoting increased enzymeactivity. Solvent may also affect enzymeactivity by direct interaction with thesubstrates or products. The solvent may affect the effectivesubstrate or product concentrations(particularly important in enzymesprone to substrate or e EnzymeEnzymes in Organic Solvents Enzymes are generally insoluble in organicsolvents (DMSO being an exception)allowing for easy recovery of enzymeafter use. One approach to nonaqueousbiocatalysis is to utilize the enzyme as afine powder suspended in the organicsolvent. Another approach is to immobilize theenzyme by adsorbing it onto an inertsupport such as celite or porous silica. Enzyme preparation, can affect enzymeproperties and activity.
Suspended Enzyme Enzyme powders suspended in organic solvents tendto be more rigid than when in an aqueous solution. It has been proposed that he unique propertiesassociated with enzymes in a nonaqueousenvironment is due in large part to this increasedrigidity. The enzymes used in organic solvents are generallyeither lyopholyzed or precipitated from an aqueoussolution with a pH optimal for enzymatic activity. In the solid powder form, the ionogenic groups of theenzymes appear to retain the ionization stateassociated with the pH of the parent aqueous solution. The pH associated with optimal performance in theorganic suspension may differ from that of the enzymein aqueous conditions. This shows some dependenceon the organic solvent.Comparison of alcohol dehydrogenase activitiespHAqueous Solution2.07.511.0Nonaqueous Solution (heptane)maximum activitymaximum activity
Adsorbed Enzyme Selection of support material is important. The support (depending on its hydrophobicity)can influence the effective substrate/productconcentration in the vicinity of the enzyme activesite. The support can also affect enzyme hydration.Enzymes adsorbed on a hydrophobic supportdemonstrate increased activity due to improvedhydration. The support can also affect the kinetics of anenzyme-catalyzed reaction. The porosity of the support can affect substrate/product diffusion and degree of enzyme loadingon the solid support.Enzymes in Organic SolventsHydrolasesOOOOH OHNHHOBoc-L-Asp-OH Under nonaqueous conditionshydrolases (lipases andproteases) catalyze esterification,transesterification and amidecondensation reactions ratherthan hydrolytic reactions. In a water-restricted environmentthe equilibrium of amidehydrolysis is shifted towards amidebond formation. Demonstrated in the enzymaticsynthesis of the dipeptide L-Asp-LPhe-OCH3 (aspartame) HOOHONN protectionAspartameOOHHHNNHOL-Asp-L-Phe-OCH3OOCH3
Enzymes in Organic Solvents Use of hydrolases in organic Hydrolasessolvents allows the enzymaticsynthesis of polymers such aspolyesters and polyacrylates. This is not possible undernaqueous conditions due tolow solubility of monomersand/or the polymer product. Use of enzymes to fabricatepolymers capitalizes on theselectivity associated withenzyme catalyzed reactions. Enzymes can be used inconjunction with traditionalcatalysts for chemoenzymatic synthesis ofpolymers.OOO nOHHOOLipaseO*OOO* CH3CHOnEnzymes in Organic SolventsOxidoreductases As in the case of hydrolases, thecatalytic activities ofoxidoreductases are affected 3-methyl-2-buten-1-ol3-methyl-2-butenalOby a nonaqueous environment.OH Most oxidoreductases requirecofactors in order to carry outredox reactions.baker's yeastNAD alcohol dehydrogenase NADH In order to function properly, it is(YADH)necessary to regenerate thecofactor.OOH YADH in heptane efficientlyisopropanolacetoneoxidizes 3-methyl-2-bute-1-ol tothe corresponding aldehyde. The cofactor (NAD ) can berecycled in situ by the additionof acetone.
Enzymes in Organic Solvents Attachment of amphiphilic polymers such as PEG toenzymes can be used to generate enzyme constructsthat are soluble in organic solvents. (PEG-modifiedenzymes are soluble in organic solvents in the mg/mlrange.) The specific activities of such PEG-enzyme constructsare often similar to those of the free enzyme underaqueous conditions. The PEG-modified enzymes generally demonstrategreater stability than the parent enzymes. May also see changes in the optimal temperature forenzyme activity. PEG-lipase catalyzing ester exchange reaction undernonaqueous conditions--- optimal temp. 70 C Lipase catalyzing the hydrolysis of esters under aqueousconditions --- optimal temp. 45 C.PEGylation of enzymes using a 2,4-bis(mPEG)-6-s-chlorotriazineNH2H2NH3CO CH2 CH2 O CH2 CH2 OnENH2NH2H3CO CH2 CH2 O CH2 CH2 OnNNClNH2NNNNH3CO CH2 CH2 O CH2 CH2 OH3CO CH2 CH2 O CH2 CH2 PeroxidaseChymotrypsinNH2SubtilisinHENH2NH2
Enzymes in Organic Chemistry! March 25th, 2013! Enzymes Enzymes are biological catalysts. They increase the rate at which equilibrium is reached, but they do not affect the equilibrium. Enzymes differ from ordinary chemical catalyst
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