] Stabilization of Enzymes by Intramolecular Cross-Linking Using Bi Functional Reagents

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<p>[55]</p> <p>E N Z Y M E S T A B I L I Z A T I O N BY C R O S S - L I N K I N G</p> <p>615</p> <p>Thermodenaturation of Native, Amidinated, and Cross-Linked P-2-ONative and cross-linked enzymes exhibit the same thermodenaturation kinetics (Fig. 11). The amidinated P-2-O preparations, however, show considerably greater thermostability. Apparently, amidination provides more opportunities for hydrogen bondings and hydrophobic interactions which may enhance the thermostability of the enzyme. The chemical modification of P-2-O with higher levels of amidination results in enzyme preparations that are I0 times more thermostable than native P-2-O. There are two effects on modification: (1) there is an elimination of the fast inactivation pattern of normal proteolyzed P-2-O; and (2) there is a 5-fold deceleration of the principal cause of thermal inactivation. Acknowledgment We thank our colleagues Mark Pemberton and Mike Kunitanifor supplyingus with purified P-2-O and glucosone, and S. Daniell for carrying out the cross-linked catalase immobilization.</p> <p>[55] S t a b i l i z a t i o n o f E n z y m e s b y I n t r a m o l e c u l a r Cross-Linking Using Bifunctional Reagents</p> <p>By KAREL MARTINEK and V. P. TORCHILINThe problem of enzyme stabilization has received considerable attention in recent years. 1-7 Enzyme immobilization has been used most frequently to solve the problem of enzyme stabilization. However, other methods have been suggested as well. 3 For example, enzyme stabilization has been achieved after (1) addition of low molecular weight compounds to enzymes free in solution, (2) chemical modification of enzymes by substitution with low molecular weight compounds, and (3) use of bifunctional reagents to produce enzymes containing artificial intramolecular cross-links. These methods are desirable in particular when the presencet K. 2 A. 3 V. 4 R. 5 K. 6 A. 7 V. Martinek, A. M. Klibanov, and I. V. Berezin, J. Solid-Phase Biochem. 2, 343 (1977). M. Klibanov, Anal. Biochem. 92, 1 (1979). P. Torchilin and K. Martinek, Enzyme Microb. Technol. 1, 74 (1979). D. Schmid, Adv. Biochem. Eng. 12, 41 (1979). Martinek and V. V. Mozhaev, Enzyme Eng.-Future Directions, p. 3 (1980). M. Klibanov, Biochem. Soc. Trans. 11, 19 (1983); Science 219, 722 (1983). V. Mozhaev and K. Martinek, Enzyme Microb. Technol. 6, 49 (1984).Copyright 1988 by Academic Press, Inc. All rights of reproduction in any form reserved,</p> <p>METHODS IN ENZYMOLOGY, VOL. 137</p> <p>616</p> <p>TECHNIQUES AND ASPECTS OF ENZYMES AND CELLS</p> <p>[55]</p> <p>of a support may decrease both the binding capacity and the reactivity of the enzyme. Also, in medical therapy applied enzyme must in many cases interact with receptors or other components of cellular membranes. In this instance, a support may change the key pathways dramatically. In this chapter, stabilization of enzymes through intramolecular crosslinking will be discussed in detail. The principles of intramolecular crosslinking are shown schematically in Fig. 1. This approach is based on diminishing the polypeptide entropy which is the principal thermody!</p> <p>2</p> <p>3</p> <p>@ ...@ - - </p> <p>IL</p> <p>SLOW It,</p> <p>5</p> <p>VERY SLOW</p> <p>A1</p> <p>2</p> <p>3</p> <p>FIG. 1. General scheme of enzyme stabilization effected by intramolecular cross-linking. (A) 1, Native oligomeric enzyme; 2, reversibly dissociated subunits; 3, irreversibly denaturated subunits; 4, cross-linked enzyme; 5, irreversibly denaturated cross-linked enzyme. (B) 1, Native monomeric enzyme; 2, denaturated enzyme; 3, cross-linked enzyme.</p> <p>[55]</p> <p>ENZYME STABILIZATION BY CROSS-LINKING</p> <p>617</p> <p>namic quantity stabilizing the denatured form. 8 In 1967 Hartman andW o l d 9 introduced the use of bifunctional reagents in protein chemistry.</p> <p>Then, Husain and Lowe l used protein cross-linking with a bifunctional reagent as a means to study the tertiary structure of an enzyme molecule consisting of a single polypeptide chain. In addition, this technique has been applied to exploring the quaternary structure of oligomeric enzymes by Davies and Stark.l~ Since the publication of these pioneering works cross-linking of proteins has become a widely used technique.~2-21 Procedures have been developed for attachment of D N A and RNA molecules to proteins 22 with the aid of cross-linking methodology. Immunoanalysis and radioactive labeling have been used for identifying proteins in crosslinked protein complexes, 23 and, recently, the use of the cross-linking approach in fundamental studies in biochemistry has been reviewed. 24</p> <p>Cross-Linking ReagentsThere are now many bifunctional compounds available for cross-linking of proteins. 12-2~ Examples of such reagents are dialdehydes, diimido esters, diisocyanates, and bisdiazonium salts. Moreover, diamines such as H2N(CH2)nNH2 may be used for cross-linking of protein carboxyl groups, if the latter have been preactivated by treatment with carbodiimide. 25 Likewise, diacids such as HOOC(CH2)nCOOH (after their preactivation with carbodiimide) could be used for cross-linking of protein8 p. j. Flory, J. Am. Chem. Soc. 78, 5222 (1956). 9 F. C. Hartman and F. Wold, Biochemistry 6, 2439 (1967). 10 S. S. Husain and G. Lowe, Biochem. J. 103, 855 (1968). ii G. E. Davies and G. R. Stark, Proc. Natl. Acad. Sei. U.S.A. 66, 651 (1970). lz H. Fasold, J. Klappenberger, and H. Remold, Angew. Chem., Int. Ed. Engl. 10, 795 (1971). ~3 F. Wold, this series, Vol. 25, p. 623. 14 O. R. Zaborsky, Enzyme Eng. 1, 211 (1972). ~5 R. E. Peeney, G. Blankenborn, and H. B. F. Dixon, Adv. Protein Chem. 29, 135 (1975). 16 R. Uy and F. Wold, in "Biomedical Applications of Immobilized Enzymes and Proteins" (T. M. C. Chang, ed.), p. 15. Plenum, New York, 1976. 17 K. Peters and F. M. Richards, Annu. Rev. Biochem. 46, 523 (1977). 18 R. B. Freedman, Trends Biochem. Sci. (Pers. Ed.) 4, 193 (1979), 19 M. Das and F. Fox, Annu. Rev. Biophys. Bioeng. 8, 165 (1979). 10 T. H. Ji, this series, Vol. 91, p. 580. 21 K.-K. Han, C. Richard, and A. Delacourte, Int. J. Biochem. 16, 129 (1984). 22 K. C. Smith, in "Aging, Carcinogenesis and Radiation Biology," p. 67. Plenum, London, 1976. 23 S. K. Sinha and K. Brew, J. Biol. Chem. 256, 4193 (1981). 24 K. Martinek and V. V. Mozhaev, Adv. Enzymol. 57, 179 (1985). z5 V. P. Torchilin, A. V. Maksimenko, A. M. Klibanov, I. V. Berezin, and K. Martinek, Biochim. Biophys. Acta 522, 277 (1978).</p> <p>618</p> <p>TECHNIQUES AND ASPECTS OF ENZYMES AND CELLS</p> <p>[55]</p> <p>amino groups. 26 Both diamines and diacids are commercially available and relatively inexpensive, factors that are of prime importance in biotechnology. In addition, application of heterobifunctional cross-linking reagents 13,2,2j offers the possibility of increasing the number of crosslinks by reacting with different functional groups of the protein to be modified. Photochemical activation provides another possibility in the use of cross-linking reagents 27 (for reviews, see Refs. 20, 21, and 28). Since cross-linking requires reaction with at least two functional groups, probably differing in chemical reactivity and/or spatial location, better control over the cross-linking reaction might be obtained in a stepwise crosslinking approach. This is possible if the reagent contains both a chemically reactive group and a light-activatable (photochemical) group (or two photochemical groups showing no overlap in their photoactivation spectra). 29 Cleavable cross-linking reagents useful in some situations contain in the molecule a chemical bond that can be split readily under mild conditions (e.g., under mild oxidation or reduction conditions)3; for reviews, see Refs. 17-21. Also, water-insoluble (hydrophobic) cross-linking reagents have been used to modify membrane proteins. 17,~8</p> <p>Reactions of Cross-Linking Reagents with ProteinsThe reaction of a bifunctional reagent with an enzyme can in principle yield three different types of products: (1) a one-point modified enzyme, (2) an intramolecular cross-linked enzyme, and (3) an intermolecular cross-linked enzyme (see Fig. 2). The yields of one-point modification and intramolecular cross-linked products will depend on the length of the bifunctional reagent used and the distance between the functional groups on the protein to be modified. To increase the number of intramolecular cross-links in a protein molecule (and hence to decrease the degree of one-point modification) one can: (1) choose an optimal length of the crosslinking molecule25.26; (2) premodify the protein by substituting the protein surface with additional reactive groups25'31; (3) exploit the potentially re26 V. P. Torchilin, V. S. Trubetskoy, and K. Martinek, J. Mol. Catal. 19, 291 (1983). 27 j. R. Knowles, Acc. Chem. Res. 5, 155 (1972). 28 p. Guire, this series, Vol. 44, p. 280. 29 p. Guire, in "Enzyme Technology and Renewable Resources," p. 55. Univ. of Virginia, Charlottesville, Virginia, 1976. 30 R. R. Taraut, A. Bollen, T. Sun, J. W. B. Hershey, J. Sundberg, and L. R. Pierce, Biochemistry 12, 3266 (1973). 31 V. P. Torchilin, A. V. Maksimenko, V. N. Smirnov, I. V. Berezin, and K. Martinek, Biochim. Biophys. Acta 568, 1 (1979).</p> <p>[55]</p> <p>ENZYME STABILIZATION" BY CROSS-LINKING Y Y--X\/)~-~ ~"X-Y ENZYME ONE-POINT MODIFICATION Y</p> <p>619</p> <p>/t</p> <p>"Et ~"a~ + v - - ~~'~J-J,~'XACTIVESI.E</p> <p>--- x - ~ k ~ # t , j - x - v - - - v - x - ~ - x~ ~P" ~.~ JO.O.-',.KING</p> <p>BIFUNCTIONAL</p> <p>CROSS-LINKING FIG. 2. Possible reactions of bifunctional reagents with enzymes.</p> <p>versible character of chemical cross-linking by applying a mixture of bifunctional reagents of different chain lengths. 31 This means that in the course of the reaction the protein molecule itself will"select" intramolecular cross-linking in preference to one-point modification. Furthermore, the probability of intermolecular cross-linking may be reduced by decreasing the enzyme concentration in the reaction medium. Alternatively, to suppress intermolecular cross-linking the protein could be attached to a solid support through a cleavable spacer arm, prior to cross-linking. After cross-linking, the spacer containing, for example, a disulfide linkage, is cleaved by reducing the S-S bond with thiol reagents. 32,33In addition, a light-initiated heterobifunctional reagent can be used for cross-linking, resulting in no intermolecular side reactions. 27 In this case, first the bifunctional reagent reacts chemically at one end of the molecule, and then, after illumination of the premodified protein, it reacts photochemically at the other end of the cross-linking reagent (containing a diazo or an azide group). On illumination, a highly reactive carbene or nitrene is produced, reacting with the closest C - H linkage of the protein.</p> <p>Thermostabilization of ot-Chymotrypsin by Intramolecular Cross-Linking Succinylation of a-Chymotrypsin, a-Chymotrypsin (EC issuccinylated according to the method of Goldstein. 34 a-Chymotrypsin32 G. P. Royer, S. Ikeda, and K. Aso, FEBS Lett. 80, 89 (1977). 33 S. Pillai and B. K. Bachhawat, J. Mol. Biol. 131, 877 (1979). 34 L. Goldstein, Biochemistry 11, 4072 (1972).</p> <p>620</p> <p>TECHNIQUES AND ASPECTS OF ENZYMES AND CELLS</p> <p>[55]</p> <p>(900 mg) is dissolved in 30 ml of 0.2 M phosphate buffer, pH 7.7. Succinic anhydride (300 mg) is then added in small portions while keeping the enzyme solution in the cold (4) and maintaining the pH at 7.7. Under these conditions over 80% of available amino groups (14-15) of the enzyme are succinylated. 25 The reaction mixture is then passed through a column (2.6 60 cm) packed with Sephadex G-50 (Pharmacia). (The column is preequilibrated with 10 mM KCI.) The elution rate is 1.5 ml/ min. The succinylated o~-chymotrypsin preparation shows both catalytic activity and thermostability that are comparable to the same properties of the native enzyme. 25</p> <p>Carbodffmide Activation of Carboxyl Groups of ~-Chymotrypsin.~-Chymotrypsin is treated with carbodiimide by a slightly modified version of the method described in Ref. 25. A solution (63 ml) containing ~-chymotrypsin (10 -6 M native or succinylated enzyme) is added to 7 ml of an aqueous solution containing 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) (10 -2 M), and the mixture is left at a constant pH of 4.5 (using a pH-stat) for 1 hr at 20. Under these conditions 15 out of 17 exposed carboxyl groups of o~-chymotrypsin are modified. On treatment of a-chymotrypsin with carbodiimide, the relative catalytic activity of the enzyme drops 3-fold.</p> <p>Reaction of Carbodiimide-Activated ~-Chymotrypsin with Diamines.The solution (10 ml) containing o~-chymotrypsin or succinylated o~-chymotrypsin preactivated by carbodiimide treatment and 20 mM phosphate buffer (4 ml), pH 8.2, is added to a solution (1 ml) containing the amine reagent. The following amine concentrations are used: 10 mM hexamethylenediamine and dodecamethylenediamine; 0.1 M ethylenediamine, tetramethylenediamine, and pentamethylenediamine; 10 v/v% of hydrazine (or 1-amino-propan-3-ol). The reaction is carried out at pH 8.2 for 1 hr at 20 . Thermoinactivation. To a solution (10 ml) containing cross-linked ot-chymotrypsin (10 -6 M), 20 mM phosphate buffer (5 ml), pH 7.0, is added, and the mixture is left at 50 . Aliquots (I ml) are withdrawn at certain time intervals, and the enzyme activity is determined. Activity Measurements of Native and Modified Enzyme, The catalytic activity of the native and modified enzyme is measured in a Radiometer TTT-ld pH-stat (Radiometer) by determining the initial rates of hydrolysis of 10 mM N-acetyl-L-tyrosine ethyl ester in 0.1 M KCI at pH 7.0, 20 (assay volume 10 ml). Results. The rate of thermoinactivation of enzyme modified with diamines of different chain lengths showed a minimum in the inactivation curve (Fig. 3) when the cross-linking reagent contained 4 methylene</p> <p>[55]</p> <p>ENZYME</p> <p>05]0.3o.ao</p> <p>STABILIZATION</p> <p>BY CROSS-LINKING</p> <p>621</p> <p>E 0.25o</p> <p>I</p> <p>Z</p> <p>o_I---</p> <p>Z</p> <p>o;~ 0.15O I-.Z Z O</p> <p>~ 0.10</p> <p>o,~ 0.05</p> <p>3</p> <p>0</p> <p>4</p> <p>8</p> <p>12</p> <p>ALKYL CHAIN LENGTH OF DIAMINES (n)</p> <p>FIG. 3. Dependence of the first-order rate constant of thermoinactivation of cross-linked ~-chymotrypsin on the chain length of the diamine reagents used for cross-linking: curve 2, cross-linked native ~-chymotrypsin; curve 3, cross-linked succinylated c~-chymotrypsin; curve 1, thermostability of native and succinylated c~-chymotrypsin. From Torchilin et a l ? s</p> <p>groups. It is worth adding that intermolecular cross-links were not formed under the experimental conditions, and that the monofunctional crosslinking analog, 1-aminopropan-3-ol, caused a certain destabilization of the enzyme. On the basis of the above, it is suggested that intramolecular cross-links were formed in o~-chymotrypsin after treatment of the carbodiimide-activated enzyme with 1,4-tetramethylenediamine. Premodification of the enzyme with succinic anhydride resulted in additional reactive carboxyl groups on the protein surface. It was found that cross-linked succinylated preparations showed an increased thermostabili...</p>