Acyl Group Transfer Proteases and Esterases
Acyl group transfer processes are plentiful in enzymatic reactions. Examples may be found in ATP-dependent ligation in chapter 11, carbon-carbon bond formation in chapter 14, and fatty acid biosynthesis in chapter 18. In this chapter, we begin by presenting the basic chemistry of acyl group transfer. We then consider four major classes of proteases that catalyze acyl group transfer in the hydrolysis of peptide bonds. Acyl group transfer is so common in organic and biochemistry that the...
O Mpv
The p-toluenesulfonyl group of TPCK mimics the leaving peptide group of a substrate, and the chloromethyl group replaces the ethoxy group of ATEE. The chloromethylketone group alkylates any nucleophilic group with which it can interact. TPCK binds to the active site of chymotrypsin in place of a substrate and alkylates His57, inactivating chymotrypsin Schoellmann and Shaw, 1970 Shaw, 1963 . Chemical degradation of the inactive, alkylated chymotrypsin showed that His57 had been alkylated. A...
N
fig. 1-25, which alkylates His57. Given that the hemiketal displays a low pAa Ficunane and Malthouse, 1992 O'Connell and Malthouse, 1995 , the internal displacement of chloride by the hemiketal oxyanion and alkylation of His57 by the epoxyketal seems most attractive. All mechanisms lead to the cross-linked active site hemiketal, with both His57 and Ser195 bonded to the inactivator. Because of the involvement of Ser195 and hemike-tal formation in the inactivation mechanism, TPCK can be regarded...
Chapter 3 Coenzymes I Organic Coenzymes 129
Structures and Functions of Nicotinamide Coenzymes, 129 Stereospecificity of Hydride Transfer, 132 NAD as a Coenzyme, 134 Thiamine Pyrophosphate, 141 Structure, 141 Reaction Mechanism, 141 a-Lipoamide, 147 Pyridoxal-5'-Phosphate, 148 Enzymatic Reactions Facilitated by Pyridoxal-5'-Phosphate, 149 Pyridoxal-5'-Phosphate-Stabilized Amino Acid Carbanions, 149 Mechanisms of Pyridoxal-5'-Phosphate-Dependent Reactions, 151 Flavin Coenzymes, 158 Structures of Flavin Coenzymes, 158 Mechanisms of Flavin...
Dehydrogenation at GlycosylC4
GalE contains one tightly bound molecule of NAD per subunit when purified to homogeneity, and it is transiently reduced to NADH when substrate is added Liu et al., 1997 Nelsestuen and Kirkwood, 1971 Wilson and Hogness, 1964 . The C4 H and C4 O of UDP-glucose are retained in the product that is, they are not exchanged with water, and tritium is transferred between NADH and UDP-4-ketoglucose at the active site Adair et al., 1973 Anderson et al., 1956 Kowalsky and Koshland, 1956 Maitra and Ankel,...
Haldane Relationships
The kinetic parameters for an enzymatic reaction are constrained by the equilibrium constant for that reaction, just as are the rate constants for a simple chemical reaction. For a simple equilibrium between reactant A and product P, the value of Keq is equal to the ratio of the forward and reverse rate constants. Similarly, an enzymatic reaction at equilibrium continues to take place in forward and reverse directions at the same rates. In terms of initial rates vf and vr, the net rate is zero...
Info Fpt
typical of binding rate constants for enzymes see chap. 2 , suggesting that substrate and product diffusion could be rate limiting. Diffusion as the rate-limiting process means that the enzyme is as highly evolved as it can be because further improvements in the chemistry of the process cannot increase the rate. This is consistent with a theory of the evolution of enzymes, which holds that incremental evolution lowering the activation barriers for individual steps will eventually lead to...
Pyruvate Kinase
The action of pyruvate kinase EC 2.7.1.40 , in addition to its importance in glycolysis, is interesting for its unique mechanistic aspects and relationships with other phosphokinases such as phosphoenolpyruvate carboxykinase EC 4.1.1.49 . Also of interest are the fundamental mechanistic differences between the actions of pyruvate kinase and pyruvate phosphate dikinase EC 2.7.9.1 . Pyruvate kinase catalyzes phosphoryl group transfer between phosphoenolpyruvate and ADP according to eq. 10-8. I _...
Proline Racemase
In early work on clostridial proline racemase, it was found to be PLP-independent Cardinale and Abeles, 1968 . The racemization of proline is chemically interesting because it is the only a-amino acid that cannot be racemized nonenzymatically by heating its copper complex with base. Proline racemase EC 5.1.1.4 is a homodimeric enzyme with a subunit molecular mass of 38 kDa Rudnick and Abeles, 1975 . Enzymatic racemization of a-deuteroproline proceeds with a primary kinetic isotope effect, and...
Info Sfd
The five-membered ring is strained, and strain is relieved by hydrolysis to hydroxyethyl phosphate. Relief of strain in the overall reaction can explain the rate difference only if the mechanism includes relief of strain in a rate-limiting step. Normally, the hydrolysis of a phosphodiester proceeds through a tight transition state. In the case of five-membered ring cyclic phosphates, the relief of strain on formation of the transition state is sufficient to allow the formation of trigonal...
Oxalate Decarboxylase
Oxalate undergoes at least three types of metabolism in various organisms, all of which lead to decarboxylation but by very different mechanisms and with the formation of very different products. Oxalate decarboxylase OxDC EC 4.1.1.23 is found in fungi and bacteria. It catalyzes the decarboxylation to form carbon dioxide and formate according to eq. 8-7. HOOC COO- CO2 HCOO- 8-7 Oxalate oxidase EC 1.2.3.4 catalyzes the reaction of oxalate with molecular oxygen to form two moles of carbon dioxide...
N Gxr
In the second step of an acyl group transfer, the leaving group from the tetrahedral intermediate would be an oxide ion O2 , a very poor leaving group. In thioesters such as CoA-esters, nucleophilic addition is much less disfavored because of little accumulation of negative charge, and there is a good leaving group. Thioesters are highly activated toward acyl transfer relative to carboxylate ions. The thioester carbonyl is also more electrophilic than the oxyester carbonyl because thioesters...
Atcase Acyl Group Transfer
Fig. 2-23. Derivation of the Hill equation. When n 2, n, the equation contains higher powers of A , and plots of v against A are nonhyperbolic. When each enzyme form reacts at a different rate, each term in the numerator will require a different rate constant, in which case, the rate equations become even more complex. A very special theoretical case of multisite binding leads to the Hill equation and the Hill coefficient, which is useful for assessing the degree of cooperativity in a real...
Enzymatic Rate Enhancement and TransitionState Binding
Accepting transition state theory as a basis for understanding reaction kinetics, it can be shown that an enzyme or any catalyst can catalyze a reaction by binding the transition state more tightly than the ground state. This concept made its appearance early in the study of enzymes Pauling, 1948 . It is a straightforward and reasonable concept because of the fact that a binding interaction should be stabilizing therefore, binding the transitionstate should stabilize it, thereby increasing the...
Acetoacetate Decarboxylase
A few decarboxylases do not require the assistance of a coenzyme. A well-studied example is the decarboxylation of acetoacetate eq. 8-4 by acetoacetate decarboxylase AAD EC 4.1.1.4 , a dodecamer of identical 40-kDa subunits from Clostridium acetobutylicum. As a P-ketoacid, acetoacetate undergoes nonenzymatic decarboxylation at a significant rate, especially under acidic conditions by the mechanism in scheme 8-3, analogous to mechanism A in fig. 8-1. Acetoacetic acid incorporates within its...
Catalysis of Multistep Reactions
Very few enzymatic reaction mechanisms involve a single chemical event and a single transition state. Most proceed in multiple steps that involve the transient formation of reactive intermediates. In a multistep reaction, an enzyme must catalyze more than one chemical process. Even as simple a reaction as peptide hydrolysis by chymotrypsin proceeds in two steps, acylation and deacylation, and the transition states for the two processes cannot be identical, although they may be very similar. In...
Reaction and Mechanism
The oxygen-dependent desaturation of fatty acids discussed in chapter 4 in connection with fatty acyl carrier protein ACP desaturase cannot account for the biosynthesis of unsaturated fatty acids under anaerobic conditions. Little is known about this subject outside of the metabolism of E. coli. However, the oxygen-independent biosynthesis of palmitoleyl-ACP has been solved. This is brought about in part through the action of P-hydroxydecanoyl ACP dehydratase Helmkamp et al., 1968 , which...
Structure of Yeast Pyruvate Decarboxylase
Several structures of PDC from yeast and bacteria reveal much about the chemical environment at the active site and the structure of TPP at this site Aijunan et al., 1996 Dobritzsch et al., 1998 Dyda et al., 1993 . The structure of yeast PDC in fig. 8-3 reveals the complexities of a tetrameric multidomain protein with several binding sites. In addition Fig. 8-3. Pyruvate decarboxylase from yeast is a homotetrameric protein that is subject to allosteric regulation. The four subunits of the...
Electrostatic Activation of Coordinated Water
In general, the catalytic effect of divalent metal ions can be attributed to their electrostatic effects on the chemical properties of ligands to which they are coordinated. A fairly clear concept of the magnitude of this effect can be acquired by considering the acid strengthening effect of a divalent metal ion on a coordinated water molecule. Table 4-3 lists the range of effects of divalent metal ion coordination on the pKa of water in a collection of divalent metal ions. The values of pKa...
Methotrexate
Aminopterin and amethopterin methotrexate are folate antagonists used in the treatment of leukemia and other malignancies. They are inhibitors of DHFR Osborn et al., 1958 Peters and Greenberg, 1958 . Structure 5-2 does not give clues about their potencies as inhibitors of DHFR. Nor did their inhibitory properties seem at first to be in accord with expectations that they would bind at the active site. 5-2 j N' gt HN Aminopterin R1 NH2 R2 H Original inhibition data are shown graphically in fig....
Info Jcx
Folate Compounds of One-Carbon Metabolism Tetrahydrofolate H4folate compounds are intermediates in one-carbon metabolism at the three oxidation states of carbon corresponding to methanol, formaldehyde, and formate. Their structures and enzymatic interconversions are outlined in fig. 3-34, where 5-methyl-H4folate is shown at the methanol level, 5,10-methylenetetrahydrofolate methylene-H4folate is at the formaldehyde level, and 10-formyltetrahydrofolate 10-formyl-H4folate is at the formate level...
Xylose Isomerase
In contrast to TIM and phosphoglucose isomerase, exchange of substrate hydrogen with solvent has never been observed in the reaction of xylose isomerase EC 5.3.1.5 , which catalyzes the interconversion of D-xylose and D-xylulose eq. 7-2 . The isomerization takes place in the noncyclic aldose ketose forms of the sugars according to eq. 7-1. The complete absence of exchange suggested that the reaction might occur by the 1,2-hydride shift mechanism along the lower pathway of scheme 7-1. This...
Reaction Mechanism
Several properties of thymidylate synthase provided clues to the mechanism of its reaction Carreras and Santi, 1995 . First, this enzyme catalyzes the exchange of tritium from pyrimidine-C5 of 5-3H dUMP into water. Second, thymidylate synthase is inactivated by 5-fluorodeoxyuridine-5'-phosphate FdUMP in the presence but not in the absence of methylene-H4folate. Third, both FdUMP and methylene-H4folate are covalently bonded to the inactivated enzyme. Fourth, covalent bonding of FdUMP in the...
Acetylenic Inactivator
Replacement of the double bond in p,y-decenoyl cysteamine with a triple bond, the acetylenic group of p,y-decynoyl cysteamine, leads to a molecule that binds at the active site in place of the product and is a suicide inactivator. The inactivation leads to covalent attachment of the inactivator to the enzyme molecule through alkylation of a histidine residue Endo et al., 1970 . The mechanism outlined in fig. 5-5 begins with the acetylenic analog of the product binding to the active site. The...
Role of Ile16
To comprehend the identification of Ile16 as an ionizing group governing pH dependence, it is necessary to understand the origin of the B chain. Mature a-chymotrypsin consists of chains A, B, and C that arise from proteolytic processing of chymotrypsinogen, the inactive proenzyme form and primary translation product. Activation of chymotrypsinogen in vitro by the action of trypsin and activated chymotrypsin leads to the species depicted in fig. 6-2. Trypsin catalyzes peptide hydrolysis at sites...
Coenzymes I Organic Coenzymes
Most enzymatic reactions proceed with chemical changes that cannot be brought about by the side chains of amino acid residues. These enzymes function in cooperation with coen-zymes and cofactors, which lend physicochemical potentialities not found in amino acids. Many coenzymes are organic molecules incorporating functional groups with chemical properties that enable them to facilitate reactions of certain types. These molecules bind to active sites tailored for them through evolution and...
Coenzymes II Metallic Coenzymes
The original coenzymes were small organic molecules that activated enzymes and participated directly in catalyzing enzymatic reactions. Most of them were derived from vitamins and were known as biologically activated forms of vitamins such as niacin, riboflavin, thiamine, and pyridoxal. Heme was in a separate category, perhaps because of its widespread biological role as an oxygen carrier, and because it was not a vitamin, it was not widely regarded as a coenzyme. However, heme was clearly an...
Glyphosate
Humans and animals do not biosynthesize phenylalanine or tryptophan, which are essential amino acids that must be obtained from their diets. Plants cannot obtain aromatic amino acids from their environments and must biosynthesize them. EPSP synthase is an essential enzyme in the life of plants, and inhibitors of aromatic amino acid biosynthesis are herbicides. Glyphosate, or N- methylphosphono glycine, is such a molecule, and it is a very specific inhibitor of EPSP synthase. Glyphosate binds to...
Other Nickel Coenzymes
Hydrogenases catalyze the production of hydrogen gas in the reduction of hydrogen ions. The overall reaction may be expressed as 2H 2e- H2 E ' -0.414 V. Most hydrogenases contain binuclear metal sulfur centers, including NiFe binuclear centers and Fe2 binuclear centers. In methanogenesis, hydrogen is the ultimate reducing agent in four of the steps, often as XH2 in the first step of fig. 4-39, as the reducing agent in the production of H2F420, and in the reduction of HTP-S-S-CoM to HTP-SH and...
Methyl Coenzyme M Reductase
In methane production, methyl coenzyme M reductase catalyzes the reaction of methyl coenzyme M CH3-CoM with coenzyme B CoB-SH to form methane and the disulfide CoM-S-S-CoB. Methyl coenzyme M reductase contains two copies of each of three subunits a2p2y2 Thauer et al., 1993 . The coenzyme F430 of methyl coenzyme M reduc-tase is related to corrin and heme but is more highly reduced and has Ni II as the coordinating metal Diekert et al., 1980 Farber et al., 1991 Pfaltz et al., 1982 . The location...
Ping Pong Mechanisms
Parallel lines in double reciprocal plots usually mean that a substrate reacts with the enzyme and produces a product independently of the presence of another substrate. In the two-substrate case, scheme 2-7 is the ping pong bi bi mechanism. In this mechanism, the leading substrate A reacts with the enzyme, changing it to a chemically different form designated F in scheme 2-7, and this process leads to the formation and release of the product P. The reaction of E F may involve an enzymatic...
Binding the Near Attack Conformation
The concept of transition-state binding to active sites cannot be regarded as representing a true, reversible binding phenomenon. Transition states cannot dissociate into solution and be measured. The concept should be regarded in the same light as that of the equilibrium between a ground state and activated complex in transition state theory as a useful theoretical construct. Another theory holds that enzymes bind substrates so as to promote the formation of near attack conformations NACs , in...
Hn Uci
Fig. 3-34. Structures and interconversions of folate compounds in one-carbon metabolism. a quaternary complex of enzyme, H4folate, MgATP, and formate as a compulsory intermediate Joyce and Himes, 1966 . Hfolate MgATP HCOO - 10-Formyl-H4folate MgADP Pi 3-30 The mechanism of formate activation and capture in the active site of the quaternary complex proceeds in a two-step mechanism shown in fig. 3-35. Formate is first phospho-rylated to formyl phosphate by MgATP. Formyl phosphate is a highly...
Transient Methods Burst Kinetics
The chymotrypsin-catalyzed hydrolysis of p-nitrophenyl acetate PNPA is a simple reaction that can be studied by a transient kinetic method and yields valuable mechanistic information. PNPA is a very poor substrate for chymotrypsin that is easily assayed Fig. 2-13. Burst kinetics in the chymotrypsin-catalyzed hydrolysis ofp-nitrophenyl acetate. Fig. 2-13. Burst kinetics in the chymotrypsin-catalyzed hydrolysis ofp-nitrophenyl acetate. spectrophotometrically because of the yellow color of one of...