Purification and Properties of Component E of Glutamate Mutase

Trapping choline oxidase in a nonfunctional conformation by freezing at low pH

Choline oxidase is a flavin-dependent enzyme that catalyzes the oxidation of choline to glycine-betaine, with oxygen as electron acceptor. Storage at pH 6 and -20 degrees C resulted in a change in the conformation of choline oxidase, which was associated with complete loss of catalytic activity when the enzyme was assayed at pH 6. Incubation of the inactive enzyme at pH values > or = 6.5 and 25 degrees C resulted in a fast and partial reactivation of the enzyme, which occurred with slow onset of steady state during enzymatic turnover. The rate of approaching steady state was independent of the concentrations of choline and enzyme, but increased to a limiting value with increasing pH, defining a pKa value of approximately 7.3 for an unprotonated group required for enzyme activation. Prolonged incubation of the inactive enzyme at pH 6 and temperatures > or = 20 degrees C, at which no hysteretic behavior was observed, resulted in the slow and full recovery of activity over 3 h, associated with a conformational change that reverted the enzyme to the native form. Activation of the enzyme at pH 6 was enthalpy-driven with deltaH(double dagger) and TdeltaS(double dagger) values of approximately 112 kJ mol(-1) and approximately 20 kJ mol(-1) determined at 25 degrees C. These data suggest that freezing the enzyme at low pH induces a localized and reversible conformational change that is associated with the complete and reversible loss of catalytic activity.

Role of Arg100 in the Active Site of Adenosylcobalamin-Dependent Glutamate Mutase

Arginine-100 is involved in recognizing the gamma carboxylate of the substrate in glutamate mutase. To investigate its role in substrate binding and catalysis, this residue was mutated to lysine, tyrosine, and methionine. The effect of these mutations was to reduce k(cat) by 120-320-fold and to increase K(m(apparent)) for glutamate by 13-22-fold; K(m(apparent)) for adenosylcobalamin is little changed by these mutations. Even at saturating substrate concentrations, no cob(II)alamin could be detected in the UV-visible spectra of the Arg100Tyr and Arg100Met mutants. However, in the Arg100Lys mutant cob(II)alamin accumulated to concentrations similar to wild-type enzyme, which allowed the pre-steady-state kinetics of adenosylcobalamin homolysis to be investigated by stopped-flow spectroscopy. It was found that homolysis of the coenzyme is slower by an order of magnitude, compared with wild-type enzyme. Furthermore, glutamate binding is significantly weakened, so much so that the reaction exhibits second-order kinetics over the range of substrate concentrations used. The Arg100Lys mutant does not exhibit the very large deuterium isotope effects that are observed for homolysis of the coenzyme when the wild-type enzyme is reacted with deuterated substrates; this suggests that homolysis is slowed relative to hydrogen abstraction by this mutation.

Pre-steady-state kinetic studies on the Glu171Gln active site mutant of adenosylcobalamin-dependent glutamate mutase

Glutamate-171 is involved in recognizing the amino group of the substrate in glutamate mutase. The effect of mutating this residue to glutamine on the ability of the enzyme to catalyze the homolysis of adenosylcobalamin has been investigated using UV-visible stopped-flow spectroscopy. Although Glu171 does not contact the coenzyme, the mutation results in the apparent rate constants for substrate-induced homolysis of the coenzyme that are slower by 7-fold and 13-fold with glutamate and methylaspartate, respectively, than those measured for the wild-type enzyme; furthermore, it weakens the binding of these substrates by approximately 50-fold and approximately 400-fold, respectively. These observations lend support to the idea that the enzyme may use substrate binding energy to accelerate homolysis of the coenzyme. The mutation also results in isotope effects on coenzyme homolysis that are much smaller than the very large effects observed when the wild-type enzyme is reacted with deuterated substrates. This observation is consistent with adenosylcobalamin homolysis being slowed relative to hydrogen abstraction from the substrate.

The role of the active site glutamate in the rearrangement of glutamate to 3-methylaspartate catalyzed by adenosylcobalamin-dependent glutamate mutase

BACKGROUND

Adenosylcobalamin (coenzyme B(12))-dependent enzymes catalyze a variety of chemically difficult reactions that proceed through the generation of free radical intermediates. A long-standing question is how proteins stabilize what are normally regarded as highly reactive organic radicals and direct them towards productive reactions. In glutamate mutase the carboxylate of Glu171 hydrogen bonds with the amino group of the substrate. We have investigated the role of this residue in the enzyme mechanism.

RESULTS

Several sterically and functionally conservative mutations were introduced at position 171. In the most impaired mutant, Glu171Gln, k(cat) is reduced 50-fold, although the K(m) for glutamate is little affected. In the wild-type enzyme activity was pH-dependent and the acidic limb of the activity curve titrated with an apparent pK(a) of 6.6 on V(max), whereas for the sluggish Glu171Gln mutant activity is independent of pH. The steady state deuterium kinetic isotope effect is reduced in the mutant enzyme, but the steady state concentration of free radical species on the enzyme (as measured by the steady state concentration of cob(II)alamin) is unaffected by the mutation.

CONCLUSIONS

The properties of the mutant proteins are consistent with the hypothesis that Glu171 acts as a general base that serves to deprotonate the amino group of the substrate during catalysis. Deprotonation is expected to facilitate the formation of the glycyl radical intermediate formed during the inter-conversion of substrate and product radicals, but to have little effect on the stability of product or substrate radicals themselves.

Interconversion of (S)-glutamate and (2S,3S)-3-methylaspartate: a distinctive B(12)-dependent carbon-skeleton rearrangement

The interconversion of (S)-glutamate and (2S,3S)-3-methylaspartate catalyzed by B(12)-dependent glutamate mutase is discussed using results from high-level ab initio molecular orbital calculations. Evidence is presented regarding the possible role of coenzyme-B(12) in substrate activation and product formation via radical generation. Calculated electron paramagnetic resonance parameters support experimental evidence for the involvement of substrate-derived radicals and will hopefully aid the future detection of other important radical intermediates. The height of the rearrangement barrier for a fragmentation-recombination pathway, calculated with a model that includes neutral amino and carboxylic acid substituents in the migrating glycyl group, supports recent experimental evidence for the interconversion of (S)-glutamate and (2S,3S)-3-methylaspartate through such a pathway. Our calculations suggest that the enzyme may facilitate the rearrangement of (S)-glutamate through (partial) proton-transfer processes that control the protonation state of substituents in the migrating group.

Rearrangement of L-2-hydroxyglutarate to L-threo-3-methylmalate catalyzed by adenosylcobalamin-dependent glutamate mutase

Adenosylcobalamin-dependent enzymes catalyze a variety of chemically difficult isomerizations in which a nonacidic hydrogen on one carbon is interchanged with an electron-withdrawing group on an adjacent carbon. We describe a new isomerization, that of L-2-hydroxyglutarate to L-threo-3-methylmalate, involving the migration of the carbinol carbon. This reaction is catalyzed by glutamate mutase, but k(cat) = 0.05 s(-)(1) is much lower than that for the natural substrate, L-glutamate (k(cat) = 5.6 s(-)(1)). EPR spectroscopy confirms that the major organic radical that accumulates on the enzyme is the C-4 radical of L-2-hydroxyglutarate. Pre-steady-state kinetic measurements revealed that L-2-hydroxyglutarate-induced homolysis of AdoCbl occurs very rapidly, with a rate constant approaching those measured previously with glutamate and methylaspartate as substrates. These observations are consistent with the rearrangement of the 2-hydroxyglutaryl radical being the rate-determining step in the reaction. The slow rearrangement of the 2-hydroxyglutaryl radical can be attributed to the poor stabilization by the hydroxyl group of the migrating glycolyl moiety of the radical transiently formed on the migrating carbon. In contrast, with the normal substrate the migrating carbon atom bears a nitrogen substituent that better stabilizes the analogous glycyl moiety. These studies point to the importance of the functional groups attached to the migrating carbon in facilitating the carbon skeleton rearrangement.

Review Article Coenzyme-B(12)-Dependent Glutamate Mutase

Adenosylcobalamin (coenzyme B12)-dependent glutamate mutase catalyzes a most unusual carbon skeleton rearrangement involving the isomerization of l-glutamate to L-threo-methylaspartate, a reaction that is without precedent in organic chemistry. This reaction proceeds through a mechanism involving free radical intermediates that are initiated by homolysis of the cobalt-carbon bond of the coenzyme. The enzyme serves as a paradigm for adenosylcobalamin-dependent catalysis and, more generally, provides insights into how enzymes generate and control reactive free radical species. This review describes how recent studies on the mechanism and structure of glutamate mutase have contributed to our understanding of adenosylcobalamin-mediated catalysis. Copyright 2000 Academic Press.

Cloning and sequencing of glutamate mutase component E from Clostridium tetanomorphum

The nucleotide sequence of the large subunit E of glutamate mutase of Clostridium tetanomorphum was determined. The protein consists of 483 amino acids and is not made in a precursor form, thus excluding the possibility of subunit E being a pyruvoyl enzyme. It shows no homology to any other protein in the database, and while binding coenzyme B12, a conspicuous B12 binding motif, shared amongst other proteins, is not detectable at the sequence level.

Cloning and sequencing of glutamate mutase component E from Clostridium tetanomorphum. Organization of the mut genes

The gene encoding component E, the large subunit, of adenosylcobalamin (coenzyme B12)-dependent glutamate mutase from Clostridium tetanomorphum has been cloned and sequenced. The mutE gene encodes a protein of 485 amino acid residues, with M(r) 53,708. The mutE gene is situated some 1,400 bp downstream of the mutS gene, which encodes the small subunit of glutamate mutase. Between the two is an open reading frame encoding a protein of 462 amino acids, with M(r) 50,171, and of unknown function. All three genes appear to be transcribed as an operon and lie immediately upstream of the gene encoding beta-methylaspartase, the next enzyme in the pathway of glutamate fermentation. Local homology exists between mutE and a region of beta-methylaspartase which contains an active-site serine residue.

Cloning and sequencing of glutamate mutase component S from Clostridium tetanomorphum. Homologies with other cobalamin-dependent enzymes

The gene encoding component S, the small subunit, of glutamate mutase, an adenosylcobalamin (coenzyme B12)-dependent enzyme from Clostridium tetanomorphum has been cloned and its nucleotide sequence determined. The mutS gene encodes a protein of 137 amino acid residues, with M(r) 14,748. The deduced amino acid sequence showed homology with the C-terminal portion of adenosylcobalamin-dependent methylmalonyl-CoA mutase [1989, Biochem. J. 260, 345-352] and a region of cobalamin-dependent methionine synthase which has been shown to bind cobalamin [1989, J. Biol. Chem 264, 13888-13895].

Glutamate mutase from Clostridium cochlearium. Purification, cobamide content and stereospecific inhibitors

Both components, E and S, of the adenosylcobalamin-(coenzyme B12)-dependent glutamate mutase from Clostridium cochlearium were purified. Component S (16 kDa) must be added to component E to obtain activity, although the latter contains substoichiometric amounts of component S besides the major 50-kDa subunit. The enzyme proved to be very similar to that of C. tetanomorphum as described by Barker et al. [Barker, H. A., Rooze, V., Suzuki, F. & Iodice, A. A. (1964) J. Biol. Chem. 239, 3260-3266] but component E of C. cochlearium was more stable and led to the first pure preparation. The pink component E showed a cobamide-like absorbance spectrum with a characteristic maximum at 470 nm indicating the presence of a cob(II)amide, probably Co alpha-[alpha-(aden-9-yl)]-cob(II)amide. A typical cob(II)amide signal at g = 2.23 with hyperfine and superhyperfine splitting was observed by EPR spectroscopy. A cobamide content of about 0.43 mol/mol 50-kDa subunit was determined by cyanolysis. Substitution of the migrating hydrogen at C-4 of glutamate by fluorine yielded the potent competitive inhibitor (2S,4S)-4-fluoroglutamate (Ki = 70 microM). (2R,3RS)-3-Fluoroglutamate (Ki = 600 microM) was also inhibitory. The competitive inhibition by 2-methyleneglutarate (Ki = 400 microM) and (S)-3-methylitaconate (Ki = 100 microM) but not by (RS)-2-methylglutarate suggested the transient formation of an sp2 center during catalysis. However, the presence of an N-terminal pyruvoyl residue was excluded and no evidence for the participation of another electrophilic center in the reaction was obtained.

Adenosylcobalamin and cob(II)alamin as prosthetic groups of 2-methyleneglutarate mutase from Clostridium barkeri

The ultraviolet/visible spectrum of the pure pink-orange 2-methyleneglutarate mutase from Clostridium barkeri between 300-600 nm showed the presence of cobalamins; notably the peaks at 470 and 528 nm were indicative of oxygen-stable cob(II)alamin and adenosylcobalamin (coenzyme B12), respectively. Using the absorption coefficients of the isosbestic points at 340, 393 and 489 nm, the total cobalamin content was estimated as 3.7 +/- 0.3 mol/mol tetrameric enzyme (m = 300 kDa). Denaturation with 8 M urea in the presence of 2 mM dithiothreitol followed by gel chromatography and renaturation afforded an inactive enzyme which contained 40-50% of the initially bound cobalamin. This preparation could be reactivated to 95-100% by addition of adenosylcobalamin. The cobalamins were removed to 85% from the mutase by denaturation with 8 M urea in the presence of 1 M cyanide (pH 12) with irreversible loss of activity. 2-Methyleneglutarate mutase was inactivated by incubation with aquo-, cyano- or methylcobalamin; up to 50% of the activity was recovered by addition of adenosylcobalamin. Upon incubation of the mutase with [5'-3H]adenosylcobalamin about 30% of the total cobalamin was exchanged by the tritium-labelled cofactor without loss of activity. During aerobic catalysis the enzyme became sensitive towards oxygen which was accompanied by loss of activity and formation of aquocobalamin from adenosylcobalamin. EPR spectroscopy demonstrated the presence of 0.8 mol base-on cob(II)alamin/mol enzyme. Upon addition of 2-methyleneglutarate a second EPR signal of about equal intensity at g = 2.13 arose. The question of whether the oxygen-stable cob(II)alamin participates in catalysis or its complex with the enzyme represents an inactive form is currently under investigation.

Vitamin B 12

In spite of the considerable progress made in recent years toward the understanding of the chemistry and biological function of the cobalt-containing B(12) group of compounds, much of the information still is more descriptive than definitive in nature. In general terms, it is known that the free vitamin forms can function as methyl group carriers and that the 5'-deoxyadenosyl or coenzyme forms serve as hydrogen carriers; but the mechanism of these processes is not understood in detail. More systematic studies of the pure chemistry of these complex molecules containing a carbon-cobalt covalent bond are needed before the biochemist can interpret many of his observations on the enzyme-catalyzed reactions. Even in relatively simple solutions it is difficult to ascertain the state of oxidation of several of the vitamin forms, and these problems are compounded when the reactive thiol compounds and complex proteins of the biological systems also are present. For example, both vitamin B(12r) (the Co(2+) form) and corresponding analogs are known to disproportionate in solution to B(12s) (Co(1+)) and B(12a) (Co(3+)) under a variety of mild conditions (12, 57). This means that in the biological systems it is exceedingly difficult to ascertain the chemical nature of many B(12) intermediates and reaction products. The role of the protein moiety of the various B(12)-linked enzymes in the catalytic processes is little known as is, also, the mode of binding of the B(12) derivative to the protein. These types of questions perhaps can be answered eventually by the crystallographers, whose art is becoming increasingly sophisticated. Note added after preparation of manuscript. In contrast to the values given in Table 4 for the molecular weights of the two dissimilar protein moieties of glycerol dehydrase, a recent report (57a), gives a value of 188,000 for the molecular weight of a stable, catalytically inactive complex of 1 mole of hydroxocobalamin and 1 mole of the apoenzyme complex of glycerol dehydrase. The latter is presumed to contain one equivalent of each of the two dissimilar protein subunits. The original estimate of 240,000 as the molecular weight of the unstable sulfhydryl protein moiety (39) was undoubtedly made on partially aggregated material.