Glutamine synthetase (L-glutamate : ammonia ligase, EC 6.3.1.2) fromPhaseolus aureus (mung bean) seedlings was purified to homogeneity by ammonium sulphate fractionation, DEAE-cellulose chromatography, Sephadex G-200 gel filtration and affinity chromatography on histidine-Sepharose. The enzyme had a molecular weight of 775,000 ± 25,000. The enzyme consisted of identical subunits with an approximate subunit molecular weight of 50,000. Hyperbolic saturation curves were obtained with the substrates, glutamate, ATP and hydroxylamine.

Antibody, raised in the rabbit, against mung bean glutamine synthetase, completely inhibited the activity of the enzyme. Preincubation of the enzyme with glutamate and ATP, prior to the addition of the antibody, partially protected the enzyme against inhibition. TheK_m values of this enzyme-antibody complex and the native enzyme were identical (glutamate, 2.5mM; ATP, 1 mM; hydroxylamine, 0.5 mM). The Km values of the partially inhibited enzyme (the enzyme pretreated with antibody prior to the addition of substrates) were 2-fold higher than those of the native enzyme. These results suggested that the substrate-induced conformational changes in the enzyme were responsible for the protection against inhibition of the enzyme activity by the antibody.

A homogenous and crystalline form of nucleotide pyrophosphatase (EC 3.6.1.9) fromPhaseolus aureus (mung bean) seedlings was used for the study of the regulation of enzyme activity by adenine nucleotides. The native dimeric form of the enzyme had a helical content of about 65% which was reduced to almost zero values by the addition of AMP. In addition to this change in the helical content, AMP converted the native dimer to a tetramer. Desensitization of AMP regulation, without an alteration of the molecular weight, was achieved either by reversible denaturation with 6 M urea or by passage through a column of Blue Sepharose but additionofp-hydroxymercuribenzoate desensitized the enzyme by dissociating the native dimer to a monomer. The changes in the quaternary structure and conformation of the enzyme consequent to AMP interaction or desensitization were monitored by measuring the helical content, EDTA inactivation and Zn²⁺ reactivation, stability towards heat denaturation, profiles of urea denaturation and susceptibility towards proteolytic digestion. Based on these results and our earlier work on this enzyme, we propose a model for the regulation of the mung bean nucleotide pyrophosphatase by association-dissociation and conformational changes. The model emphasizes that multiple mechanisms are operative in the desensitization of regulatory proteins.

Retinol-binding protein and prealbumin were isolated from duck plasma by chromatography on DEAE-cellulose-and DEAE-Sephadex A-50, gel filtration on Sephadex G-100 and preparative Polyacrylamide gel electrophoresis. The molecular weights of the retinol-binding protein-prealbumin complex, prealbumin and retinol-binding protein were found to be 75,000, 55,0000 and 20,000, respectively. On sodium dodecyl sulphate Polyacrylamide gel electrophoresis, prealbumin dissociated into identical subunits exhibiting a molecular weight of 13,500. Retinol-binding protein exhibited microheterogeneity on electrophoresis, whereas prealbumin moved as a single band unlike the multiple bands observed in chicken and rat. The ultraviolet and fluorescence spectra of the two proteins were similar to those isolated from other species. No carbohydrate moiety was detected in either retinol-binding protein or prealbumin. Duck retinol-binding protein and prealbumin showed cross-reactivity with their counterparts in chicken but differed immunologically from those of goat and man. Retinol-binding protein and prealbumin could be dissociated at low ionic strength, in 2M urea, by CM-sephadex chromatography or on preparative electrophoresis. Although the transport of retinol in duck plasma is mediated by carrier proteins as in other species, it is distinguished by the absence of microheterogeneity in prealbumin and of an apo-retinol-binding protein form that could be transported in the plasma.

The far-ultraviolet region circular dichroic spectrumof serine hydroxymethyltransferase from monkey liver showed that the protein is in an α-helical conformation. The near ultraviolet circular dichoric spectrum revealed two negative bands originating from the tertiary conformational environment of the aromatic amino acid residues. Addition of urea or guanidinium chloride perturbed the characteristic fluorescence and far ultraviolet circular dichroic spectrum of the enzyme. The decrease in (θ)₂₂₂ and enzyme activity followed identical patterns with increasing concentrations of urea, whereas with guanidinium chloride, the loss of enzyme activity preceded the loss of secondary structure. 2-Chloroethanol, trifluoroethanol and sodium dodecyl sulphate enhanced the mean residue ellipticity values. In addition, sodium dodecyl sulphate also caused a perturbation of the fluorescence emission spectrum of the enzyme. Extremes of pH decreased the — (θ)₂₂₂ value. Plots of — (θ)₂₂₂and enzyme activity as a function of pH showed maximal values at pH 7.4–7.5. These results suggested the prevalence of “conformational flexibility” in the structure of serine hydroxymethyltransferase.

The homogeneous serine hydroxymethyltransferase from monkey liver was optimally activate at 60°C and the Arrhenius plot for the enzyme was nonlinear with a break at 15°C. The monkey liver enzyme showed high thermal stability of 62°C, as monitored by circular dichroism at 222 nm, absorbance at 280 nm and enzyme activity. The enzyme exhibited a sharp co-operative thermal transition in the range of 50°–70°(T_m= 65°C), as monitored by circular dichroism. L-Serine protected the enzyme against both thermal inactivation and thermal disruption of the secondary structure. The homotropic interactions of tetrahydrofolate with the enzyme was abolished at high temperatures (at 70°C, the Hill coefficient value was 1.0). A plot ofh values vs. assay temperature of tetrahydrofolate saturation experiments, showed the presence of an intermediate conformer with anh value of 1.7 in the temperature range of 45°–60°C. Inclusion of a heat denaturation step in the scheme employed for the purification of serine hydroxymethyltransferase resulted in the loss of cooperative interactions with tetrahydrofolate. The temperature effects on the serine hydroxylmethyltransferase, reported for the first time, lead to a better understanding of the heat induced alterations in conformation and activity for this oligomeric protein.

Free proline content in Ragi (Eleusine coracana) leaves increased markedly (6 to 85 fold) as the degree of water stress, created by polyethylene gylcol treatment, was prolonged There was also a marginal increase in soluble proteins in the stressed leaves as compared to that in the controls. Water stress stimulated the activities of ornithine aminotransferase and pyrroline-5-carboxylate reductase, the enzymes of proline biosynthesis and markedly inhibited the enzymes involved in proline degradation viz., proline oxidase and pyrroline-5-carboxylate dehydrogenase. These results suggest that increase in free proline content of Ragi leaves could be due to enhanced activities of the enzymes synthesizing proline but more importantly due to severe inhibition of the enzymes degrading proline. These observations establish for the first time, the pathway of proline metabolism in plants by way of detection of the activities of all the enzymes involved and also highlight the role of these enzymes in proline accumulation during water stress.

Serine hydroxymethyltransferase (EC 2.1.2.1) was purified from the cytosolic fraction of sheep liver by ammonium sulphate fractionation, CM-Sephadex chromatography, gel filtration using Ultrogel ACA 34 and Blue Sepharose affinity chromatography. The homogeneity of the enzyme was rigorously established by Polyacrylamide gel and sodium dodecyl sulphate-polyacrylamide gel electrophoresis, isoelectrofocusing, ultracentrifugation, immunodiffusion and Immunoelectrophoresis. The enzyme was a homotetramer with a molecular weight of 210,000 ±5000. The enzyme showed homotropic cooperative interactions with tetrahydrofolate (n_H =2.8) and a hyperbolic saturation pattern with L-serine. At the lowest concentration of tetrahydrofolate used (0.2 mM), only 5% of the added folate was oxidized during preincubation and assay. TheⁿH value was independent of the time of preincubation. Preincubation of the enzyme with serine resulted in a partial loss of the cooperative interactions (n_H =1.6) with tetrahydrofolate. The enzyme was regulated allosterically by interaction with nicotinamide nucleotides; NADH was a positive effector while NAD+ was a negative allosteric effector. The subunit interactions were retained even at the temperature optimum of 60‡C unlike in the case of the monkey liver enzyme, where these interactions were absent at higher temperatures. D-Cycloserine, a structural analogue of serine caused a sigmoid pattern of inhibition, in contrast with the observations on the monkey liver enzyme. Cibacron blue F3GA completely inhibited the enzyme and this inhibition could be reversed by tetrahydrofolate. Unlike in the monkey liver enzyme, NAD+ and NADH gave considerable protection against this inhibition. The sheep liver enzyme differs significantly in its kinetic and regulatory properties from the serine hydroxymethyltransferases isolated from other sources.

5,10-Methylenetetrahydrofolate reductase (EC 1.1.1.68) was purified from the cytosolic fraction of sheep liver by (NH₄)₂ SO₄ fractionation, acid precipitation, DEAE-Sephacel chromatography and Blue Sepharose affinity chromatography. The homogeneity of the enzyme was established by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, ultracentrifugation and Ouchterlony immunodiffusion test. The enzyme was a dimer of molecular weight 1,66,000 ± 5,000 with a subunit molecular weight of 87,000 ±5,000. The enzyme showed hyperbolic saturation pattern with 5-methyltetrahydrofolate.K_0.5 values for 5-methyltetrahydrofolate menadione and NADPH were determined to be 132 ΜM, 2.45 ΜM and 16 ΜM. The parallel set of lines in the Lineweaver-Burk plot, when either NADPH or menadione was varied at different fixed concentrations of the other substrate; non-competitive inhibition, when NADPH was varied at different fixed concentrations of NADP; competitive inhibition, when menadione was varied at different fixed concentrations of NADP and the absence of inhibition by NADP at saturating concentration of menadione, clearly established that the kinetic mechanism of the reaction catalyzed by this enzyme was ping-pong.

The specific activity of glutamine synthetase (L-glutamate: ammonia ligase, EC 6.3.1.2) in surface grownAspergillus niger was increased 3–5 fold when grown on L-glutamate or potassium nitrate, compared to the activity obtained on ammonium chloride. The levels of glutamine synthetase was regulated by the availability of nitrogen source like NH₄⁺, and further, the enzyme is repressed by increasing concentrations of NH₄⁺. In contrast to other micro-organisms, theAspergillus niger enzyme was neither specifically inactivated by NH₄⁺ or

L-glutamine nor regulated by covalent modification. Glutamine synthetase fromAspergillus niger was purified to homogenity. The native enzyme is octameric with a molecular weight of 385,000±25,000. The enzyme also catalyses Mn²⁺ or Mg²⁺-dependent synthetase and Mn²⁺-dependent transferase activity.

Aspergillusniger glutamine synthetase was completely inactivated by two mol of phenyl-glyoxal and one mol of N-ethylmaleimide with second order rate constants of 3.8 M^-1 min^-1 and 760 M^-1 min^-1 respectively. Ligands like Mg. ATP, Mg. ADP, Mg. AMP, L-glutamate NH₄⁺, Mn²⁺ protected the enzyme against inactivation. The pattern of inactivation and protection afforded by different ligands against N-ethylamaleimide and phenylglyoxal was remarkably similar. These results suggest that metal ATP complex acts as a substrate and interacts with an arginine ressidue at the active site. Further, the metal ion and the free nucleotide probably interact at other sites on the enzyme affecting the catalytic activity.

Water stress resulted in a specific response leading to a large and significant increase (80-fold) in free proline content of ragi (Eleusine coracana leaves and seedlings. L-Proline protected ornithine aminotransferase, an enzyme in the pathway for proline biosynthesis, isolated from normal and stressed ragi leaves against heat inactivation and denaturation by urea and guanidinium chloride. The protection of the stressed enzyme by L-proline was much more complete than that of the enzyme isolated from normal leaves. While L-ornithine, one of the substrates, protected the stressed enzyme against inactivation, it enhanced the rate of inactivation of the normal enzyme. α-Ketoglutarate protected both the normal and stressed enzyme against inactivation and denaturation. These results support the suggestion that ornithine aminotransferase has undergone a structural alteration during water stress. In view of the causal relationship between elevated temperature and water stress of plants under natural conditions, the protection afforded by proline against inactivation and denaturation of the enzyme from stressed leaves assumes significance. These results provide an explanation for a possible functional importance of proline accumulation during water stress.

Aspartate transcarbamylase (EC 2.1.3.2) was purified to homogeniety from germinated mung bean seedlings by treatment with carbamyl phosphate. The purified enzyme was a hexamer with a subunit molecular weight of 20,600. The enzyme exhibited multiple activity bands on Polyacrylamide gel electrophoresis, which could be altered by treatment with carbamyl phosphate or UMP indicating that the enzyme was probably undergoing reversible association or dissociation in the presence of these effectors. The carbamyl phosphate stabilized enzyme did not exhibit positive homotropic interactions with carbamyl phosphate and hysteresis. The enzyme which had not been exposed to carbamyl phosphate showed a decrease in specific activity with a change in the concentration of both carbamyl phosphate and protein. The carbamyl phosphate saturation and UMP inhibition patterns were complex with a maximum and a plateau region. The partially purified enzyme also exhibited hysteresis and the hysteretic response, a function of protein concentration, was abolished by preincubation with carbamyl phosphate and enhanced by preincubation with UMP. All these observations are compatible with a postulation that the enzyme activity may be regulated by slow reversible association-dissociation dependent on the interaction with allosteric ligands

The fluorescein dye, rose bengal in the dark: (i) inhibited the activity of mung bean aspartate transcarbamylase (EC 2.1.3.2) in a non-competitive manner, when aspartate was the varied substrate; (ii) induced a lag in the time course of reaction and this hysteresis was abolished upon preincubation with carbamyl phosphate; and (iii) converted the multiple bands observed on polyacrylamide gel electrophoresis of enzyme into a single band. The binding of the dye to the enzyme induced a red shift in the visible spectrum of dye suggesting that it was probably interacting at a hydrophobic region in the enzyme. The dye, in the presence of light, inactivated the enzyme and the inactivation was not dependent on pH. All the effects of the dye could be reversed by UMP, an allosteric inhibitor of the enzyme. The loss of enzyme activity on photoinactivation and the partial protection afforded by N-phosphonoacetyl-L-aspartate, a transition state analog and carbamyl phosphate plus succinate, a competitive inhibitor for aspartate, as well as the reversal of the dye difference spectrum by N-phosphonoacetyl-L-aspartate suggested that in the mung bean aspartate transcarbamylase, unlike in the case ofEscherichia coli enzyme, the active and allosteric sites may be located close to each other.

The activity of glutamine synthetase fromAspergillus niger was significantly lowered under conditions of citric acid fermentation. The intracellular pH of the organism as determined by bromophenol blue dye distribution and fluorescein diacetate uptake methods was relatively constant between 6·0–6·5, when the pH of the external medium was varied between 2·3–7·0.Aspergillus niger glutamine synthetase was rapidly inactivated under acidic pH conditions and Mn²⁺ ions partially protected the enzyme against this inactivation. Mn²⁺-dependent glutamine synthetase activity was higher at acidic pH (6·0) compared to Mg²⁺-supported activity. While the concentration of Mg²⁺ required to optimally activate glutamine synthetase at pH 6·0 was very high (≥ 50 mM), Mn²⁺ was effective at 4 mM. Higher concentrations of Mn²⁺ were inhibitory. The inhibition of both Mn²⁺ and Mg²⁺-dependent reactions by citrate, 2-oxoglutarate and ATP were probably due to their ability to chelate divalent ions rather than as regulatory molecules. This suggestion was supported by the observation that a metal ion chelator, EDTA also produced similar effects. Of the end-products of the pathway, only histidine, carbamyl phosphate, AMP and ADP inhibitedAspergillus niger glutamine synthetase. The inhibitions were more pronounced when Mn²⁺ was the metal ion activator and greater inhibition was observed at lower pH values. These results permit us to postulate that glutamine synthesis may be markedly inhibited when the fungus is grown under conditions suitable for citric acid production and this block may result in delinking carbon and nitrogen metabolism leading to acidogenesis

Cibacron Blue F3G-A, a probe used to monitor nucleotide binding domains in enzymes, inhibited sheep liver 5,10-methylenetetrahydrofolate reductase competitively with respect to 5-methyltetrahydrofolate and NADPH. TheK_i values obtained by kinetic methods and theK_d value for the binding of the dye to the enzyme estimated by protein fluorescence quenching were in the range 0.9–1.2 μM. Another triazine dye, Procion Red HE-3B interacted with the enzyme in an essentially similar manner to that observed with Cibacron Blue F3G-A. These results as well as the interaction of the dye with the enzyme monitored by difference spectroscopy and intrinsic protein fluorescence quenching methods indicated that the dye was probably interacting at the active site of the enzyme by binding at a hydrophobic region.

Equilibrium unfolding studies of sheep liver tetrameric serine hydroxymethyltransferase (SHMT, EC 2.1.2.1) revealed that the enzyme assumed apparent random coil structure above 3 M guanidine hydrochloride (GdnHCl). In the presence of non-ionic detergent Brij-35 and polyethylene glycol, the 6 M GdnHCI unfolded enzyme could be completely (> 95%) refolded by a 40-fold dilution. The refolded enzyme was fully active and had kinetic constants similar to the native enzyme. The midpoint of inactivation (0.12 M GdnHCl) was well below the midpoint of unfolding (1.6±0.1 M GdnHCl) as monitored by far UV CD at 222 nm. In the presence of PLP, the midpoint of inactivation shifted to a higher concentration of GdnHCl (0.6 M) showing that PLP stabilizes the quaternary structure of the enzyme. However, 50% release of pyridoxal-5′-phosphate (PLP) from the active site occurred at a concentration (0.6 M) higher than the midpoint of inactivation suggesting that GdnHCl may also act as a competitive inhibitor of the enzyme at low concentrations which was confirmed by activity measurements. PLP was not required for the initiation of refolding and inactive tetramers were the end products of refolding which could be converted to active tetramers upon the addition of PLP. Size exclusion chromatography of the apoenzyme showed that the tetramer unfolds via the intermediate formation of dimers. Low concentrations (0.3–0.6 M) of GdnHCl stabilized at least one intermediate which was in slow equilibrium with the dimer. The binding of ANS was maximum at 0.4–0.6 M GdnHCl suggesting that the unfolding intermediate that accumulates at this concentration is less compact than the native enzyme.

Serine hydroxymethyltransferase (SHMT), a pyridoxal-5′-phosphate (PLP) dependent enzyme catalyzes the interconversion of L-Ser and Gly using tetrahydrofolate as a substrate. The gene encoding for SHMT was amplified by PCR from genomic DNA ofBacillus stearothermophilus and the PCR product was cloned and overexpressed inEscherichia coli. The purified recombinant enzyme was isolated as a mixture of dimer (90%) and tetramer (10%). This is the first report demonstrating the existence of SHMT as a dimer and tetramer in the same organism. The specific activities at 37°C of the dimeric and tetrameric forms were 6.7 U/mg and 4.1 U/mg, respectively. The purified dimer was extremely thermostable with aT_m of 85°C in the presence of PLP and L-Ser. The temperature optimum of the dimer was 80°C with a specific activity of 32.4 U/mg at this temperature. The enzyme catalyzed tetrahydrofolate-independent reactions at a slower rate compared to the tetrahydrofolate-dependent retro-aldol cleavage of L-Ser. The interaction with substrates and their analogues indicated that the orientation of PLP ring ofB. stearothermophilus SHMT was probably different from sheep liver cytosolic recombinant SHMT (scSHMT).