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R J Yon

Publications and source records attributed to R J Yon.

32 records · Page 2Linked to original sources
PubMed1979

Wheat-germ aspartate transcarbamoylase. Steady-state kinetics and stereochemistry of the binding site for L-aspartate.

1. The steady-state kinetics of the bisubstrate reaction catalysed by aspartate transcarbamoylase purified from wheat (Triticum vulgare)-germ have been studied at 25 degrees C, pH 8.5 AND I 0.10-0.12. Initial-velocity and product-inhibition results are consistent with an ordered sequential mechanism in which carbamoyl phosphate is the first substrate to bind, followed by L-aspartate, and carbamoyl aspartate is the first product to leave, followed by Pi. The order of substrate addition is supported by dead-end inhibition studies using pyrophosphate and maleate as inhibitory analogues of the substrates. Product inhibition permitted a minimum value for the dissociation constant of L-aspartate from the ternary complex to be estimated. This minimum is of the same order as the dissociation constant (Ki) of succinate. 2. A range of dicarboxy analogues of L-aspartate were tested as possible inhibitors of the enzyme. These studies suggested that L-aspartate is bound with its carboxy groups in the eclipsed configuration, and that the stereochemical constraints around the binding site are very similar to those reported for the catalytic subunit of the enzyme from Escherichia coli [Davies, Vanaman & Stark (1970) J. Biol. Chem. 245, 1175-1179].

Aspartate Carbamoyltransferase↗
PubMed1979

The adsorption of proteins and protein--dodecyl sulphate complexes on N-(3-carboxypropionyl)aminodecyl-Sepharose.

Equilibrium and kinetic aspects of the binding of several proteins to N-(3-carboxypropionyl)aminodecyl-Sepharose, an amphiphilic ampholytic adsorbent, were studied at 22 degrees C, pH 7.0, I 0.10--0.12. In the absence of detergents Scatchard plots are linear for human haemoglobin and soya-bean trypsin inhibitor, but non-linear for bovine serum albumin, which is also adsorbed more tightly than the other two proteins. The introduction [corrected] of 3.5mM-sodium dodecyl sulphate causes dramatic increases in the amounts and affinities of serum albumin and haemoglobin adsorbed, but has relatively little effect on the trypsin inhibitor. At concentrations of sodium dodecyl sulphate greater than about 10mM there is a fall in the binding of all proteins, owing to competition from the detergent for binding sites on the adsorbent, and a tendency towards more uniform behaviour by different proteins. Kinetic experiments suggest that in the absence of the detergent haemoglobin and serum albumin are adsorbed initially by mainly ionic forces, but that subsequently hydrophobic forces become dominant. Addition of 3.5 mM-sodium dodecyl sulphate causes pronounced changes in the time course of adsorption of haemoglobin and serum albumin, the nature of the changes being different for each protein. The significance of these results is discussed.

Adsorption↗
PubMed1977

Protein chromatography on adsorbents with hydrophobic and ionic groups. Purification of human erythrocyte glycophorin.

Human erythrocyte glycophorin was purified rapidly by (a) chromatography of a Triton X-100 extract of erythrocyte 'ghosts' on N-(3-carboxypropionyl)aminodecyl-Sepharose in buffers containing Triton X-100 or sodium dodecyl sulphate, or (b) chromatography of whole 'ghosts', solubilized in sodium dodecyl sulphate, on dodecyl-Sepharose, in buffers containing sodium dodecyl sulphate. The products contained 85-95% glycophorin (electrophoretic band PAS-1) and the major contaminants were glycoproteins PAS-2 (possibly a subunit of glycophorin) and PAS-3.

Chromatography, Gel↗
PubMed1977

Biospecific-elution chromatography with 'imphilytes' as stationary phases.

Six out of seven enzymes tested (four of them nicotinamide nucleotide-dependent dehydrogenases) showed differences in chromatographic behaviour in the presence and absence of their biospecific ligands, when chromatographed on immobilized amphipathic ampholytes ('imphilytes') as stationary phases. Some enzymes were adsorbed more tightly, others less tightly, in the presence of ligands. These results have implications for enzyme purification in general, and for some types of affinity chromatography in particular.

Alkaline Phosphatase↗
PubMed1976

Protein chromatography on adsorbents with hydrophobic and ionic groups. Chromatography of dodecyl sulphate-solubilized proteins of the human erythrocyte membrane on N-(3-carboxypropionyl)aminodecyl-sepharose.

Human erythrocyte 'ghosts' were solubilized in 0.5% (w/v) sodium dodecyl sulphate at pH 4.0(I = 0.012 mol/I). At a loading of 1-2 mg of protein/ml of column volume, all of membrane proteins were adsorbed to a column of CPAD [N-(3-carboxypropionyl)-aminodecyl]-Sepharose at pH 4.0 (I = 0-012 mol/1) and room temperature (22 degrees C). Many proteins were subsequently desorbed by raising the pH or by including sodium dodecyl sulphate continuously in the eluting buffer. Experiments with a series of adsorbents homologous with CPAD-Sepharose, in which the length of the hydrocarbon chain was varied, provided strong evidence of hydrophobic interactions, in addition to ionic interactions, in the binding of these proteins to CPAD-Sepharose. Elution with increasing-pH gradients at different concentrations of sodium dodecyl sulphate showed that glycophorin (the major sialoglycoprotein) was eluted in the void volume, at recoveries close to 100%, when the detergent concentration was greater than or equal to 0.3% (w/v). Protein E, the major protein, was desorbed late in the pH gradient even at a high (0.5%, w/v) concentration of the detergent, and was always incompletely desorbed, the maximum recovery recorded being 40%. Spectrin (the high-molecular-weight polypeptide pair) did not behave in a well-defined manner, and was found widely distributed among the effluent fractions under all the conditions that were tested.

Alanine↗
PubMed1975

Protein chromatography on adsorbents with hydrophobic and ionic groups. Some properties of N-(3-carboxypropionyl)aminodecyl-sepharose and its interaction with wheat-germ aspartate transcarbamoylase.

1. The charge state of two derivatives of Sepharose prepared by the CNBr activation method were studied by acid-base titration and by ion-exchange chromatography. Dodecyl-Sepharose exhibited cationic groups (21mumol/ml of settled gel; pKa=9.6) that were tentatively assigned to the coupling isourea group. 2. CPAD-Sepharose [N-(3-carboxypropionyl)aminodecyl-Sepharose] has anionic (carboxyl) groups (pKa=4.5) and cationic groups (pKa=9.6) in roughly equal concentrations (e coupling group. CPAD-Sepharose is slightly negatively charged at pH 7.0 and substantially negatively charged at pH 8.5. 3. The pKa values of dodecyl-Sepharose and CPAD-Sepharose are unaffected by a 100-fold increase in the concentration of KCl. 4. CPAD-Sepharose has considerable affinity for wheat-germ aspartate transcarbamoylase at pH 8.5 when the adsorbent and enzyme are both negatively charged. The interaction involves the C10 chain but is relatively moderate compared with C10 chains associated only with positive charge. 5. Desorption of the enzyme adsorbed to CPAD-Sepharose can be achieved by raising the pH to increase the electrostatic repulsion, or by introducing the detergent sodium deoxycholate. Acetone and butan-1-ol also weaken the adsorption at pH 8.5. 6. High concentrations of sodium acetate or sodium phosphate induced the enzyme to bind more tightly to CPAD-Sepharose. 7. These results are discussed in terms of a 'repulsion-controlled' model or hydrophobic chromatography.

Adsorption↗
PubMed1974

Enzyme purification by hydrophobic chromatography: an alternative approach illustrated in the purification of aspartate transcarbamoylase from wheat germ.

Two adsorbents containing similar numbers of hydrocarbon (C(10)) chains but different numbers of carboxyl groups were made by chemical modification of Sepharose. The use of these adsorbents to purify proteins, under conditions where hydrophobic adsorption is partly resisted by electrostatic repulsion, is illustrated in the purification of aspartate transcarbamoylase (EC 2.1.3.2) from wheat germ.

Adsorption↗
PubMed1973

Wheat-germ aspartate transcarbamoylase. The effects of ligands on the inactivation of the enzyme by trypsin and denaturing agents.

In the absence of added ligands aspartate transcarbamoylase (EC 2.1.3.2) from wheat germ is inactivated fairly rapidly by trypsin, by heat (60 degrees C), by highly alkaline conditions (pH11.3) and by sodium dodecyl sulphate. Addition of UMP alone, at low concentrations, decreases the rate of inactivation by each of these agents significantly. Carbamoyl phosphate alone does not alter the rate of inactivation by trypsin and by the detergent, but it antagonizes the effect of UMP in protecting the enzyme against these agents. These results have been interpreted to mean that two conformational states are reversibly accessible to the enzyme, namely an easily inactivated state favoured in the presence of carbamoyl phosphate and a more resistant state favoured in the presence of UMP. In the absence of ligands the enzyme is in the easily inactivated conformation. At very high concentrations l-aspartate also protects the enzyme but to a smaller extent than UMP. Some implications of these results are discussed.

Aspartate Carbamoyltransferase↗
PubMed1972

Wheat-germ aspartate transcarbamoylase. Kinetic behaviour suggesting an allosteric mechanism of regulation.

1. Some kinetic properties of aspartate transcarbamoylase (EC 2.1.3.2), that had been purified approx. 20-fold from wheat germ, were studied. 2. A plot of enzyme activity against pH showed a low maximum at pH8.4 and a second, higher, maximum at pH10.5. A plot of percentage inhibition by 0.2mm-UMP against pH was approximately parallel to the plot of activity against pH, except that between pH6.5 and 7.5 the enzyme was insensitive to 0.2mm-UMP. 3. Kinetics were studied in detail at pH10.0 and 25 degrees C. In the absence of UMP, initial-rate plots were hyperbolic when the concentration of either substrate was varied. UMP decreased both V(max.) and K(m) in plots of initial rate against l-aspartate concentration, but the plots remained hyperbolic. However, UMP converted plots of initial rate against carbamoyl phosphate concentration into a sigmoidal shape, without significantly affecting V(max.). Plots of initial rate against UMP concentration were also sigmoidal. 4. The theoretical model proposed by Monod et al. (1965) gave a partial explanation of these results. When quasi-equilibrium conditions were assumed analysis in terms of this model suggested a trimeric enzyme binding the allosteric ligands, carbamoyl phosphate and UMP, nearly exclusively to the R and T conformational states respectively, and existing predominantly in the R state when ligands were absent. However, the values of the Hill coefficients for the co-operativity of each allosteric ligand were somewhat less than those predicted by the theory. 5. Some of the implications of these results are discussed, and the enzyme is contrasted with the well-known aspartate transcarbamoylase of Escherichia coli.

Allosteric Regulation↗