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A Cornish-Bowden

Publications and source records attributed to A Cornish-Bowden.

At least 19 recordsLinked to original sources
PubMed1992

Response coefficients of interconvertible enzyme cascades towards effectors that act on one or both modifier enzymes.

Explicit expressions have been derived for the response coefficients for the effect of activator and inhibitor concentrations on the fraction in the active state of the target enzyme of a monocyclic interconvertible enzyme cascade. These allow one to assess the adequacy of such a cascade for producing a highly sensitive response to an effector. Numerical studies indicate that this type of system can readily generate response coefficients of about seven, even without requiring both modification reactions to be modulated simultaneously, and without requiring all of the parameters that characterize the system to have their optimum values. Thus, a monocyclic cascade can constitute a highly effective on/off switching device in a linear pathway.

Catalysis↗
PubMed1991

Quantitative assessment of regulation in metabolic systems.

We show how metabolic regulation as commonly understood in biochemistry can be described in terms of metabolic control analysis. The steady-state values of the variables of metabolic systems (fluxes and concentrations) are determined by a set of parameters. Some of these parameters are concentrations that are set by the environment of the system; they can act as external regulators by communicating changes in the environment to the metabolic system. How effectively a system is regulated depends both on the degree to which the activity of the regulatory enzyme with which a regulator interacts directly can be altered by the regulator (its regulability) and on the ability of the regulatory enzyme to transmit the changes to the rest of the system (its regulatory capacity). The regulatory response of a system also depends on its internal organisation around key variable metabolites that act as internal regulators. The regulatory performance of the system can be judged in terms of how sensitivity the fluxes respond to the external stimulus and to what degree homeostasis in the concentrations of the internal regulators is maintained. We show how, on the level of both external and internal regulation, regulability can be quantified in terms of an elasticity coefficient and regulatory capacity in terms of a control coefficient. Metabolic regulation can therefore be described in terms of metabolic control analysis. The combined response relationship of control analysis relates regulability and regulatory capacity and allows quantification of the regulatory importance of the various interactions of regulators with enzymes in the system. On this basis we propose a quantitative terminology and analysis of metabolic regulation that shows what we should measure experimentally and how we should interpret the results. Analysis and numerical simulation of a simple model system serves to demonstrate our treatment.

Animals↗
PubMed1991

Failure of channelling to maintain low concentrations of metabolic intermediates.

Computer modelling has been used to investigate the effect of direct transfer of metabolites between consecutive enzymes (channelling) on the free concentrations of the channelled metabolites. When a channelled intermediate cannot participate in any other reactions, any increase in channelling tends to increase its free concentration, albeit very slightly, unless the increase in net flux brought about by the channel is compensated for by a simultaneous decrease in the activity of the route through the free intermediate, in which case channelling has no effect at all on the free steady-state concentration of the channelled intermediate. If the free intermediate is capable of participating in side reactions, channelling can decrease these side reactions, but only slightly unless virtually all of the final product results from flux through the channel and the rate constants for the direct pathway are virtually zero. In general, channelling appears not to provide a useful mechanism for maintaining intermediate concentrations at low levels.

Computer Simulation↗
PubMed1991

Hexokinase and 'glucokinase' in liver metabolism.

Rat liver contains four hexokinase isoenzymes, one of which, despite often being called 'glucokinase', is no more specific for glucose than the others. However, it does differ from them in displaying a sigmoid kinetic response to glucose, requiring much higher glucose concentrations for activity, and being insensitive to physiological concentrations of glucose 6-phosphate.

Animals↗
PubMed1991

MetaModel: a program for modelling and control analysis of metabolic pathways on the IBM PC and compatibles.

MetaModel is a user-friendly program for calculating steady-state fluxes and metabolite concentrations of metabolic systems on the IBM PC and compatible computers. For any steady state that is obtained, one can then calculate a matrix of elasticity coefficients at that steady state, or a matrix of control and response coefficients. It thus offers a simple way to calculate the control structure of a pathway: it provides not only an educational tool that allows the student to verify empirically the classic summation relationships of metabolic control analysis but also a research tool for addressing 'what if?' questions about the behaviour of metabolic systems. Results can not only be printed or stored in a file, but can also be written to a special file that can be read by popular spreadsheet programs, thereby giving access to rapid, flexible and powerful methods for subsequent analysis and plotting of these results.

Algorithms↗
PubMed1990

Very large response coefficients in interconvertible enzyme cascades.

Explicit expressions have been derived for the response coefficients for the effect of activator and inhibitor concentrations on the fraction in the active state of the target enzyme of a monocyclic interconvertible enzyme cascade. These allow one to assess the adequacy of such a cascade for producing a highly sensitive response to an effector. Numerical studies indicate that this type of system can readily generate response coefficients of about 7, even without requiring both modification reactions to be modulated simultaneously, and without requiring all of the parameters that characterize the system to have their optimum values.

Biotransformation↗
PubMed1989

Metabolic control therapy and biochemical systems theory: different objectives, different assumptions, different results.

The claim by Savageau et al. (1987 a, b, Math. Biosci. 86, 127-145, 147-167) that the theory of metabolic control associated with Kacser & Burns (1973, Symp. Soc. Exp. Biol. 27, 65-104) and with Heinrich & Rapoport (1974, Eur. J. Biochem. 42, 89-102) is no more than a special case of the biochemical systems theory of Savageau and colleagues is examined. It is shown to be based on a misconception of the objectives and assumptions of metabolic control theory. In particular, the control and elasticity coefficients that play a central role in metabolic control theory are not constants and cannot be treated as constants. Consequently they cannot in general be equated with the kinetic orders that appear in biochemical systems theory, though they do correspond at the point where the two theories are tangential to one another.

Animals↗
PubMed1979

Mammalian hexokinases: a system for the study of co-operativity in monomeric enzymes.

Kinetic and structural studies have been carried out of two isoenzymes of hexokinase from the rat, hexokinase II and glucokinase. Although both enzymes are monomeric, hexokinase II has a molecular weight double that of glucokinase and resembles a dimer of glucokinase. The co-operativity of glucokinase, which is not observed for hexokinase II, appears to be kinetic in origin rather than the consequence of ineractions between distinct glucose-binding sites.

Animals↗
PubMed1978

Evaluation of rate constants for enzyme-catalysed reactions by the jackknife technique. Application to liver alcohol dehydrogenase.

Steady-state measurements of enzyme-catalysed reactions are capable of providing more information about the rate constants of the individual steps than is commonly obtained. We have applied a combination of the jackknife and non-linear regression techniques to measurements of the rate of oxidation of ethanol by NAD+, catalysed by alcohol dehydrogenase from horse liver. This has permitted values and confidence intervals to be assigned to the eight rate constants that characterize the binding of ethanol and NAD+ in random order to the enzyme, and to the net rate constant kcat. for the breakdown of the ternary complex.

Alcohol Oxidoreductases↗
PubMed1978

Purification and properties of nitrite reductase from Escherichia coli K12.

NADH-nitrite oxidoreductase (EC 1.6.4) was purified to better than 95% homogeneity from batch cultures of Escherichia coli strain OR75Ch15, which is partially constitutive for nitrite reductase synthesis. Yields of purified enzyme were low, mainly because of a large loss of activity during chromatography on DEAE-cellulose. The quantitative separation of cytochrome c-552 from nitrite reductase activity resulted in an increase in the specific activity of the enzyme: this cytochrome is not therefore an integral part of nitrite reductase. The subunit molecular weights of nitrite reductase and of a haemoprotein contaminant, as determined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, were 88000 and 80000 respectively. The sedimentation coefficient was calculated to be in the range 8.5-9.5S, consistent with a mol.wt. of 190000. It is suggested therefore that the native enzyme is a dimer with two identical or similar-sized subunits. Purest samples contained 0.4 mol of flavin/mol of enzyme, but no detectable haem. Catalytic activity was totally inhibited by 20 micron-p-chloromercuribenzoate and 1 mM-cyanide, slightly inhibited by 1 micron-sulphite and 10mM-arsenite, but insensitive to 1 mM-2,2'-bipyridine, 4mM-1,10-phenanthroline and 10mM-NaN3. Three molecules of NADH were oxidized for each NO2-ion reduced: the product of the reaction is therefore assumed to be NH4+. The specific activity of hydroxylamine reductase increased at each step in the purification of nitrite reductase, and the elution profiles for these two activities during chromatography on DEAE-Sephadex were coincident. It is likely that a single enzyme is responsible for both activities.

Centrifugation, Density Gradient↗
PubMed1978

Activation of nitrite reductase from Escherichia coli K12 by oxidized nicotinamide-adenine dinucleotide.

Nitrite reductase from Escherichia coli K12 requires the presence of NAD+, one of the products of the reduction of NO2-by NADH, for full activity. The effect is observed with both crude extracts and purified enzyme. NAD+ also acts as a product inhibitor at high concentrations, and plots of initial rate against NAD+ concentration are bell-shaped. The maximum occurs at about 1 mM-NAD+, but increases with increasing NADH concentration. In the presence of 1 mM-NAD+ and saturating NO2-(2mM) the Michaelis constant for NADH is about 16 micron. The Michaelis constant for NO2-is about 5 micron and is largely independent of the NAD+ concentration. Similar but more pronounced effects of NAD+ are observed with hydroxylamine as electron acceptor instead of NO2-. The maximum rate of NADH oxidation by hydroxylamine is about 5.4 times greater than the maximum rate of NADH oxidation by NO2- when assayed with the same volume of the same preparation of purified enzyme. The Michaelis constant for hydroxylamine is 5.3 mM, however, about 1000 times higher than for NO2-. These results are consistent with a mechanism in which the same enzyme-hydroxylamine complex occurs as an intermediate in both reactions.

Enzyme Activation↗