Enzyme-enzyme interactions and their metabolic role

The dynamics of local kinetic parameters of glutamate dehydrogenase in rat liver

Kinetic parameters of glutamate dehydrogenase (GDH, EC 1.4.1.2) for glutamate were determined in periportal and pericentral zones of adult male and female rat liver lobules under normal fed conditions and after starvation for 24 h. GDH activity was measured as formazan production over time against a range of glutamate concentrations in serial cryostat sections using image analysis. Captured gray value images were transformed to absorbance images and local initial velocities (Vini) were calculated. A hyperbolic function was used to describe the relationship between substrate concentration and local Vini. Under fed conditions, Vmax values were similar in male and female rats (8 +/- 2 and 16 +/- 2 mumol min-1 cm-3 liver tissue in periportal and pericentral zones, respectively). Starvation increased Vmax, especially in pericentral zones of females (to 27 +/- 1 mumol min-1 cm-3 liver tissue). Under fed conditions, the affinity of GDH for glutamate was similar in male and female rats (2.5 +/- 0.5 mM and 3.5 +/- 0.8 mM in periportal and pericentral zones, respectively). Starvation had no effect on K(m) values in male rats, but in female rats affinity for glutamate decreased significantly in both zones (K(m) values of 4.0 +/- 0.1 mM and 8.6 +/- 0.8 mM, respectively). These local changes in the kinetic parameters of GDH indicate that conversion of glutamate to alpha-oxoglutarate cannot be predicted on the basis of GDH concentrations or zero-order activity in the different zones of liver lobules alone.

Why and when channelling can decrease pool size at constant net flux in a simple dynamic channel

Cornish-Bowden and Cárdenas (Cornish-Bowden, A. and Cárdenas M.L. (1993) Eur. J. Biochem. 213, 87-92) have suggested that simulation results peviously published by us (Mendes, P., Kell, D.B. and Westerhoff, H.V. (1992) Eur. J. Biochem. 204, 255-266) which had demonstrated that large reductions of intermediate pool sizes could be accompanied by increasing channel flux in a model metabolic pathway, were an artefact of changes in the pathway's overall flux of the order of 0.0075%, or of inappropriate alterations of enzyme activities. They also asserted to prove that the "channelling of an intermediate cannot affect its free concentration at constant net flux". We consider the co-response of the intermediate metabolite concentration ('pool') and the channel flux to changes in kinetic (or thermodynamic) parameters. Both by analytical proofs and by numerical examples we show that this co-response can be positive, negative or null, depending on the parameter change. In particular, we prove that there is always a number of ways of changing parameters such that the intermediate metabolite concentration decreases with increasing channel flux, whether the total flux varies or is constant. We also show that increased stability of the (dynamic) enzyme-intermediate-enzyme complex, as well as a single parameter change that similarly displays no cross-over effects, can lead to decreased intermediate metabolite concentration and increased channel flux at constant total flux. In general, a non-zero co-response of the intermediate metabolite concentration ('pool') and the channel flux to changes in kinetic (or other) parameters is the rule rather than the exception. More specifically: (i) The algebraic analysis ('general proof') given in Cornish-Bowden and Cárdenas (1993) contains the constraint that the elasticities of various steps to the modulation parameters which were used to vary the channel flux at constant net flux were unity. This is an unfortunate and unnecessary constraint which, when lifted, means that the concentration of the pool in the general case can indeed change at constant net flux. A 'simplified proof' given in Cornish-Bowden and Cárdenas (1993) also fails, due in addition to the consequent failure to include mass conservation relations for some of the enzymes. (ii) In the systems studied by Cornish-Bowden and Cárdenas (1993), flux is properly to be considered as a variable (since it varies during the transition to the steady state), and not a parameter, and as such cannot per se affect the magnitude of other variables in the steady state. (iii) By relaxing the constraint referred to in (i), above, and by making dual modulations (i.e., of more than one parameter at once) which are different from those carried out in Cornish-Bowden and Cárdenas (1993) we find many instances in which channelling (described by a parameter p) does significantly affect the concentration of the pool intermediate C at constant total flux. (iv) In the same pathways, but in which the flux is held constant by setting it via a zero-order flux-generating reaction, the addition of a channel is also able to significantly to modulate the size of the pool at constant total flux. Our results show that the effectiveness of channelling in decreasing a pool, even at constant flux, is very much a reality.

Control theory of metabolic channelling

Various factors appear to control muscle energetics, often in conjunction. This calls for a quantitative approach of the type provided by Metabolic Control Analysis for intermediary metabolism and mitochondrial oxidative phosphorylation. To the extent that direct transfer of high energy phosphates and spatial organization plays a role in muscle energetics however, the standard Metabolic Control Theory does not apply, neither do its theorems regarding control. This paper develops the Control Theory that does apply to the muscle system. It shows that direct transfer of high energy phosphates bestows a system with enhanced control: the sum of the control exerted by the participating enzymes on the flux of free energy from the mitochondrial matrix to the actinomyosin may well exceed the 100% mandatory for ideal metabolic pathways. It is also shown how sequestration of high energy phosphates may allow for negative control on pathway flux. The new control theory gives method functionally to diagnose the extent to which channelling and metabolite sequestration occur.

Microcompartmentation, metabolic channelling and carbohydrate metabolism

The inter-organelle cytoplasm of eukaryotic cells was once considered to be a homogeneous solution in which many of the enzymes of intermediary metabolism are soluble; however, advances in cell biology have revealed an intricate picture at the microscopic level of cytoplasm structure. Consequently, a great deal of constraint is required when extrapolating to the intact cell from enzyme studies in vitro, a point made frequently in the literature of the last decade or so. The idea of spatial organization is now accepted and covers a wide variety of local microenvironments and possibly localized metabolic channelling. The latter, although accepted as a phenomenon, is controversial in terms of its physiological significance. This review covers evidences showing that both glycolytic and glycogenolytic enzymes are microcompartmentalized. The potential significance of this compartmentation appears to involve metabolic chanelling, a process by which rearrangement of enzymes on a dynamic cytomatrix leads to "channels" in which metabolic substrates are passed from one enzyme to the next. The combined effects of such enzyme proximity and their activation as a result of the altered kinetic properties conferred upon the enzymes by their cytoskeletal associations favours maximal rate of reaction. These and other aspects of microcompartmentation and metabolic channelling are discussed.

Analysis of enzyme reactions in situ

Estimations of metabolic rates in cells and tissues and their regulation on the basis of kinetic properties of enzymes in diluted solutions may not be applicable to intact living cells or tissues. Enzymes often behave differently in living cells because of the high cellular protein content that can lead to homologous and heterologous associations of protein molecules. These associations often change the kinetics of enzymes as part of post-translational regulation mechanisms. An overview is given of these interactions between enzyme molecules or between enzyme molecules and structural elements in the cell, such as the cytoskeleton. Biochemical and histochemical methods are discussed that have been developed for in vivo and in situ analyses of enzyme reactions, particularly for the study of effects of molecular interactions. Quantitative (histochemical) analysis of local enzyme reactions or fluxes of metabolites has become increasingly important. At present, it is possible to calculate local concentrations of substrates in cells or tissue compartments and to express local kinetic parameters in units that are directly comparable with those obtained by biochemical assays of enzymes in suspensions. In situ analysis of the activities of a number of enzymes have revealed variations in their kinetic properties (Km and Vmax) in different tissue compartments. This stresses the importance of in vivo or in situ analyses of cellular metabolism. Finally, histochemical determinations of enzyme activity in parallel with immunohistochemistry for the detection of the total number of enzyme molecules and in situ hybridization of its messenger RNA allow the analysis of regulation mechanisms at all levels between transcription of the gene and post-translational activity modulation.

Developmental variations in the interactions of pyruvate kinase and glyceraldehyde-3-phosphate dehydrogenase with subcellular structure in cavian tissues

The activities and interactions with cellular structure of glyceraldehyde-3-phosphate dehydrogenase and pyruvate kinase have been studied in the major tissues of the guinea pig during development. The extent of activity variation in these tissues is described along with the putative physiological determinants of these alterations in activity. As to binding, overall the present data provide a firm indication that the extent of enzyme-structure interactions is appreciable at all ontogenic stages, and when viewed in conjunction with other parallel studies on other enzymes and other animals, serve to confirm the broad biological significance of enzyme-structure associations in the compartmentation of glycolysis. The existence and significance of genetic and epigenetic modifications of these enzymes during development is also discussed.

Control theory of metabolic channelling

Various factors appear to control muscle energetics, often in conjunction. This calls for a quantitative approach of the type provided by Metabolic Control Analysis for intermediary metabolism and mitochondrial oxidative phosphorylation. To the extent that direct transfer of high energy phosphates and spatial organization plays a role in muscle energetics however, the standard Metabolic Control Theory does not apply, neither do its theorems regarding control. This chapter develops the Control Theory that does apply to the muscle system. It shows that direct transfer of high energy phosphates bestows a system with enhanced control: the sum of the control exerted by the participating enzymes on the flux of free energy form the mitochondrial matrix to the actinomyosin may well exceed the 100% mandatory for ideal metabolic pathways. It is also shown how sequestration of high energy phosphates may allow for negative control on pathway flux. The new control theory gives methods functionally to diagnose the extent to which channelling and metabolite sequestration occur.

Coordination of catalytic activities within enzyme complexes

If two enzymes are physically and permanently associated as a bi-enzyme complex and if these enzymes catalyze non-consecutive chemical reactions, either of these reactions may inhibit or activate the other. If these reactions belong to two different metabolic cycles, the functioning of one of these cycles will control the fine tuning of the other. Thus simple kinetic considerations lead to the conclusion that, owing to the spatial organization of enzymes as multimolecular complexes, a fine tuning and a coordination of different metabolic networks, or cycles, may be exerted. It thus appears that channelling of reaction intermediates within a multienzyme complex does not represent the only functional advantage brought about by this type of spatial molecular organization.

Plasma arginine kinetics in adult man: response to an arginine-free diet

To explore the response of whole-body arginine metabolism to a change in arginine intake, plasma arginine kinetics were investigated in eight healthy adult men who received an L-amino acid diet supplying an Arg-rich or Arg-free intake for 6 days before undergoing a tracer study on day 7. The tracer protocol lasted for 8 hours. For the first 3 hours subjects remained in the postabsorptive (fasted) state, and during the following 5 hours they consumed small meals at 30-minute intervals. Primed continuous intravenous infusions of L-[guanidino-13C]arginine, L-[5,5,5-2H3]leucine, and [15N2]urea were administered to estimate plasma amino acid fluxes and the rate of urea production. For the fasted and fed states, plasma arginine fluxes (mumol.kg-1.h-1, mean +/- SD) were 69 +/- 8 and 87 +/- 12 (P < .01), respectively, for the Arg-rich diet and 63 +/- 14 and 51 +/- 7 (P < .01, from Arg-rich) for the Arg-free diet. Compared with the Arg-rich results, fed-state plasma arginine and ornithine concentrations were decreased (P < .01) and citrulline concentration was increased (P < .01) during the Arg-free diet period. Leucine fluxes and rates of urea production did not differ between the diet groups. The lower fed-state arginine flux in subjects receiving the Arg-free compared with the Arg-rich diet appears to be entirely due to the decreased rate of entry of arginine from the intestine in the former group.(ABSTRACT TRUNCATED AT 250 WORDS)

Immobilized enzymes as tools for the demonstration of metabolon formation. A short overview

In recent years it has become clear that a cell cannot be visualized as a 'bag' filled with enzymes dissolved in bulk water. The aqueous-phase properties in the interior of a cell are, indeed, essentially different from those of an ordinary aqueous solution. Large amounts of water are believed to be organized in layers at the surface of intracellular structural proteins and membranes. Such considerations prompt us to reconsider the operation and regulation of metabolic pathways. Enzymes of metabolic pathways are nowadays thought to be clustered and operate as 'metabolons'. Very often interactions between enzymes of a pathway can exclusively be evidenced in vitro in media which are known to reduce the water concentration in the vicinity of the proteins. Immobilized enzyme preparations have been shown to be excellent tools for this type of research. We describe here some recent studies where immobilized enzymes have been used in various applications to investigate associations among enzymes of a number of different metabolic pathways (glycolysis/gluconeogenesis, citric acid cycle and its connection to the electron transport chain, aspartate-malate shuttle, glyoxylate cycle). Advantages and disadvantages of the different techniques are also discussed.

Structural and functional properties of a multi-enzyme complex from spinach chloroplasts. 2. Modulation of the kinetic properties of enzymes in the aggregated state

The carboxylase activity of free ribulose 1,5-bisphosphate carboxylase-oxygenase has been compared to that of the five-enzyme complex present in chloroplasts. Kinetic results have shown that the V/active site is lower for the free enzyme than for the complex. Conversely the Km is smaller for the complex than for the free enzyme. This implies that the catalytic activity of the enzyme is enhanced when it is embedded in the complex. Under reducing conditions and in the presence of reduced thioredoxin, inactive oxidized phosphoribulokinase, free in solution or inserted in the multi-enzyme complex, becomes active. The kinetics of this activation process has been studied and shown to be exponential. The time constant of this exponential decreases, for the free enzyme, as thioredoxin concentration is increased. Alternatively, for the enzyme embedded in the complex, this time constant increases with thioredoxin concentration almost in a linear fashion. This implies that the complex is much more rapidly activated by reduced thioredoxin than is the free phosphoribulokinase. The variation of the amplitude of this activation process as a function of thioredoxin concentration is a hyperbola. The concentration of thioredoxin which results in half the asymptotic value of this hyperbola is smaller for the complex than for the free enzyme. A kinetic model has been proposed and the dynamic equations resulting from this model have been derived. They fit the experimental results exactly. From the variation of the amplitude of the activation process one may derive the binding constants of thioredoxin on either the oxidized enzyme or on a partly dithiothreitol-reduced enzyme (both of them free or inserted in the complex). In either case, the affinity of reduced thioredoxin is larger for the complex than for the free enzyme. The individual values of some of the rate constants have also been estimated from the variation of the time constants as a function of thioredoxin concentration. Taken together, these results show that at least two enzymes, ribulose 1,5-bisphosphate carboxylase-oxygenase and phosphoribulokinase, have quite different kinetic properties depending on whether they are in free solution or embedded in the multi-enzyme complex.

Dietary arginine uptake by the splanchnic region in adult humans

To determine the uptake of dietary arginine and leucine by the splanchnic region, two experiments were carried out, each involving four healthy young adult men who received a diet supplying 1 g protein.kg-1.day-1 for 7 and 10 days before conducting a primed constant tracer infusion protocol. In study 1, subjects received for 8 h (3-h fast; 5-h fed state, achieved by a constant intragastric infusion of the diet formula) L-[5,5-2H2; guanidino-15N2]arginine ([M4]Arg), L-[guanidino-13C]arginine ([13C]Arg), and L-[5,5,5-2H3]leucine ([2H3]Leu) simultaneously by an intragastric infusion on day 7 and a repeat of this protocol on day 10 except with tracer administration given by vein. Plasma arginine fluxes were essentially the same for the two arginine tracers but differed significantly with route of administration. In study 2 the subjects received on day 7 a constant intravenous infusion of [13C]Arg and [2H3]Leu and a simultaneous intragastric infusion of [M4]Arg and [1-13C]leucine. On day 10 the routes of administration of these tracer pairs were reversed. During the fed state in study 1, splanchnic uptake of dietary arginine was 31 +/- 10 and 34 +/- 8%, based on the [13C]Arg and [M4]Arg tracers, respectively, and it was significantly higher (P < 0.01) than for leucine, which was 10 +/- 6%. In study 2, splanchnic uptake of dietary arginine, estimated from a series of tracer-protocol combinations for the fed state, was approximately 38% compared with a lower (P < 0.01) value of approximately 15% for leucine.

Metabolic channelling and control of the flux

Metabolic control theory is extended to include channelled metabolism in general. A simple relationship between the flux control by the enzymes and the degree of metabolite channelling is derived. This relationship suggests experiments in which modulation of gene expression allows one to quantify channelling.

Splanchnic metabolism of dietary arginine in relation to nitric oxide synthesis in normal adult man

Urinary nitrate (NO3) is the stable end product of nitric oxide, which is formed, in turn, from a guanidino nitrogen of arginine. We have conducted two experiments, each in four healthy adult men receiving a low nitrate diet for 7-10 days, to investigate the in vivo conversion of arginine to nitrate. In the first study [guanidino-15N2, 5,5-2H2]arginine was given on day 7 via a primed continuous intravenous infusion for 8 h. In the second study, the labeled arginine was given for 8 h by the intragastric route on day 7 and by the intravenous route on day 10. Measurement of 15NO3 output in urine collected for 24 h beginning at the time of the arginine tracer infusion revealed a more extensive transfer of 15N when the arginine tracer was given intragastricly. From the comparative labeling of 15NO3 after administration of the tracer arginine via the intragastric and intravenous routes, we estimate that 16% +/- 2% of the daily production of nitrate arises from the metabolism of dietary arginine that is taken up during its "first pass" in the splanchnic region. Hence, nitric oxide production occurs, to a measurable extent, in this area in healthy subjects, raising the question as to how various pathophysiological states might alter the relations between exogenous and endogenous sources of arginine as precursors of NO. and the relative contributions made by various organs to whole body (NO.) NO3 formation. These results also raise important questions about the use of nitric oxide synthase inhibitors in animal and human studies.

Evidence for an interaction between cytosolic aldolase and the ATP-and pyrophosphate-dependent phosphofructokinases in carrot storage roots

Immunoaffinity chromatography was employed to identify potential plant cytosolic aldolase (ALDc) binding proteins. A clarified homogenate of carrot storage root was chromatographed on a column of protein-A-Sepharose that had been covalently coupled to anti-(carrot root ALDc) immunoglobulin G. The column was washed with phosphate-buffered saline (PBS), followed by step-wise elution with increasing concentrations of NaCl in PBS. Several proteins were eluted following application of the salt gradient. Western blotting identified the major eluting proteins to be the PPi-dependent phosphofructokinase (PFP) and the cytosolic form of the ATP-dependent phosphofructokinase (PFKc), enzymes that are metabolically sequential to ALDc. The results suggest that ALDc may specifically interact with PFP and PFKc in carrots.

Ontogenic characteristics of cavian aldolase

In order to extend the available information on the ontogenic significance of the interactions between aldolase and cellular structure, the nature and extent of these associations have been studied in the tissues of the guinea pig during development, along with analyses of the isozyme status in the bound and soluble compartments. In all tissues investigated, a significant degree of binding was evident, along with a considerable variation in the degree of association of aldolase with structure during development. Binding was particularly extensive in the early foetal stages and, in general, binding preference was directed towards A-type activity over the B- and C-type of enzyme. The significance of these ontogenic phenomena have been discussed in relation to the variations in phenotype of individual tissues during maturation and the metabolic correlations of this biphasic micro-organization.

Microenvironmental factors and the binding of glycolytic enzymes to contractile filaments

1. In reviewing the microenvironmental factors involved in the binding of the glycolytic enzymes to contractile filaments, consideration has been given to the significance of molecular crowding in maintaining these interactions under cellular conditions, and the influence of hormones, metabolites, pH and enzyme modifications on these phenomena. 2. Overall, these data serve to emphasize the biological reality of these associations, and their micro-organizational adaptations during physiological activities.

Channelling can decrease pool size

It is widely considered that a possible advantage of metabolite channelling, in which a product of an enzyme is transferred to the next enzyme in a metabolic pathway without being released to the 'bulk' solution, is that channelling can decrease the steady-state concentrations of 'pool' intermediates. This then spares the limited solvent capacity of the cell, and reduces the loss of pathway flux due to leakage or instability of the free intermediate. Recently, however, based on simulations of a particular model of a 'dynamic' channel, Cornish-Bowden ["Failure of channelling to maintain low concentrations of metabolic intermediates" (1991) Eur. J. Biochem. 195, 103-108] has argued that this is not in fact the case; his simulations indicated that the channel was rather ineffective at decreasing the concentration of the pool intermediate, and in some cases actually increased it. However, although his simulations were restricted to very specific thermodynamic and kinetic parameters, he generalised his conclusions, arguing that "channelling has no effect on the free concentration of a channelled intermediate in a pathway". By showing that, for a number of kinetic cases, the concentration of the pool intermediate did decrease substantially with increased channelling, we demonstrate here that the conclusion of Cornish-Bowden is not correct. In particular, if the reaction catalysed by the enzymes forming the channel has an equilibrium constant K higher than 1, and if the enzyme removing the product of the channel reaction is kinetically competent, channelling in the model system studied by Cornish-Bowden (1991) can decrease the steady-state concentration of the pool by a factor of 1000, independently of the mechanism of the terminal reaction and under conditions of essentially constant overall flux. If the channel is a 'static' channel, the decrease in the pool can be to arbitrarily low levels. This conclusion also holds for a system in which other reactions may consume the pool intermediate. Thus, channelling can maintain metabolite concentrations at low levels.

Interactions between mitochondria and cytoplasm in isolated hepatocytes

Results from a wide variety of metabolic studies have provided indirect support for conclusions derived from enzymological approaches that the enzymes of the so-called soluble cytoplasm (and the mitochondrial matrix) exist within the cell and function in the form of multienzyme complexes and that metabolite channeling takes place between the enzymes of each complex. Our studies support the possibility that the enzymes of glycolysis in liver are segregated from those of gluconeogenesis. Thus, the segregation and aggregation of Krebs cycle enzymes in the mitochondrial matrix, elucidated by Paul Srere, may be an example of a general pattern of enzyme organization pertaining to all metabolic pathways.

Human kidney hexosaminidase A and hexosaminidase B form a complex

The isolation and purification of human kidney hexosaminidases A and B was carried out. Regulation of the supramolecular organization and catalytic activity of hexosaminidases was investigated in the bis(2-ethylhexyl)sulphosuccinate reversed micellar system modeling the enzyme microenvironment in the lysosomes. It was shown that hexosaminidases A and B associate forming 280-300-kDa dimeric complexes under these conditions. At pH 4.75, the hexosaminidases A and B can be isolated from kidney tissue only in the form of this complex.