Interactions between soluble enzymes and subcellular structure

Site-specific crosslinking reveals Phosphofructokinase-L inhibition drives self-assembly and attenuation of protein interactions

Phosphofructokinase is the central enzyme in glycolysis and constitutes a highly regulated step. The liver isoform (PFKL) compartmentalizes during activation and inhibition in vitro and in vivo, respectively. Compartmentalized PFKL is hypothesized to modulate metabolic flux consistent with its central role as the rate limiting step in glycolysis. PFKL tetramers self-assemble at two interfaces in the monomer (interface 1 and 2), yet how these interfaces contribute to PFKL compartmentalization and drive protein interactions remains unclear. Here, we used site-specific incorporation of noncanonical photocrosslinking amino acids to identify PFKL interactors at interface 1, 2, and the active site. Tandem mass tag-based quantitative interactomics reveals interface 2 as a hotspot for PFKL interactions, particularly with cytoskeletal, glycolytic, and carbohydrate derivative metabolic proteins. Furthermore, PFKL compartmentalization into puncta was observed in human cells using citrate inhibition. Puncta formation attenuated crosslinked protein-protein interactions with the cytoskeleton at interface 2. This result suggests that PFKL compartmentalization sequesters interface 2, but not interface 1, and may modulate associated protein assemblies with the cytoskeleton.

Site-Specific Crosslinking Reveals Phosphofructokinase-L Inhibition Drives Self-Assembly and Attenuation of Protein Interactions

Phosphofructokinase is the central enzyme in glycolysis and constitutes a highly regulated step. The liver isoform (PFKL) compartmentalizes during activation and inhibition in vitro and in vivo respectively. Compartmentalized PFKL is hypothesized to modulate metabolic flux consistent with its central role as the rate limiting step in glycolysis. PFKL tetramers self-assemble at two interfaces in the monomer (interface 1 and 2), yet how these interfaces contribute to PFKL compartmentalization and drive protein interactions remains unclear. Here, we used site-specific incorporation of noncanonical photocrosslinking amino acids to identify PFKL interactors at interface 1, 2, and the active site. Tandem mass tag-based quantitative interactomics reveals interface 2 as a hotspot for PFKL interactions, particularly with cytoskeletal, glycolytic, and carbohydrate derivative metabolic proteins. Furthermore, PFKL compartmentalization into puncta was observed in human cells using citrate inhibition. Puncta formation attenuated crosslinked protein-protein interactions with the cytoskeleton at interface 2. This result suggests that PFKL compartmentalization sequesters interface 2, but not interface 1, and may modulate associated protein assemblies with the cytoskeleton.

Stable and Temporary Enzyme Complexes and Metabolons Involved in Energy and Redox Metabolism

Significance: Alongside well-characterized permanent multimeric enzymes and multienzyme complexes, relatively unstable transient enzyme-enzyme assemblies, including metabolons, provide an important mechanism for the regulation of energy and redox metabolism. Critical Issues: Despite the fact that enzyme-enzyme assemblies have been proposed for many decades and experimentally analyzed for at least 40 years, there are very few pathways for which unequivocal evidence for the presence of metabolite channeling, the most frequently evoked reason for their formation, has been provided. Further, in contrast to the stronger, permanent interactions for which a deep understanding of the subunit interface exists, the mechanism(s) underlying transient enzyme-enzyme interactions remain poorly studied. Recent Advances: The widespread adoption of proteomic and cell biological approaches to characterize protein-protein interaction is defining an ever-increasing number of enzyme-enzyme assemblies as well as enzyme-protein interactions that likely identify factors which stabilize such complexes. Moreover, the use of microfluidic technologies provided compelling support of a role for substrate-specific chemotaxis in complex assemblies. Future Directions: Embracing current and developing technologies should render the delineation of metabolons from other enzyme-enzyme complexes more facile. In parallel, attempts to confirm that the findings reported in microfluidic systems are, indeed, representative of the cellular situation will be critical to understanding the physiological circumstances requiring and evoking dynamic changes in the levels of the various transient enzyme-enzyme assemblies of the cell. Antioxid. Redox Signal. 35, 788-807.

Metabolons, enzyme-enzyme assemblies that mediate substrate channeling, and their roles in plant metabolism

Metabolons are transient multi-protein complexes of sequential enzymes that mediate substrate channeling. They differ from multi-enzyme complexes in that they are dynamic, rather than permanent, and as such have considerably lower dissociation constants. Despite the fact that a huge number of metabolons have been suggested to exist in plants, most of these claims are erroneous as only a handful of these have been proven to channel metabolites. We believe that physical protein-protein interactions between consecutive enzymes of a pathway should rather be called enzyme-enzyme assemblies. In this review, we describe how metabolons are generally assembled by transient interactions and held together by both structural elements and non-covalent interactions. Experimental evidence for their existence comes from protein-protein interaction studies, which indicate that the enzymes physically interact, and direct substrate channeling measurements, which indicate that they functionally interact. Unfortunately, advances in cell biology and proteomics have far outstripped those in classical enzymology and flux measurements, rendering most reports reliant purely on interactome studies. Recent developments in co-fractionation mass spectrometry will likely further exacerbate this bias. Given this, only dynamic enzyme-enzyme assemblies in which both physical and functional interactions have been demonstrated should be termed metabolons. We discuss the level of evidence for the manifold plant pathways that have been postulated to contain metabolons and then list examples in both primary and secondary metabolism for which strong evidence has been provided to support these claims. In doing so, we pay particular attention to experimental and mathematical approaches to study metabolons as well as complexities that arise in attempting to follow them. Finally, we discuss perspectives for improving our understanding of these fascinating but enigmatic interactions.

Passing the Baton: Substrate Channelling in Respiratory Metabolism

Despite species-specific differences in the pathways of respiratory metabolism are remarkably conserved across the kingdoms of life with glycolysis, the tricarboxylic acid cycle, and mitochondrial electron transport chain representing the major components of the process in the vast majority of organisms. In addition to being of critical importance in fueling life itself these pathways serve as interesting case studies for substrate channelling with research on this theme having been carried out for over 40 years. Here we provide a cross-kingdom review of the ample evidence for protein-protein interaction and enzyme assemblies within the three component pathways as well as describing the scarcer available evidence for substrate channelling itself.

Revisiting the microtrabecular lattice

The 'microtrabecular lattice' (MTL) that Keith Porter described in the 1970s and 1980s is reconsidered as a proposed fundamental cytoplasmic structure of eukaryotic cells. Although considered to be an artefact by most cell biologists of his time (and probably ours), the case is made that something like the MTL may well exist, but in a much more dynamic form than images from electron microscopy imply and convey.

Simple method for isolation of glyceraldehyde 3-phosphate dehydrogenase and the improvement of myofibril gel properties

Porcine glycolytic enzyme, glyceraldehyde 3-phosphate dehydrogenase (G3PD) was prepared effectively by a combination of ethylene diamine tetra-acetate (EDTA) pretreatment and affinity purification. After salting out of porcine sarcoplasmic proteins (SP) with ammonium sulfate at 75% saturation, the obtained supernatant (SP-f3) was treated with EDTA, leaving G3PD in the supernatant (G3PD-E) and most other SPs in the precipitate. At that time, the separation of G3PD-E required more than 20 mmol/L EDTA. G3PD-E was then subjected to affinity purification by batchwise method using blue-sepharose CL-6B, and purified G3PD (G3PD-AP) was obtained using 2 mol/L potassium chloride (KCl) as an eluent. Texture analysis showed that the hardness, adhesiveness and gumminess of the myofibril gel at 0.2-mol/L NaCl increased with the addition of G3PD-AP. Scanning electron microscopy revealed that the G3PD-AP reinforced the gel network of the myofibril. However, scanning electron micrograph analysis showed that the network-structure of the gel by the addition of G3PD-AP developed in a different manner from that by adding 0.6 mol/L NaCl. These results showed that glycolytic enzyme, G3PD, contributes to the improvement of the rheological properties of meat products.

Application of free-flow IEF to identify protein candidates changing under microgravity conditions

Using antibody-related methods, we recently found that human thyroid cells express various proteins differently depending on whether they are cultured under normal gravity (1g) or simulated microgravity (s-microg). In this study, we performed proteome analysis in order to identify more gravity-sensitive thyroid proteins. Cells cultured under 1g or s-microg conditions were sonicated. Proteins released into the supernatant and those remaining in the cell fragments were fractionated by free-flow IEF. The fractions obtained were further separated by SDS-gel electrophoresis. Selected gel pieces were excised and their proteins were determined by MS. A total of 235 different proteins were found. Out of 235 proteins, 37 appeared to be first identifications in human thyroid cells. Comparing SDS gel lanes of equally numbered free-flow IEF fractions revealed similar patterns with a number of identical bands if proteins of a distinct cell line had been applied, irrespective of whether the cells had been cultured under 1g or s-microg. Most of the identical band pairs contained identical proteins. However, the concentrations of some types of proteins were different within the two pieces of gel. Proteins that concentrated differently in such pieces of gel are considered as candidates for further investigations of gravitational sensitivity.

Effect of ammonium, sodium, and potassium ions on rabbit muscle phosphofructokinase-1 and adenylate kinase activities

This report shows that 30 nM PFK-1 and 30 nM AK were both affected by the presence of NH(4)(+), Na(+), and K(+) salts but with opposite consequences. Low concentrations of PFK-1 lose about half of its activity as a result of dilution and become susceptible to further activity losses owing to the presence of monovalent salts. On the other hand low concentrations of AK lose about 75 percent of its activity but regains activity losses owing to the presence of monovalent salts. It was determined that regain of AK activity did not appear to be a reflection of a major effect on the K(m) value of either AMP or ATP. Dilution to 30 nM AK resulted in no increase K(m) values compared to K(m) values at 140 nM AK. Dilution caused major decreases in the maximum velocities, V(max), when ATP or fructose 6-phosphate was the variable substrate. It was shown in earlier reports that these same low concentrations of PFK-1 and AK were susceptible inhibitions by ascorbate. These attributes are discussed as they may relate to the role of ascorbate facilitation glycogen synthesis in resting muscle and the role that the cytoskeleton infrastructure scaffold may play is also discussed.

Some characteristics of rabbit muscle phosphofructokinase-1 inhibition by ascorbate

These studies relate to a working hypothesis that glycogen storage is facilitated in resting muscle by inhibiting glycolysis via inhibition of LDH, AK, and PFK-1 by ascorbate; when muscle is active, these isozymes combine with muscle proteins and are released and protected from inhibition by ascorbate and glycolysis proceeds. Focus in these studies is on the ability of G-actin and aldolase to prevent PFK-1 inhibition by ascorbate. We found that inhibition by ascorbate was PFK-1 concentration dependent; ascorbate does not inhibit above 200 nM PFK-1. We conclude that ascorbate inhibits PFK-1 dimers (and perhaps monomers) but not PFK-1 tetramers. Separation of PFK-1 dimers from tetramers was achieved with centrifugal filter devices and differences in their sensitivity to ascorbate inhibition were demonstrated. Some comparisons are made with attributes of AK inhibitions by ascorbate that, like PFK-1, are also enzyme concentration dependent. Discussions relate findings to cellular infrastructure and the role of ascorbate in glycogen synthesis.

Investigation of the interaction of pig muscle lactate dehydrogenase with acidic phospholipids at low pH

Interaction of pig muscle lactate dehydrogenase (LDH) with acidic phospholipids is strongly dependent on pH and is most efficient at pH values<6.5. The interaction is ionic strength sensitive and is not observed when bilayer structures are disrupted by detergents. Bilayers made of phosphatidylcholine (PC) do not bind the enzyme. The LDH interaction with mixed composition bilayers phosphatidylserine/phosphatidylcholine (PS/PC) and cardiolipin/phosphatidylcholine (CL/PC) leads to dramatic changes in the specific activity of the enzyme above a threshold of acidic phospholipid concentration likely when a necessary surface charge density is achieved. The threshold is dependent on the kind of phospholipid. Cardiolipin (CL) is much more effective compared to phosphatidylserine, which is explained as an effect of availability of both phosphate groups in a CL molecule for interaction with the enzyme. A requirement of more than one binding point on the enzyme molecule for the modification of the specific activity is postulated and discussed. Changes in CD spectra induced by the presence of CL and PS vesicles evidence modification of the conformational state of the protein molecules. In vivo qualitative as well as quantitative phospholipid composition of membrane binding sites for LDH molecules would be crucial for the yield of the binding and its consequences for the enzyme activity in the conditions of lowered pH.

The need for metabolic mapping in living cells and tissues

The ultimate activity of an enzyme depends on many regulatory steps from transcription of the gene up to complex formation of the enzyme. Therefore, gene expression (mRNA levels) or protein expression (protein levels) are not reliable parameters to predict the functional activity of an enzyme. Activity measurements in cell homogenates or in frozen or fixed (and thus dead) cell preparations are not appropriate either because post-translational regulation mechanisms that exist in living cells may be lost by homogenization or freezing or chemical fixation of cells. Therefore, metabolic mapping in living cells or, in other words, visualization and quantification using microscopy and image analysis of enzyme reactions in living cells is the approach of choice to understand the functional role of enzymes in vivo as is demonstrated here with a number of examples in recent literature.

Frog oocyte glycogen synthase: enzyme regulation under in vitro and in vivo conditions

Frog oocyte glycogen synthase properties differ significantly under in vitro or in vivo conditions. The K(mapp) for UDP-glucose in vivo was 1.4mM (in the presence or absence of glucose-6-P). The in vitro value was 6mM and was reduced by glucose-6-P to 0.8mM. Under both conditions (in vitro and in vivo) V(max) was 0.2 m Units per oocyte in the absence of glucose-6-P. V(max) in vivo was stimulated 2-fold by glucose-6-P, whereas, in vitro, a 10-fold increase was obtained. Glucose-6-P required for 50% activation in vivo was 15 microM and, depending on substrate concentrations, 50-100 microM in vitro. The prevailing enzyme obtained in vitro was the glucose-6-P-dependent form, which may be converted to the independent species by dephosphorylation. This transformation could not be observed in vivo. We suggest that enzyme activation by glucose-6-P in vivo is due to allosteric effects rather than to dephosphorylation of the enzyme. Regulatory mechanisms other than allosteric activation and covalent phosphorylation are discussed.

Coupling of creatine kinase to glycolytic enzymes at the sarcomeric I-band of skeletal muscle: a biochemical study in situ

The specific interaction of muscle type creatine-kinase (MM-CK) with the myofibrillar M-line was demonstrated by exchanging endogenous MM-CK with an excess of fluorescently labeled MM-CK in situ, using chemically skinned skeletal muscle fibers and confocal microscopy. No binding of labeled MM-CK was noticed at the I-band of skinned fibers, where the enzyme is additionally located in vivo, as shown earlier by immunofluorescence staining of cryosections of intact muscle. However, when rhodamine-labeled MM-CK was diffused into skinned fibers that had been preincubated with phosphofructokinase (PFK), a glycolytic enzyme known to bind to actin, a striking in vivo-like interaction of Rh-MM-CK with the I-band was found, presumably mediated by binding of Rh-MM-CK to the glycolytic enzyme. Aldolase, another actin-binding glycolytic enzyme was also able to bind Rh-MM-CK to the I-band, but formation of the complex occurred preferably at long sarcomere length (> 3.0 microm). Neither pyruvate kinase, although known for its binding to actin, nor phosphoglycerate kinase (PGK), not directly interacting with the I-band itself, did mediate I-band targeting of MM-CK. Anchoring of MM-CK to the I-band via PFK, but not so via aldolase, was strongly pH-dependent and occurred below pH 7.0. Labeling performed at different sarcomere length indicated that the PFK/MM-CK complex bound to thin filaments of the I-band, but not within the actomyosin overlap zones. The physiological consequences of the structural interaction of MM-CK with PFK at the I-band is discussed with respect to functional coupling of MM-CK to glycolysis, metabolic regulation and channeling in multi-enzyme complexes. The in situ binding assay with skinned skeletal muscle fibers described here represents a useful method for further studies of specific protein-protein interactions in a structurally intact contractile system under various precisely controlled conditions.

Macromolecular compartmentation and channeling

One of the accepted characterizations of the living state is that it is complex to an extraordinary degree. Since our current understanding of the living condition is minimal and fragmentary, it is not surprising that our first descriptions are simplistic. However, in certain areas of metabolism, especially those that have been amenable to experimentation for the longest period of time, the simplistic explanations have been the most difficult to revise. For example, current texts of general biochemistry still view metabolism as occurring by a series of independent enzymes dispersed in a uniform aqueous environment. This notion has been shown to be deeply flawed by both experimental and theoretical considerations. Thus, there is ample evidence that, in many metabolic pathways, specific interactions between sequential enzymes occur as static and/or dynamic complexes. In addition, reversible interactions of enzymes with structural proteins and membranes is a common occurrence. The interactions of enzymes give rise to a higher level of complexity that must be accounted for when one wishes to understand the regulation of metabolism. One of the phenomena that occurs because of sequential enzyme interactions is the process of channeling. This article discusses enzyme interactions and channeling and summarizes experimental and theoretical results from a few well-studied examples.

Channelling of intermediates in the biosynthesis of phosphatidylcholine and phosphatidylethanolamine in mammalian cells

Previous studies with electropermeabilized cells have suggested the occurrence of metabolic compartmentation and Ca2+-dependent channeling of intermediates of phosphatidylcholine (PC) biosynthesis in C6 rat glioma cells. With a more accessible permeabilization technique, we investigated whether this is a more general phenomenon also occurring in other cell types and whether channeling is involved in phosphatidylethanolamine (PE) synthesis as well. C6 rat glioma cells, C3H10T12 fibroblasts and rat hepatocytes were permeabilized with Staphylococcus aureus alpha-toxin, and the incorporation of the radiolabelled precursors choline, phosphocholine (P-choline), ethanolamine and phosphoethanolamine (P-EA) into PC and PE were measured both at high and low Ca2+ concentrations. In glioma cells, permeabilization at high Ca2+ concentration did not affect [14C]choline or [14C]P-choline incorporation into PC. However, reduction of free Ca2+ in the medium from 1.8 mM to <1 nM resulted in a dramatic increase in [14C]P-choline incorporation into permeabilized cells, whereas [14C]choline incorporation remained unaffected. Also, in fibroblasts, reduction of extracellular Ca2+ increased [14C]P-choline and [14C]P-EA incorporation into PC and PE respectively. In hepatocytes, a combination of alpha-toxin and low Ca2+ concentration severely impaired [14C]choline incorporation into PC. Therefore, alpha-toxin-permeabilized hepatocytes are not a good model in which to study channeling of intermediates in PC biosynthesis. In conclusion, our results indicate that channeling is involved in PC synthesis in glioma cells and fibroblasts. PE synthesis in fibroblasts is also at least partly dependent on channeling.

Interaction of M1 and M2 isozymes pyruvate kinase from human tissues with phospholipids

The effect of pH and the presence of FBP on the interaction of skeletal muscle (PK-M1) and kidney or tumor meningioma (PK-M2) pyruvate kinase with the phospholipids liposomes were investigated by ultracentrifugation and steady-state kinetics and were compared with those results obtained using the bovine heart (PK-M1) isoenzyme which we previously studied. Pyruvate kinase specific activity increases upon the interaction with liposomes. The activation is specifically sensitive to presence of phosphatidylserine (PS) in liposomes. Liposomes made of phosphatidylcholine + phosphatidylserine mixture are good adsorptive systems for both human and bovine of M-type isozymes at low ionic strength. Interaction of PK-M1 with PS liposomes results in the change of Vmax and K(m) values for PEP without marked effect on Hill coefficients. Addition of PS liposomes to PK-M2 induces hyperbolic saturation curves for PEP.

Localized firefly luciferase probes ATP at the surface of mitochondria

The concentration of ATP generated by yeast mitochondria and consumed by yeast hexokinase was monitored using native firefly luciferase in solution, or recombinant luciferase localized at the surface of mitochondria. In the absence of hexokinase, both probes perform similarly in detecting exogenous or mitochondrially-generated ATP. The steady-state concentrations of ATP can be reduced in a dose-dependent manner by hexokinase. With hexokinase added in large excess, the localized probe reports substantial ATP concentrations while none is detectable by soluble luciferase. Thus, ATP accumulates near the membrane where it appears, relatively to solution, and vice versa for ADP. The extent of nucleotide gradients is shown to be correlated with the specific activity of oxidative phosphorylation and with the viscosity of the medium, but independent of the concentration of the organelles. A simple model involving diffusional restrictions is presented to describe this behavior. The metabolic and evolutionary implications of cellular catalysis limitation by physical processes are discussed.

Alternative binding of two sequential glycolytic enzymes to microtubules. Molecular studies in the phosphofructokinase/aldolase/microtubule system

Simultaneous binding of two sequential glycolytic enzymes, phosphofructokinase and aldolase, to a microtubular network was investigated. The binding of the phosphofructokinase to microtubules and its bundling activity has been previously characterized (Lehotzky, A., Telegdi, M., Liliom, K., and Ovádi, J. (1993) J. Biol. Chem. 268, 10888-10894). Aldolase binding to microtubules at near physiological ionic strength is weak (Kd = 20 microM) as compared with that of the kinase (Kd = 1 microM). The interactions of both enzymes with microtubules are modulated by their common intermediate, fructose-1,6-bisphosphate. Pelleting and electron microscopic measurements have revealed that the aldolase binding interferes with that of phosphofructokinase, although they have distinct binding domains on microtubules. The underlying molecular mechanism responsible for this finding is that in the solution phase aldolase and phosphofructokinase form a bienzyme complex that does not bind to the microtubule. The bienzyme complex formation does not influence the catalytic activity of aldolase, however, it inhibits the dissociation-induced inactivation of the kinase by stabilizing a catalytically active molecular form. The present data suggest the first experimental evidence that two sequential glycolytic enzymes do not associate simultaneously to microtubules, but their complexation in solution provides kinetic advantage for glycolysis.

Metabolic consequences of enzyme interactions

The fact that enzyme complexes, stable, quasi-stable, and dynamic, exist in cells can no longer be ignored. Experimental evidence done with a variety of techniques has demonstrated these interactions in vitro and in vivo. There is scarcely a single known metabolic pathway in which no interactions of any of its enzymes exist (see reference 27 for a list of these). Such interactions are not only ubiquitous throughout metabolism, but they exist in all cell types, procaryote and eucaryote. In many of these systems the advantages of and regulatory power of enzyme-enzyme or enzyme-structural protein interactions has been amply demonstrated. The more difficult task is to assess accurately quantitative aspects of a system that varies between the solid, gel, and aqueous state. It is clear that the metabolic paradigm of soluble randomly dispersed metabolic elements in cells must be replaced, and new theoretical and experimental approaches introduced into this vital area of biological research.

Metabolic adaptations supporting anoxia tolerance in reptiles: recent advances

Animal survival during severe hypoxia and/or anoxia is enhanced by a variety of biochemical adaptations including adaptations of fermentative pathways of energy production and, most importantly, the ability to sharply reduce metabolic rate by 5-20 fold and enter a hypometabolic state. The biochemical regulation of metabolic arrest is proving to have common molecular principles that extend across phylogenetic lines and that are conserved in different types of arrested states (not only anaerobiosis but also estivation, hibernation, etc.). Our new studies with anoxia-tolerant vertebrates have identified a variety of regulatory mechanisms involved in both metabolic rate depression and in the aerobic recovery process using as models the freshwater turtle Trachemys scripta elegans and garter snakes Thamnophis sirtalis parietalis. Mechanisms include: 1) post-translational modification of cellular and functional proteins by reversible phosphorylation and changes in protein kinase (PKA, PKC) and/or phosphatase activities to regulate this, 2) reversible enzyme binding associations with subcellular structural elements, 3) differential gene expression and/or mRNA translation producing new mRNA variants and new protein products, 4) changes in protease activity, particularly the multicatalytic proteinase complex, and 5) both constitutive and anoxia-induced modifications to cellular antioxidant systems to deal with oxidative stress during the anoxic-aerobic transition of recovery.

A novel quantitative histochemical assay to measure endogenous substrate concentrations in tissue sections. Fundamental aspects

A quantitative histochemical assay has been developed for measurement of endogenous substrate concentrations in cryostat sections using a colorimetric visualization technique. Model sections of frozen gelatin solutions with known concentrations of glucose-6-phosphate (G6P) were sandwiched with a second cryostat section containing glucose-6-phosphate dehydrogenase (G6PDH) and all other compounds (with the exception of G6P) that are necessary for the demonstration of G6PDH activity with a tetrazolium salt method. After 60 min of incubation, G6P was converted with concomitant formazan production. The amount of formazan generated was measured cytophotometrically and used as a parameter of the G6P concentration in the first section. A calibration graph was obtained with a high correlation coefficient, allowing the conversion of mean integrated absorbance values into absolute substrate concentrations. The method was highly reproducible, and the recovery of G6P was 85 +/- 4% irrespective section thickness (4-20 microns) and G6P concentration (0.08-1.6 mM) in the sections. The sensitivity of the tetrazolium-linked method appeared to be 100 microM in 20 microns thick sections. This sensitivity enables the measurement of physiological substrate concentrations in tissue sections. Spatial resolution was approximately 150 microns, indicating a relatively high rate of diffusion of G6P during the reaction. The model study shows that the method described here allows the quantitative determination of substrate concentrations in tissue sections. These endogenous substrate concentrations are necessary for the calculation of local metabolic fluxes when determined in combination with local enzyme activities and kinetics, thus giving a more accurate reflection of in situ metabolic heterogeneity of tissues.

Interaction of bovine heart pyruvate kinase with phospholipids

The interaction between bovine heart pyruvate kinase and liposomes was investigated for various phospholipids as function of pH, and salt concentration using steady-state kinetics and ultracentrifugation. Liposomes made from erythrocyte total lipid fraction and individual phospholipids were used. Pyruvate kinase specific activity increases upon the interaction with the phospholipids. The activation is specifically sensitive to presence of phosphatidylserine in liposomes. L-serine, and phospho-L-serine which are main components of phosphatidylserine head group show also some activation effect. Efficient adsorption of pyruvate kinase to phosphatidylserine liposomes occurs in the pH range 6.0-8.0 and at low ionic strength. Interaction with phosphatidylserine liposomes results in the change of Vmax and Km values for phospho enol pyruvate without marked effect on Km value for ADP, and Hill coefficients for both substrates. The interaction does not seem to influence the cooperativity between binding sites.

Heterogeneity of kinetic parameters of enzymes in situ in rat liver lobules

In the present review, metabolic compartmentation in liver lobules is discussed as being dynamic and more complex than thus far assumed on the basis of numbers of mRNA or protein molecules or the capacity (zero-order activity) of enzymes. Isoenzyme distribution patterns and local kinetic parameters of enzymes may vary over the different zones of liver lobules. As a consequence, metabolic fluxes in vivo at physiological substrate concentrations may be completely different from those that are assumed on the basis of the number of molecules or the capacity of enzymes present in zones of liver lobules. For a more correct estimation of the levels of metabolic processes in the different compartments of liver tissue, local kinetic parameters and substrate concentrations have to be determined to calculate local metabolic fluxes. Direct measurements of metabolic fluxes in vivo with the use of noninvasive techniques is a promising alternative and the techniques will become increasingly important in future metabolic research.

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.

Lipid dynamics and peripheral interactions of proteins with membrane surfaces

A large body of evidence strongly indicates biomembranes to be organized into compositionally and functionally specialized domains, supramolecular assemblies, existing on different time and length scales. For these domains and intimate coupling between their chemical composition, physical state, organization, and functions has been postulated. One important constituent of biomembranes are peripheral proteins whose activity can be controlled by non-covalent binding to lipids. Importantly, the physical chemistry of the lipid interface allows for a rapid and reversible control of peripheral interactions. In this review examples are provided on how membrane lipid (i) composition (i.e., specific lipid structures), (ii) organization, and (iii) physical state can each regulate peripheral binding of proteins to the lipid surface. In addition, a novel and efficient mechanism for the control of the lipid surface association of peripheral proteins by [Ca2+], lipid composition, and phase state is proposed. The phase state is, in turn, also dependent on factors such as temperature, lateral packing, presence of ions, metabolites and drugs. Confining reactions to interfaces allows for facile and cooperative large scale integration and control of metabolic pathways due to mechanisms which are not possible in bulk systems.

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.

Metabolic channeling versus free diffusion: transition-time analysis

Metabolic channeling is the term used to describe the restricted flow of substrates and products in multienzyme systems. It has been argued for some time that free diffusion is sufficiently rapid to obviate the need for channeling and, furthermore, that it is also fast enough to prevent competing side reactions from interfering with the metabolic flow. In this article we argue that a thorough consideration of the temporal behavior of metabolite pools suggests that channeling is important in many cases.

Myofibrillar creatine kinase and cardiac contraction

This article is a review on the organization and function of myofibrillar creatine kinase in striated muscle. The first part describes myofibrillar creatine kinase as an integral structural part of the complex organization of myofibrils in striated muscle. The second part considers the intrinsic biochemical and mechanical properties of myofibrils and the functional coupling between myofibrillar CK and myosin ATPase. Skinned fiber studies have been developed to evidence this functional coupling and the consequences for cardiac contraction. The data show that creatine kinase in myofibrils is effective enough to sustain normal tension and relaxation, normal Ca sensitivity and kinetic characteristics. Moreover, the results suggest that myofibrillar creatine kinase is essential in maintaining adequate ATP/ADP ratio in the vicinity of myosin ATPase active site to prevent dysfunctioning of this enzyme. Implications for the physiology and physiopathology of cardiac muscle are discussed.

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.

Enzyme-to-enzyme tunnelling between phosphoglucoisomerase and phosphofructokinase

1. Cross-linked and permeabilized rat erythrocytes were incubated for 2-5 min at 37 degrees C in the presence of ATP and either D-[U-14C]glucose 6-phosphate (3 mM) mixed with unlabelled D-fructose 6-phosphate (1 mM) or D-[U-14C]fructose 6-phosphate (1 mM) mixed with unlabelled D-glucose 6-phosphate (3 mM). 2. The contribution of molecules derived from the radioactive ketohexose ester relative to the total amount of newly formed D-fructose 1,6-bisphosphate was lower than the time-related average value for such a relative contribution in the pool of D-fructose 6-phosphate. 3. From such a difference, it was calculated that, under the present experimental conditions, 13.1 +/- 2.0% of the molecules of D-fructose 1,6-bisphosphate formed during incubation are directly derived from D-glucose 6-phosphate by a process of enzyme-to-enzyme channelling between phosphoglucoisomerase and phosphofructokinase, rather than originating from the free pool of D-fructose 6-phosphate. 4. A comparable value of 13.2 +/- 3.2% was reached when the process of enzyme-to-enzyme tunnelling was judged from the 3H/14C ratio in D-fructose 1,6-bisphosphate formed by permeabilized erythrocytes exposed for 5-15 min to D-glucose 6-phosphate (3 or 5 mM) mixed with tracer amounts of both D-[1-14C]glucose 6-phosphate and D-[2-3H]glucose 6-phosphate.

Ontogenic activities and interactions of the lactate dehydrogenase isozymes with cellular structure in the guinea pig

The LDH activities and isozyme distributions associated with soluble and particulate fractions of five major tissues have been followed during the development of the guinea pig. Evidence has been provided for an appreciable degree of interaction between LDH and cellular structure in all these tissues (liver, kidney, skeletal muscle, brain and heart) at all stages of development, but particularly in the early foetal stages. These data have been discussed in relation to the nature and extent of this binding and the correlations with the metabolic emphases in these tissue situations during development.

Control of glycolytic enzyme binding: effect of changing enzyme substrate concentrations on in vivo enzyme distributions

The effect of changing concentrations of glycolytic intermediates on the binding of phosphofructokinase, aldolase and pyruvate kinase to cellular particulate matter was investigated. Concentrations of glycolytic intermediates were altered by adding 2 mM iodoacetic acid (IAA) to an incubation medium containing tissues isolated from the channelled whelk Busycon canaliculatum. Iodoacetic acid inhibited glyceraldehyde 3-phosphate dehydrogenase activity causing a 100-400 fold increase in the concentration of fructose 1,6-bisphosphate as well as 3-20 fold increases in glucose 6-phosphate, fructose 6-phosphate, and dihydroxyacetone phosphate levels depending on the experimental protocol. Cellular pH values were not statistically different in the presence of IAA. Measurement of enzyme binding to particulate matter showed that the binding of phosphofructokinase, aldolase and pyruvate kinase was unaffected by iodoacetic acid under any experimental condition. These results show that changes in the tissue concentrations of enzyme substrates and products do not regulate enzyme binding to particulate matter in the cell.

Patterns of spatiotemporal organization in an "ambiquitous" enzyme model

Many enzymes in pathways such as glycolysis associate reversibly with cellular substructures. The spatiotemporal behavior of a "limit-cycle" oscillation model is studied under the condition that the "ambiquitous" oscillophor, phosphofructokinase, is partitioned between "bulk-phase" and "bound" forms in a heterogeneous system. Computer simulation demonstrates the occurrence of sustained, wave-like spatiotemporal patterns of chemical concentration in the bulk medium. Kinetic dissimilarity among the localized populations of bound enzyme leads to a "polarity" effect in the wave phenomenon. It is suggested that a key physiological role of the limit-cycle regime is to engender a rapid, site-to-site, signal-transmission modality in large eukaryotic (e.g., mammalian) cells.

Hepatocyte swelling leads to rapid decrease of the G-/total actin ratio and increases actin mRNA levels

Exposure of isolated rat hepatocytes to hypotonic (190 mosmol/l) incubation media lowered the cellular G-actin level without affecting the total actin content: here the G-/total actin ratio decreased by 15.5 +/- 1.4% (n = 7). Similar effects were observed following isotonic cell swelling by either addition of glutamine (10 mM) or insulin (100 nM), resulting in a decrease of the G-/total actin ratios by 13.5 +/- 2.1% (n = 5) and 14.1 +/- 1.1% (n = 11), respectively. The effects of hypotonic exposure, glutamine and insulin on the G-/total actin ratio largely occurred within 1 min and persisted for at least 2 h in presence of the respective effectors. After a 120 min exposure to hypotonic media, glutamine or insulin the actin mRNA levels were increased 2.4-, 2.0- and 3.6-fold, respectively. Hypertonic exposure lowered the G-/total actin ratio by only 4.9 +/- 2.5% (n = 4) and increased actin mRNA levels only 1.2-fold. There was a close relationship between glutamine- and hypotonicity-induced cell swelling and the decrease of G-/total actin ratios. The data suggest that cell swelling exerts rapid and marked effects on the state of actin polymerization and increases actin mRNA levels. Thus, cytoskeletal alterations in response to cell swelling may be involved in the regulation of hepatic metabolism by cell volume.

Local probes and heterogeneous catalysis: a case study of a mitochondria-luciferase-hexokinase coupled system

Biological systems are characterized by a high degree of structural organization. In the intracellular context, this introduces physical constraints which are not considered in the standard biochemical analysis of isolated systems, aimed towards mechanistic studies. A major challenge in cellular biology is thus to integrate the structural and mechanistic information and reach an adequate representation of the modes of operation in situ. We present an approach to this problem which takes advantage of a localized probe to study heterogeneous coupled system, as minimal models for cellular operation. The system consists of ATP production at the surface of mitochondria, and ATP consumption in solution by the hexokinase reaction. Soluble or biologically localized firefly luciferase is used to continuously monitor ATP concentration either in the bulk solution or at the surface of the organelle, respectively. The general system of a surface source and a bulk sink is mathematically modeled, and an analytic steady-state solution for local and bulk ATP is presented. The results are validated by experiment and differ from the expected behavior of an equivalent homogeneous system in solution. The model is further adapted to evaluate the effect of mixing. In addition, two limiting cases of heterogeneous distribution of hexokinase are analyzed, in which the soluble enzyme adsorbs non-specifically to mitochondria, or binds selectively to the site of ATP appearance on the membrane. The results are discussed in terms of their significance to the analysis of bulk measurements in vitro and their relevance to better description of cellular situations.

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.

Regulation of the GM1-galactosidase supramolecular structure and catalytic activity in vitro

Regulation of the supramolecular organization and the catalytic activity of GM1-galactosidase (EC 3.2.1.23) and neuraminidase (EC 3.2.1.18) from human kidney was studied in a system of hydrated reversed micelles of Aerosol OT in octane. It was shown that both the catalytic activity and the oligomeric structure of the GM1-galactosidase in reversed micelles depend on the [H2O]/[Aerosol OT] molar ratio (w(o)). GM1-galactosidase 64-67 kDa monomers, 260 kDa tetramers, and 660 kDa octamers were obtained in systems with w(o) = 0-20, 25-30 and 30-40, respectively. The association of GM1-galactosidase monomers into an octamer results in the cooperative increase in enzymatic activity. 'Protective protein', a component of the GM1-galactosidase-neuraminidase native complex, was found to improve this association significantly.

Interaction in vitro of scallop muscle arginine kinase with filamentous actin

Scallop muscle arginine kinase binds to F-actin from mollusc and rabbit muscle in vitro. One site of interaction appears to be located in residues 305-325 of a C-terminal fragment (residues 285-375) of actin. The binding is hindered in the presence of arginine, Mg(2+)-ADP and NO3-, which form a dead-end complex with the enzyme. F-actin inhibits the enzyme activity non-competitively with respect to Mg(2+)-ATP. As a function of arginine concentration, the inhibition is of the mixed type, where Km is affected more than Vmax.

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.

Investigation of lens glycolytic enzymes: species distribution and interaction with supramolecular order

Distribution of several glycolytic enzymes in the lenses of different vertebrate species and their organization in the calf lenses were studied. Though no general pattern of enzyme activities in different species is discernible, high activities of TPI followed, in decreasing order, by GAPDH, enolase, PK, LDH and aldolase appear to be more common. Our observation on the unusually high activities of aldolase in the pig, enolase in the sheep and LDH in the duck lens are interesting in view of the already known dual function of LDH as an enzyme and a structural protein (epsilon-crystallin) in duck. Controlled treatment with detergents Brij-58 and Triton X-100 caused distinctly differential purturbations in the lens cells. In spite of fiber membrane disruption and partial actin dissolution by Brij-58, no significant increase in the release of glycolytic enzymes compared to control was observed. This suggests that none of the enzymes existed as a completely soluble and freely diffusible fraction. But treatment with a strong detergent (Triton X-100) caused the release of higher amounts of enzymes suggesting either a direct or indirect interaction with the cytomatrix components. Aldolase appears to be maximally bound in the cytosol followed by TPI, GAPDH, LDH and PK in decreasing order. Although thin lens slices were incubated with the detergents for a total period of 40 min and the loss of fiber architecture and organization confirmed by light microscopy, in the Triton X-100 treated tissues less than 25% of the total activity of any enzyme except TPI appeared in the bathing medium.(ABSTRACT TRUNCATED AT 250 WORDS)