Organization of soluble enzymes in the cell. Relay at the surface

Compartmentation of GAPDH

The concept of the cytosol as a space that contains discrete zones of metabolites is discussed relative to the contribution of GAPDH. GAPDH is directed to very specific cell compartments. This chapter describes the utilization of GAPDH's enzymatic function for focal demands (i.e. ATP/ADP and NAD(+)/NADH), and offers a speculative role for GAPDH as perhaps moderating local concentrations of inorganic phosphate and hydrogen ions (i.e. co-substrate and co-product of the glycolytic reaction, respectively). Where known, the structural features of the binding between GAPDH and the compartment components are discussed. The nuances, which are associated with the intracellular distribution of GAPDH, appear to be specific to the cell-type, particularly with regards to the various plasma membrane proteins to which GAPDH binds. The chapter includes discussion on the curious observation of GAPDH being localized to the external surface of the plasma membrane in a human cell type. The default perspective has been that GAPDH localization is synonymous with compartmentation of glycolytic energy. The chapter discusses GAPDH translocation to the nucleus and to non-nuclear cellular structures, emphasizing its glycolytic function. Nevertheless, it is becoming clear that alternate functions of GAPDH play a role in compartmentation, particularly in the translocation to the nucleus.

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.

Mechanisms for cytoplasmic organization: an overview

One of the basic characteristics of life is the intrinsic organization of cytoplasm, yet we know surprisingly little about the manner in which cytoplasmic macromolecules are arranged. It is clear that cytoplasm is not the homogeneous "soup" it was once envisioned to be, but a comprehensive model for cytoplasmic organization is not available in modern cell biology. The premise of this volume is that phase separation in cytoplasm may play a role in organization at the subcellular level. Other mechanisms for non-membrane-bounded intracellular organization have previously been proposed. Some of these will be reviewed in this chapter. Multiple mechanisms, involving phase separation, specific intracellular targeting, formation of macromolecular complexes, and channeling, all could well contribute to cytoplasmic organization. Temporal and spatial organization, as well as composition, are likely to be important in defining the characteristics of cytoplasm.

Metabolic compartmentation in living cells: structural association of aldolase

The glycolytic enzyme aldolase is concentrated in a domain around stress fibers in living Swiss 3T3 cells, but the mechanism by which aldolase is localized has not been revealed. We have recently identified a molecular binding site for F-actin on aldolase, and we hypothesized that this specific binding interaction, rather than a nonspecific mechanism, is responsible for localizing aldolase in vivo. In this report, we have used fluorescent analog cytochemistry of a site-directed mutant of aldolase to demonstrate that actin-binding activity localizes this molecule along stress fibers in quiescent cells and behind active ruffles in the leading edge of motile cells. The specific cytoskeletal association of aldolase could play a structural role in cytoplasm, and it may contribute to metabolic regulation, metabolic compartmentation, and/or cell motility. Functional duality may be a widespread feature among cytosolic enzymes.

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.

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.

Glucose metabolism and the channeling of glycolytic intermediates in permeabilized L-929 cells

L-929 cells (mouse fibroblasts) permeabilized with dextran sulfate (DSP cells) carry out vigorous and linear rates of glycolysis when supplied with a suitable incubation medium. Glycolysis in DSP cells is pH dependent, being strongly inhibited at pH 6.5. Compared to their nonpermeabilized counterparts, DSP cells exhibit faster glycolytic rates, but tend to convert a smaller proportion of the glucose utilized to lactate. [14C]Glucose is converted to lactate by DSP cells without dilution from endogenous substrates. When exogenous 12C-labeled glycolytic intermediates (12C-I) are added to glycolyzing DSP cells the [14C]lactate produced from [14C]glucose is diluted to varying extents, depending on the intermediate. However, the extent of that dilution (reduced specific activity) is not that expected from the complete mixing of exogenous 12C-I with their corresponding 14C-labeled intermediates coming from [14C]-glucose. DSP cells also respire and convert glucose to CO2. The amount of 14CO2 produced from [14C]glucose is also reduced by addition of most 12C-I, an interesting exception being pyruvate, which had no measurable effect on 14CO2 production and caused only a modest stimulation of respiration in glycolyzing DSP cells. These results suggest that channeling, or some other form of coupling, takes place between the glycolytic production of pyruvate and its further oxidation. These observations confirm previously published data and add further support to the proposition that channeling of glycolytic intermediates occurs in DSP cells but is of the "leaky" type. Although abundant evidence in the literature indicates that various glycolytic enzymes associate with F-actin, as well as other elements of the cytomatrix, we observed no effect of cytochalasin D on lactate production even at very high concentrations of this compound. Our results are compared with those from other laboratories and discussed in the context of metabolic organization.

Glycolysis compared in intact, permeabilized and sonicated L-929 cells

The effects of incubation time and cell density on glycolytic rate were examined in suspensions of intact, permeabilized and sonicated L-929 cells. Sonicates exhibited strong dependence on cell density and a distinct lag in glycolytic rate, while intact cells showed no cell density dependence and linear glycolytic rates. Permeabilized cells exhibited linear glycolytic rates, but sometimes showed dependence on cell density. Rates of lactate production (nmol at 30 min/10(6) cells) were highest in sonicates and lowest in intact cells. These results are interpreted as support for the previously proposed hypothesis that enzymes of the glycolytic pathway are highly organized in intact L-929 cells.

Effects of reduced cell volume and water content on glycolysis in L-929 cells

Mouse L-929 cells were subjected to increasing concentrations of sorbitol, which remove cell water and reduce volume osmotically. The rate of lactate production from glucose was significantly higher in osmotically perturbed cells than in controls, both in monolayers and in suspensions. L cells can apparently use sorbitol as a glycolytic substrate; however, studies using other solutes (trehalose and sucrose) and permeabilized cells showed that the major effect of sorbitol on glycolysis in intact cells is mediated through a reduction in cell water content and volume. It is possible to explain some of these results by an increase in the chemical potentials of dissolved components of the glycolytic pathway caused by water loss; however, the relationship between water loss and glycolytic rate increase in not a simple linear one, suggesting that the situation is more complex than would result merely from increased concentrations of pathway components. Whatever the complete explanation might be, these studies show that glycolysis continues in an orderly fashion in cells that have lost about 85% of their original water content, suggesting that the operation of this pathway is not unduly sensitive to events taking place in the bulk aqueous phase.