Effect of polyethylene glycol on the activity, intrinsic fluorescence, and oligomeric structure of castor seed cytosolic fructose-1,6-bisphosphatase

Poly(ethylene glycol)-mediated conformational alteration of α-chymotrypsin prevents inactivation of insulin by stabilizing active intermediates

Proteolytic enzymes in the gut represent one of the biggest barriers against oral delivery of therapeutic proteins and peptides. In the current study, we explored the effect of poly(ethylene glycol) 400 (PEG 400), a commonly used crowding agent, on insulin degradation mediated by α-chymotrypsin (α-CT). Without PEG 400, insulin was quickly cleaved by α-CT to generate inactive degradation products. In comparison, incorporation of PEG 400 resulted in reaction mixtures with retained biological activity. The analysis on the conformation of α-CT and the local environment of the enzyme's active site unraveled that PEG 400 altered the conformation of α-CT to prevent the inactivation of insulin via stabilization of active intermediates. These findings indicated that PEG 400 may provide a promising addition toward oral delivery of insulin.

Interaction of polyethylene glycol-6000 with C4 phosphoenolpyruvate carboxylase in crude leaf extracts as well as in purified protein form from Amaranthus hypochondriacus L.: evidence for oligomerization of PEPC in vitro and in vivo

The interaction of phosphoenolpyruvate carboxylase (PEPC) with a compatible solute, PEG-6000, was examined using crude leaf extracts as well as the purified protein from leaves of Amaranthus hypochondriacus, a NAD-malic enzyme type C4 plant. The inclusion in the assay medium of PEG-6000 stimulated the activity of purified PEPC by about 2.5-fold over control. The addition of PEG during both extraction and assay, stimulated PEPC activity by almost 5.0 fold in crude extracts. The stimulation by PEG of the dark-form of PEPC (2.4 fold) was more than that of the light-form (1.7 fold). Gel filtration of PEPC in leaf extracts on Sephadex G-200, showed the existence of three different oligomeric forms: tetramer, dimer and monomer. The exclusion of PEG and glycerol during extraction and elution on Sephadex resulted in a marked shift of the enzyme into dimer and/or monomer, with a very small proportion of tetramer but on the contrary, the inclusion of PEG and glycerol resulted in the enzyme maintaining predominantly a tetrameric shape. Thus, the activity and the structural properties of PEPC can be influenced by the presence or absence of compatible solutes (PEG or glycerol), obviously due to changes in the microenvironment of the enzyme.

The interaction of FBPase with aldolase: a kinetic and fluorescence investigation on chicken muscle enzymes

Fructose-1,6-bisphosphatase (FBPase; EC 3.1.3.11) is strongly inhibited by AMP in vitro and, therefore, at physiological concentrations of substrate and AMP, FBPase should be completely inhibited. Desensitization of rabbit muscle FBPase against AMP inhibition was previously observed in the presence of rabbit muscle aldolase. In this study, we analysed the kinetics of an FBPase catalyzed reaction and interaction between chicken muscle FBPase and chicken muscle aldolase. The initial rate of FBPase reaction vs. substrate concentration shows a maximum activity at a concentration of 20 microM Fru-1,6P2 and then decreases. Assuming rapid equilibrium kinetics, the enzyme-catalyzed reaction was described by the substrate inhibition model, with Ks approximately 5 microM and Ksi approximately 39 microM and factor beta approximately 0.2, describing change in the rate constant (k) of product formation from the ES and ESSi complexes. Based on ultracentrifugation studies, aldolase and FBPase form a hetero-complex with approximately 1:1 stoichiometry with a dissociation constant (Kd) of 3.8 microM. The FBPase-aldolase interaction was confirmed via fluorescence investigation. The aldolase-FBPase interaction results in aldolase fluorescence quenching and its maximum emission spectrum shifting from 344 to 356 nm. The Kd of the FBPase-aldolase complex, determined on the basis of fluorescence changes, is 0.4 microM at 25 degrees C with almost 1:1 stoichiometry. This interaction increases the I(0.5) for the AMP inhibition of FBPase threefold, and slightly affects FBPase affinity to magnesium ions, increasing the Ka and Hill coefficient (n). No effect of aldolase on the FBPase pH optimum was observed. Thus, the decrease in FBPase sensitivity to AMP inhibition enables FBPase to function in vivo thanks to aldolase.

Biochemical characterization of cytosolic fructose-1,6-bisphosphatase from apple (Malus domestica) leaves

Cytosolic fructose-1,6-bisphosphatase was purified to apparent homogeneity from the leaves of apple, a sorbitol synthesizing species. The enzyme was a homotetramer with a subunit mass of 37 kDa, and was highly specific for fructose 1,6-bisphosphate (F1,6BP) with a Km of 3.1 micro M and a Vmax of 48 units (mg protein)(-1). Either Mg2+ or Mn2+ was required for its activity with a Km of 0.59 mM and 62 micro M, respectively. Li+, Ca2+, Zn2+, Cu2+ and Hg2+ inhibited whereas Mn2+ enhanced the Mg2+ activated enzyme activity. Fructose 6-phosphate (F6P) was found to be a mixed type inhibitor with a Ki of 0.47 mM. Fructose 2,6-bisphosphate (F2,6BP) competitively inhibited the enzyme activity and changed the substrate saturation curve from hyperbolic to sigmoidal. AMP was a non-competitive inhibitor for the enzyme. F6P interacted with F2,6BP and AMP in a synergistic way to inhibit the enzyme activity. Dihydroxyacetone phosphate slightly inhibited the enzyme activity in the presence or absence of F2,6BP. Sorbitol increased the susceptibility of the enzyme to the inhibition by high concentrations of F1,6BP. High concentrations of sorbitol in the reaction mixture led to a reduction in the enzyme activity.

Kinetic and structural analysis of the ultrasensitive behaviour of cyanobacterial ADP-glucose pyrophosphorylase

The kinetic and (supra)molecular properties of the ultrasensitive behaviour of ADP-glucose pyrophosphorylase (AGPase) from Anabaena PCC 7120 (a cyanobacterium) were exhaustively studied. The response of the enzyme toward the allosteric activator 3-phosphoglycerate (3PGA) occurs with ultrasensitivity as a consequence of the cross-talk with the inhibitor P(i). Molecular 'crowding' renders AGPase more sensitive to the interplay between the allosteric regulators and, consequently, enhances the ultrasensitive response. In crowded media, and when orthophosphate is present, the activation kinetics of the enzyme with 3PGA proceed with increased co-operativity and reduced affinity toward the activator. Under conditions of ultrasensitivity, the enzyme's maximal activation takes place in a narrow range of 3PGA concentrations. Moreover, saturation kinetics of the enzyme with respect to its substrates, glucose 1-phosphate and ATP, were different at low or high 3PGA levels in crowded media. Only under the latter conditions did AGPase exhibit discrimination between low or high levels of the activator, which increased the affinity toward the substrates and the maximal activity reached by the enzyme. Studies of fluorescence emission of tryptophan residues, fourth-derivative spectroscopy and size-exclusion chromatography indicated that the ultrasensitive behaviour is correlated with intramolecular conformational changes induced in the tertiary structure of the homotetrameric enzyme. The results suggest a physiological relevance of the ultrasensitive response of AGPase in vivo, since the enzyme could be subtly sensing changes in the levels of allosteric regulators and substrates, and thus determining the flux of metabolites toward synthesis of storage polysaccharides.

Enzymes as molecular automata: a stochastic model of self-oscillatory glycolytic cycles in cellular metabolism

A stochastic model based on the molecular automata approach was developed to simulate the cyclic dynamics of glycolysis-gluconeogenesis in cell energy metabolism. The stochastic algorithm, based on Gillespie's method, simulates the master equation associated with any network of enzymatically controlled reactions. This model of the glycolytic-gluconeogenetic cycle was developed by assembling the stochastic kinetic reactions controlled by two enzymes: phosphofructokinase (PFKase) and fructose-1, 6-biphosphatase (FBPase). In order to obtain the hysteresis behaviour predicted by classical Sel'kov analysis, a minimum complexity is required in the allosteric regulations. This implies that the FBPase cannot have a single binding site for its transition to the inactive state (a minimum of two or three binding sites is necessary). Given the multimeric structure of this enzyme, this kinetic hypothesis is tenable. Other possible applications of the stochastic automata approach for different cases of channels, receptors and enzymatic control are suggested.

PEG-variant biomaterials as selectively adhesive protein templates: model surfaces for controlled cell adhesion and migration

Our study focused on the role of poly(ethylene glycol) (PEG) in actively regulating the biological responsiveness of protein-adsorbed biomaterials. To this end, we designed PEG-variant biomaterials from a family of tyrosine/PEG-derived polycarbonates to present surfaces ranging from low to intermediate levels of PEG concentration, below the PEG level requisite for complete abolition of protein adsorption. We analyzed the effect of PEG concentration on the amount, conformation and bioactivity of an adsorbed model protein, fibronectin, and on the attachment, adhesion strength and motility of L929 fibroblasts. Our results demonstrate that low levels of PEG can regulate not only the extent but also the conformation and specific bioactivity of adsorbed fibronectin. As the PEG concentration was increased from 0 to 6 mol%, the amount of adsorbed fibronectin decreased linearly yet the fibronectin conformation was altered such that the overall bioactivity of adsorbed fibronectin was uncompromised. We report that the degree of cell attachment varied with PEG concentration in a manner similar to the dependence of fibronectin bioactivity on PEG. In contrast, the nature of cell adhesion strength dependence on PEG paralleled the pattern observed for fibronectin surface concentration. Our studies also indicated that the rate of cell migration was inversely correlated with PEG concentration over a narrow range of PEG concentration. Overall, these results highlight the striking ability of PEG-variant biomaterials to systematically regulate the behavior of adsorbed cell adhesion proteins and, consequently, effect cell functions.

A fluorescence study of ligand-induced conformational changes in cytosolic fructose-1,6-bisphosphatase from germinating castor oil seeds

The intrinsic fluorescence of homogeneous castor oil seed cytosolic fructose-1,6-bisphosphatase (FBPasec) was used as an indicator of conformational changes due to ligand binding. Binding of the substrate and the inhibitor fructose-2,6-bisphosphate (F-2,6-P2) was quantitatively compared to their respective kinetic effects on enzymatic activity. There are two distinct types of substrate interaction with FBPasec, corresponding to catalytic and inhibitory binding, respectively. Inhibitory substrate binding shares several characteristics with F-2,6-P2 binding which indicates that both ligands bind at the same site. However, F-2,6-P2 does not prevent fluorescence transitions attributed to catalytic substrate binding. The marked synergistic inhibition of FBPasec by AMP and F-2,6-P2 appears to arise via AMP's promotion of F-2,6-P2 binding. Based on the X-ray crystal structure of porcine kidney FBPase our modelling studies suggest the existence of a distinct F-1,6-P2/F-2,6-P2 inhibitory binding site which partially overlaps with the enzyme's catalytic site. We propose that a pronounced allosteric transition mediated by AMP binding increases access of F-1,6-P2 and F-2,6-P2 to this common inhibitory binding site.

THE ORGANIZATION AND REGULATION OF PLANT GLYCOLYSIS

This review discusses the organization and regulation of the glycolytic pathway in plants and compares and contrasts plant and nonplant glycolysis. Plant glycolysis exists both in the cytosol and plastid, and the parallel reactions are catalyzed by distinct nuclear-encoded isozymes. Cytosolic glycolysis is a complex network containing alternative enzymatic reactions. Two alternate cytosolic reactions enhance the pathway's ATP yield through the use of pyrophosphate in place of ATP. The cytosolic glycolytic network may provide an essential metabolic flexibility that facilitates plant development and acclimation to environmental stress. The regulation of plant glycolytic flux is assessed, with a focus on the fine control of enzymes involved in the metabolism of fructose-6-phosphate and phosphoenolpyruvate. Plant and nonplant glycolysis are regulated from the "bottom up" and "top down," respectively. Research on tissue- and developmental-specific isozymes of plant glycolytic enzymes is summarized. Potential pitfalls associated with studies of glycolytic enzymes are considered. Some glycolytic enzymes may be multifunctional proteins involved in processes other than carbohydrate metabolism.