The fluid mosaic model of the structure of cell membranes

A fluid mosaic model is presented for the gross organization and structure of the proteins and lipids of biological membranes. The model is consistent with the restrictions imposed by thermodynamics. In this model, the proteins that are integral to the membrane are a heterogeneous set of globular molecules, each arranged in an amphipathic structure, that is, with the ionic and highly polar groups protruding from the membrane into the aqueous phase, and the nonpolar groups largely buried in the hydrophobic interior of the membrane. These globular molecules are partially embedded in a matrix of phospholipid. The bulk of the phospholipid is organized as a discontinuous, fluid bilayer, although a small fraction of the lipid may interact specifically with the membrane proteins. The fluid mosaic structure is therefore formally analogous to a two-dimensional oriented solution of integral proteins (or lipoproteins) in the viscous phospholipid bilayer solvent. Recent experiments with a wide variety of techniqes and several different membrane systems are described, all of which abet consistent with, and add much detail to, the fluid mosaic model. It therefore seems appropriate to suggest possible mechanisms for various membrane functions and membrane-mediated phenomena in the light of the model. As examples, experimentally testable mechanisms are suggested for cell surface changes in malignant transformation, and for cooperative effects exhibited in the interactions of membranes with some specific ligands. Note added in proof: Since this article was written, we have obtained electron microscopic evidence (69) that the concanavalin A binding sites on the membranes of SV40 virus-transformed mouse fibroblasts (3T3 cells) are more clustered than the sites on the membranes of normal cells, as predicted by the hypothesis represented in Fig. 7B. T-here has also appeared a study by Taylor et al. (70) showing the remarkable effects produced on lymphocytes by the addition of antibodies directed to their surface immunoglobulin molecules. The antibodies induce a redistribution and pinocytosis of these surface immunoglobulins, so that within about 30 minutes at 37 degrees C the surface immunoglobulins are completely swept out of the membrane. These effects do not occur, however, if the bivalent antibodies are replaced by their univalent Fab fragments or if the antibody experiments are carried out at 0 degrees C instead of 37 degrees C. These and related results strongly indicate that the bivalent antibodies produce an aggregation of the surface immunoglobulin molecules in the plane of the membrane, which can occur only if the immunoglobulin molecules are free to diffuse in the membrane. This aggregation then appears to trigger off the pinocytosis of the membrane components by some unknown mechanism. Such membrane transformations may be of crucial importance in the induction of an antibody response to an antigen, as well as iv other processes of cell differentiation.

Optimal computer folding of large RNA sequences using thermodynamics and auxiliary information

This paper presents a new computer method for folding an RNA molecule that finds a conformation of minimum free energy using published values of stacking and destabilizing energies. It is based on a dynamic programming algorithm from applied mathematics, and is much more efficient, faster, and can fold larger molecules than procedures which have appeared up to now in the biological literature. Its power is demonstrated in the folding of a 459 nucleotide immunoglobulin gamma 1 heavy chain messenger RNA fragment. We go beyond the basic method to show how to incorporate additional information into the algorithm. This includes data on chemical reactivity and enzyme susceptibility. We illustrate this with the folding of two large fragments from the 16S ribosomal RNA of Escherichia coli.

Denaturant m values and heat capacity changes: relation to changes in accessible surface areas of protein unfolding

Denaturant m values, the dependence of the free energy of unfolding on denaturant concentration, have been collected for a large set of proteins. The m value correlates very strongly with the amount of protein surface exposed to solvent upon unfolding, with linear correlation coefficients of R = 0.84 for urea and R = 0.87 for guanidine hydrochloride. These correlations improve to R = 0.90 when the effect of disulfide bonds on the accessible area of the unfolded protein is included. A similar dependence on accessible surface area has been found previously for the heat capacity change (delta Cp), which is confirmed here for our set of proteins. Denaturant m values and heat capacity changes also correlate well with each other. For proteins that undergo a simple two-state unfolding mechanism, the amount of surface exposed to solvent upon unfolding is a main structural determinant for both m values and delta Cp.

Hydrogen bridges in crystal engineering: interactions without borders

A hydrogen bond, X-H...A, is an interaction wherein a hydrogen atom is attracted to two atoms, X and A, rather than just one and so acts like a bridge between them. This attraction always increases with increasing electronegativity of X and A, and in the classical view all hydrogen bonds are highly electrostatic and sometimes even partly covalent. Gradually, the concept of a hydrogen bond became more relaxed to include weaker interactions, provided some electrostatic character remains. In the limit, these weak hydrogen bonds have considerable dispersive-repulsive character and merge into van der Waals interactions. A great variety of hydrogen bonds are observed in the solid state and the aim of this article is to highlight some features common to all these bonds and further to suggest that the term hydrogen bridge is perhaps a better descriptor for them. Such a description recognizes an interaction without borders and one that admits of much variation in its relative covalent, electrostatic, and van der Waals content.

The effect of insulin on the disposal of intravenous glucose. Results from indirect calorimetry and hepatic and femoral venous catheterization

The effect of insulin on the disposal of intravenous glucose was examined employing the euglycemic insulin clamp technique in 24 normal subjects. When the plasma insulin concentration was raised by approximately 100 μU/ml, total glucose metabolism rose to 6.63 ± 0.38 mg/kg · min. Basal splanchnic (hepatic venous catheter technique) glucose production, 2.00 increased only slightly. These results suggest that the ability of higher doses of insulin to further stimulate glucose metabolism is primarily the result of increased glucose storage by peripheral tissues, most likely muscle. 0.15 ± mg/kg · min, reverted to a small net glucose uptake which averaged 0.33 mg/kg · min over the ensuing 2 h. This represented only 5% of the total glucose metabolized. In contrast, leg (femoral venous catheterization) glucose uptake rose from 1.18 ± 0.14 to 8.40 ± 1.06 mg/kg of leg wt. per min. If all muscles in the body respond similarly to those in the leg, muscle would account for 85% of the total glucose metabolism. To determine the relative contributions of glucose oxidation versus glucose storage by peripheral tissues following hyperinsulinemia, we performed euglycemic insulin clamp studies in combination with indirect calorimetry. Basal glucose oxidation, 1.21 ± 0.10 mg/kg min, rose to 2.28 ± 0.16 (P < 0.01), and this increase above baseline accounted for only 20% of the total glucose metabolized, 5.44 ± 0.38 mg/kg · min. Following insulin, glucose storage increased to 3.18 ± 0.34 mg/kg min and was responsible for 59% of the total glucose metabolized. These results indicate that the primary effect of insulin on muscle tissue is to enhance glucose storage, presumably as glycogen. When a higher degree of hyperinsulinemia (163 ± 19 μl/ml) was created while maintaining euglycemia, total glucose metabolism (7.99 ± 0.58) and glucose storage (5.30 ± 0.80) both increased (P < 0.01) compared with the lower dose insulin clamp study, but glucose oxidation (2.70 ± 0.16 mgμkg min)increased only slightly. These results suggest that the ability of higher doses of insulin to further stimulate glucose metabolism is primarily the result of increased glucose storage by peripheral tissues, most likely muscle.

The theoretical bases of indirect calorimetry: a review

Indirect calorimetry is the method by which the type and rate of substrate utilization, and energy metabolism are estimated in vivo starting from gas exchange measurements. This technique provides unique information, is noninvasive, and can be advantageously combined with other experimental methods to investigate numerous aspects of nutrient assimilation, thermogenesis, the energetics of physical exercise, and the pathogenesis of metabolic diseases. Since its use as a research tool in metabolism is growing, the theoretical bases of indirect calorimetry are here reviewed in a detailed and orderly fashion. Special cases, such as the occurrence of net lipid synthesis or gluconeogenesis, are formally considered with derivation of explicit stoichiometric equations. The limitations of indirect calorimetry, both theoretical and technical, are discussed in the context of circumstances of clinical interest in metabolism.

Brain metabolism during fasting

Catheterization of cerebral vessels in three obese patients undergoing 5-6 wk of starvation demonstrated that beta-hydroxybutyrate and acetoacetate replaced glucose as the predominant fuel for brain metabolism. A strikingly low respiratory quotient was also observed, suggesting a carboxylation mechanism as a means of disposing of some of the carbon of the consumed substrates.

Identification and structural characterization of the ATP/ADP-binding site in the Hsp90 molecular chaperone

Hsp90 molecular chaperones in eukaryotic cells play essential roles in the folding and activation of a range of client proteins involved in cell cycle regulation, steroid hormone responsiveness, and signal transduction. The biochemical mechanism of Hsp90 is poorly understood, and the involvement of ATP in particular is controversial. Crystal structures of complexes between the N-terminal domain of the yeast Hsp90 chaperone and ADP/ATP unambiguously identify a specific adenine nucleotide binding site homologous to the ATP-binding site of DNA gyrase B. This site is the same as that identified for the antitumor agent geldanamycin, suggesting that geldanamycin acts by blocking the binding of nucleotides to Hsp90 and not the binding of incompletely folded client polypeptides as previously suggested. These results finally resolve the question of the direct involvement of ATP in Hsp90 function.

Fibroblast growth factor 21 reverses hepatic steatosis, increases energy expenditure, and improves insulin sensitivity in diet-induced obese mice

OBJECTIVE

Fibroblast growth factor 21 (FGF21) has emerged as an important metabolic regulator of glucose and lipid metabolism. The aims of the current study are to evaluate the role of FGF21 in energy metabolism and to provide mechanistic insights into its glucose and lipid-lowering effects in a high-fat diet-induced obesity (DIO) model.

RESEARCH DESIGN AND METHODS

DIO or normal lean mice were treated with vehicle or recombinant murine FGF21. Metabolic parameters including body weight, glucose, and lipid levels were monitored, and hepatic gene expression was analyzed. Energy metabolism and insulin sensitivity were assessed using indirect calorimetry and hyperinsulinemic-euglycemic clamp techniques.

RESULTS

FGF21 dose dependently reduced body weight and whole-body fat mass in DIO mice due to marked increases in total energy expenditure and physical activity levels. FGF21 also reduced blood glucose, insulin, and lipid levels and reversed hepatic steatosis. The profound reduction of hepatic triglyceride levels was associated with FGF21 inhibition of nuclear sterol regulatory element binding protein-1 and the expression of a wide array of genes involved in fatty acid and triglyceride synthesis. FGF21 also dramatically improved hepatic and peripheral insulin sensitivity in both lean and DIO mice independently of reduction in body weight and adiposity.

CONCLUSIONS

FGF21 corrects multiple metabolic disorders in DIO mice and has the potential to become a powerful therapeutic to treat hepatic steatosis, obesity, and type 2 diabetes.

Consideration of the Possibility that the slow step in protein denaturation reactions is due to cis-trans isomerism of proline residues

A model is proposed to account for the observation that the denaturation of small proteins apparently occurs in two kinetic phases. It is suggested that only one of these phases--the fast one--is actually an unfolding process. The slow phase is assumed to arise from the cis-trans isomerism of proline residues in the denaturated protein. From model compound data, it is shown that the expected rate for isomerism is in satisfactory agreement with the rates actually observed for protein folding. It is also shown that a simple model of protein unfolding based on the isomerism concept is very successful in accounting for many known experimental characteristics of the kinetics and thermodynamic of protein denaturation. Thus, the model is able to predict that two kinetic phases will be seen in the transition region while none are seen in the base-line regions, that both the fast and slow refolding phases lead to the native protein as the product, that the fast phase becomes the only observable phase for jumps ending far in the denatured base-line region, that most or all small proteins show a limiting low-temperature activation energy of ca. 20,000 cal, and that the relaxtion time for the slow phase seen in cytochrome c denaturation is much shorter than for all other small proteins. By utilizing "double-jump" experiments, it is shown directly that the slow phase is not part of the unfolding process but that it corresponds to a transition among two or more denatured forms which have identical spectroscopic (286.5 nm) properties. Thus, the slow relaxation is "invisible" except in the transition region where it couples to the fast unfolding equilibrium. Finally, since the present model assumes that only one of the major kinetic phases seen in denaturation reactions is concerned with the denaturation process per se, it is in agreement with numerous thermodynamic studies which show consistency with the two-state model for unfolding.

Orexin activation counteracts decreases in nonexercise activity thermogenesis (NEAT) caused by high-fat diet

Overweight and obesity result from an imbalance between caloric intake and energy expenditure, including expenditure from spontaneous physical activity (SPA). Changes in SPA and resulting changes in non-exercise activity thermogenesis (NEAT) likely interact with diet to influence risk for obesity. However, previous research on the relationship between diet, physical activity, and energy expenditure has been mixed. The neuropeptide orexin is a driver of SPA, and orexin neuron activity can be manipulated using DREADDs (Designer Receptors Exclusively Activated by Designer Drugs). We hypothesized that HFD decreases SPA and NEAT, and that DREADD-mediated activation of orexin neuron signaling would abolish this decrease and produce an increase in NEAT instead. To test these ideas, we characterized behaviors to determine the extent to which access to a high-fat diet (HFD) influences the proportion and probability of engaging in food intake and activity. We then measured NEAT following access to HFD and following a DREADD intervention targeting orexin neurons. Two cohorts of orexin-cre male mice were injected with an excitatory DREADD virus into the caudal hypothalamus, where orexin neurons are concentrated. Mice were then housed in continuous metabolic phenotyping cages (Sable Promethion). Food intake, indirect calorimetry, and SPA were automatically measured every second. For cohort 1 (n=8), animals were given access to chow, then switched to HFD. For cohort 2 (n=4/group), half of the animals were given access to HFD, the other access to chow. Then, among animals on HFD, orexin neurons were activated following injections of clozapine n-oxide (CNO). Mice on HFD spent significantly less time eating (p<0.01) and more time inactive compared to mice on chow (p<0.01). Following a meal, mice on HFD were significantly more likely to engage in periods of inactivity compared to those on chow (p<0.05). NEAT was decreased in animals on HFD, and was increased to the NEAT level of control animals following activation of orexin neurons with DREADDs. Food intake (kilocalories) was not significantly different between mice on chow and HFD, yet mice on chow expended more energy per unit of SPA, relative to that in mice consuming HFD. These results suggest that HFD consumption reduces SPA and NEAT, and increases inactivity following a meal. Together, the data suggest a change in the efficiency of energy expenditure based upon diet, such that SPA during HFD burns fewer calories compared to SPA on a standard chow diet.

Mechanical and chemical unfolding of a single protein: a comparison

Is the mechanical unraveling of protein domains by atomic force microscopy (AFM) just a technological feat or a true measurement of their unfolding? By engineering a protein made of tandem repeats of identical Ig modules, we were able to get explicit AFM data on the unfolding rate of a single protein domain that can be accurately extrapolated to zero force. We compare this with chemical unfolding rates for untethered modules extrapolated to 0 M denaturant. The unfolding rates obtained by the two methods are the same. Furthermore, the transition state for unfolding appears at the same position on the folding pathway when assessed by either method. These results indicate that mechanical unfolding of a single protein by AFM does indeed reflect the same event that is observed in traditional unfolding experiments. The way is now open for the extensive use of AFM to measure folding reactions at the single-molecule level. Single-molecule AFM recordings have the added advantage that they define the reaction coordinate and expose rare unfolding events that cannot be observed in the absence of chemical denaturants.

Application of conducting polymers to biosensors

Recently, conducting polymers have attracted much interest in the development of biosensors. The electrically conducting polymers are known to possess numerous features, which allow them to act as excellent materials for immobilization of biomolecules and rapid electron transfer for the fabrication of efficient biosensors. In the present review an attempt has been made to describe the salient features of conducting polymers and their wide applications in health care, food industries, environmental monitoring etc.

Determinants of 24-hour energy expenditure in man. Methods and results using a respiratory chamber

Daily human energy requirements calculated from separate components of energy expenditure are inaccurate and usually in poor agreement with measured energy intakes. Measurement of energy expenditure over periods of 24 h or longer is needed to determine more accurately rates of daily energy expenditure in humans. We provide a detailed description of a human respiratory chamber and methods used to determine rates of energy expenditure over 24-h periods in 177 subjects. The results show that: fat-free mass (FFM) as estimated by densitometry is the best available determinant of 24-h energy expenditures (24EE) and explains 81% of the variance observed between individuals (24EE [kcal/d] = 597 + 26.5 FFM); 24EE in an individual is very reproducible (coefficient of variation = 2.4%); and even when adjusted for differences in FFM, there is still considerable interperson variability of the daily energy expenditure. A large portion of the variability of 24EE among individuals, independent of differences in body size, was due to variability in the degree of spontaneous physical activity, i.e., "fidgeting," which accounted for 100-800 kcal/d in these subjects.

Discrepancy between self-reported and actual caloric intake and exercise in obese subjects

BACKGROUND AND METHODS

Some obese subjects repeatedly fail to lose weight even though they report restricting their caloric intake to less than 1200 kcal per day. We studied two explanations for this apparent resistance to diet--low total energy expenditure and underreporting of caloric intake--in 224 consecutive obese subjects presenting for treatment. Group 1 consisted of nine women and one man with a history of diet resistance in whom we evaluated total energy expenditure and its main thermogenic components and actual energy intake for 14 days by indirect calorimetry and analysis of body composition. Group 2, subgroups of which served as controls in the various evaluations, consisted of 67 women and 13 men with no history of diet resistance.

RESULTS

Total energy expenditure and resting metabolic rate in the subjects with diet resistance (group 1) were within 5 percent of the predicted values for body composition, and there was no significant difference between groups 1 and 2 in the thermic effects of food and exercise. Low energy expenditure was thus excluded as a mechanism of self-reported diet resistance. In contrast, the subjects in group 1 underreported their actual food intake by an average (+/- SD) of 47 +/- 16 percent and overreported their physical activity by 51 +/- 75 percent. Although the subjects in group 1 had no distinct psychopathologic characteristics, they perceived a genetic cause for their obesity, used thyroid medication at a high frequency, and described their eating behavior as relatively normal (all P < 0.05 as compared with group 2).

CONCLUSIONS

The failure of some obese subjects to lose weight while eating a diet they report as low in calories is due to an energy intake substantially higher than reported and an overestimation of physical activity, not to an abnormality in thermogenesis.

Structural basis for inhibition of the Hsp90 molecular chaperone by the antitumor antibiotics radicicol and geldanamycin

The cellular activity of several regulatory and signal transduction proteins, which depend on the Hsp90 molecular chaperone for folding, is markedly decreased by geldanamycin and by radicicol (monorden). We now show that these unrelated compounds both bind to the N-terminal ATP/ADP-binding domain of Hsp90, with radicicol displaying nanomolar affinity, and both inhibit the inherent ATPase activity of Hsp90 which is essential for its function in vivo. Crystal structure determinations of Hsp90 N-terminal domain complexes with geldanamycin and radicicol identify key aspects of their nucleotide mimicry and suggest a rational basis for the design of novel antichaperone drugs.