Nontoxic toxins: the energetics of coevolution

I propose that the relative productivity of an organism in its community importantly influences the kinds of predator defences that are most effective relative to their costs. Two classes of defence mechanisms are distinguished: those that give some energy to the predator (tribute defences) and those that do not (non-tribute defences). The pattern of distribution of these two types of defences both geographically and among different taxonomic groups of organisms support the hypothesis that the productivity of a species relative to others in its community is an important determinant of the type ol defence a species adopts.     

The comparative energetics of rigid endothermy: the Arvicolidae

Rate of metabolism and body temperature were examined in 24 species of arvicolid rodents, for 15 of which data are presented here, to determine the factors that influence their level of energy expenditure. Arvicolids are characterized by a high, precisely-regulated body temperature, a high basal rate of metabolism by general mammalian standards, and a standard thermal conductance, except at large masses, when low conductances can occur. No evidence of entrance into torpor is known for any arvicolid; its absence is associated with especially high basal rates at masses smaller than 52 g. Arvicolids that live in cold climates, i.e. at high altitudes and latitudes, have higher basal rates than species that live in other environments. Basal rates, however, appear to be independent of food habits, presumably because of the small mass of most species and possibly because all species are herbivorous. A small size in combination with herbivory has permitted arvicolids to maintain continuous endothermy, which in turn has permitted them to exploit cool-to-cold environments by means of a high rate of production. The use of daily or seasonal torpor might well have prevented arvicolids from attaining the pivotal position in energy transfer that they presently occupy in many high-latitude communities.     

Locomotor energetics in primates: gait mechanics and their relationship to the energetics of vertical and horizontal locomotion

All primates regularly move within three-dimensional arboreal environments and must often climb, but little is known about the energetic costs of this critical activity. Limited previous work on the energetics of incline locomotion suggests that there may be differential selective pressures for large compared to small primates in choosing to exploit a complex arboreal environment. Necessary metabolic and gait data have never been collected to examine this possibility and biomechanical mechanisms that might explain size-based differences in the cost of arboreal movement. Energetics and kinematics were collected for five species of primate during climbing and horizontal locomotion. Subjects moved on a treadmill with a narrow vertical substrate and one with a narrow horizontal substrate at their maximum sustainable speed for 10–20 min while oxygen consumption was monitored. Data during climbing were compared to those during horizontal locomotion and across size. Results show that climbing energetic costs were similar to horizontal costs for small primates (<0.5 kg) but were nearly double for larger species. Spatio-temporal gait characteristics suggest that the relationship between the cost of locomotion and the rate of force production changes between the two locomotor modes. Thus, the main determinants of climbing costs are fundamentally different from those during horizontal locomotion. These new results combining spatiotemporal and energetic data confirm and expand on our previous argument (Hanna et al.: Science 320 (2008) 898) that similar costs of horizontal and vertical locomotion in small primates facilitated the successful occupation of a fine-branch arboreal milieu by the earliest primates.     

Minireview: demystifying microbial reaction energetics

The biology literature is rife with misleading information on how to quantify catabolic reaction energetics. The principal misconception is that the sign and value of the standard Gibbs energy () define the direction and energy yield of a reaction; they do not. is one part of the actual Gibbs energy of a reaction (ΔG_r ), with a second part accounting for deviations from the standard composition. It is also frequently assumed that applies only to 25 °C and 1 bar; it does not. is a function of temperature and pressure. Here, we review how to determine ΔG_r as a function of temperature, pressure and chemical composition for microbial catabolic reactions, including a discussion of the effects of ionic strength on ΔG_r and highlighting the large effects when multi-valent ions are part of the reaction. We also calculate ΔG_r for five example catabolisms at specific environmental conditions: aerobic respiration of glucose in freshwater, anaerobic respiration of acetate in marine sediment, hydrogenotrophic methanogenesis in a laboratory batch reactor, anaerobic ammonia oxidation in a wastewater reactor and aerobic pyrite oxidation in acid mine drainage. These examples serve as templates to determine the energy yields of other catabolic reactions at environmentally relevant conditions.     

Microbial energetics

An understanding of the mechanisms by which bacteria derive their energy is clearly important for the prediction of growth yields. Bacteria can synthesize ATP by a variety of routes, by fermentation, by oxidative phosphorylation, and possibly by the excretion of metabolic end products. The bacterium Escherichia coli has been studied extensively and a great deal is now known about the different membrane-bound multi-enzyme complexes that are responsible for oxidative phosphorylation. The efficiency of oxidative phosphorylation can vary not only between different bacteria that have adapted to particular ecological niches but also in an individual bacterium grown under different conditions or modified genetically by mutation with respect to its parent. Clearly, the concept that bacteria always grow with maximum thermodynamic efficiency is erroneous and it is important, therefore, to be able to assess the efficiency of energy conversion as well as the biochemical and genetical factors that regulate the physiological expression of energy-yielding reactions if they are to be manipulated by the investigator.     

Incubation energetics of the Laysan Albatross

Summary The energy expenditure of incubating and foraging Laysan Albatross (Diomedea immutabilis, mean body weight 3.07 kg) was estimated by means of the doubly-labelled water technique. During incubation, the energy expenditure was similar to that of resting birds that were not incubating an egg. The energy expenditure of foraging albatross (2072 kJ/day) was 2.6 times that of resting birds. It was concluded that the energy expenditure of the tropical Laysan Albatross was not less than that of species foraging over cold, high-latitude oceans. An energy budget compiled for an incubating pair of albatross revealed that the energy expenditure of the female was greater than that of the male bird, during the incubation period.     

Dissecting the energetics of the apoflavodoxin-FMN complex

Many flavoproteins are non-covalent complexes between FMN and an apoprotein. To understand better the stability of flavoproteins, we have studied the energetics of the complex between FMN and the apoflavodoxin from Anabaena PCC 7119 by a combination of site-directed mutagenesis, titration calorimetry, equilibrium binding constant determinations, and x-ray crystallography. Comparison of the strength of the wild type and mutant apoflavodoxin-FMN complexes and that of the complexes between wild type apoflavodoxin and shortened FMN analogues (riboflavin and lumiflavin) allows the dissection of the binding energy into contributions associated with the different parts of the FMN molecule. The estimated contribution of the phosphate is greatest, at 7 kcal mol(-1); that of the isoalloxazine is of around 5-6 kcal mol(-1) (mainly due to interaction with Trp-57 and Tyr-94 in the apoprotein) and the ribityl contributes least: around 1 kcal mol(-1). The stabilization of the complex is both enthalpic and entropic although the enthalpy contribution is dominant. Both the phosphate and the isoalloxazine significantly contribute to the enthalpy of binding. The ionic strength does not have a large effect on the stability of the FMN complex because, although it weakens the phosphate interactions, it strengthens the isoalloxazine-protein hydrophobic interactions. Phosphate up to 100 mM does not affect the strength of the riboflavin complex, which suggests the isoalloxazine and phosphate binding sites may be independent in terms of binding energy. Interestingly, we find crystallographic evidence of flexibility in one of the loops (57-62) involved in isoalloxazine binding.     

The energetics of coprophagy: a theoretical analysis

Lagomorphs, many rodents and some other small mammals eat their faeces during the part of the day when they are not foraging for fresh food. One of the possible benefits of this habit of coprophagy is that it may enable them to extract more energy from their food. A computer model is used to assess the likely benefits and explore their relationships to food, feeding rate and gut morphology. The predicted benefits are much larger for hindgut fermenters than for foregut fermenters, and especially large for hindgut fermenters with relatively small fermentation chambers. They are larger for poor foods (with lower proportions of cell contents) than for richer ones. At low feeding rates the energetic advantage of coprophagy may disappear if the faeces from food eaten during one feeding period emerge largely during the next, but this can be avoided by adjusting the rate of passage of gut contents during the intervening rest period.     

Standard energetics of phyllostomid bats: the inadequacies of phylogenetic-contrast analyses

The basal rates of metabolism (BMR) of bats belonging to the family Phyllostomidae are re-examined after an earlier correlation with food habits was rejected because it did not take phylogeny into consideration. This rejection was based on an erroneous attribution of food habits and on an analytical method, phylogenetic contrasts, that ignores interactions that occur among character states and preferentially attributes responsibility for character states to phylogeny. The re-examination made here was based on analysis of covariance, which makes no a priori assumptions on the relative impact of factors that influence character states and permits factor interactions to be identified. A resulting model, based on variation in body mass, food habits, occurrence with respect to elevation, and residence on islands or continents, accounts for 99.4% of the variation in the BMR of 30 species of phyllostomids. Basal rate is also correlated with subfamily, but only if food habits are excluded because they are correlated with subfamily affiliation, as is residence on islands and continents, two examples of factor interaction. The preference to assign the effects of food habits and island residence on basal rate to subfamily affiliation (and phylogeny) is not justified. The concept that quantitative physiological characters can be transmitted via phylogeny without regard to the habits of animals and the characteristics of their environments cannot be defended. Phylogeny is the historical context in which the evolution of character states occurs, not the 'cause' of their evolution.     

The energetics of bacterial growth: a reassessment

The growth yield of microbial cultures can be used to estimate the efficiency of energy generation during a fermentation or respiration, in the past, the assessment of this efficiency in organisms carrying out a respiration has been the subject of many heated debates. This has partly been caused by the complexity of microbial respiratory chains. Strains of Escherichia coli specifically modified in their respiratory chain have been used recently to re-evaluate the energetic efficiency of the bacterial respiration using chemostat cultures. The different strains indeed show different growth efficiencies. The physiological significance of energetically less-efficient branches of the respiratory chain is discussed.     

The energetics of obesity

Although there is little argument about the state of energy imbalance that produces weight gain, there is considerable argument about the respective role of genetics, diet and physical activity in achieving obesity. In the USA, obesity has increased in the last decades despite a concomitant decrease in total energy and fat intake suggesting that there has been a dramatic drop in total energy expenditure. In this review, we investigated the respective role of resting metabolic rate, post-prandial thermogenesis, and activity energy expenditure in this lower energy output, and provided evidence that physical inactivity is the major contributor. Based on Jean Mayer original observation (Mayer et al., 1954), we hypothesize that there is a level of physical activity below which mechanisms of body mass regulation are impaired. The increasing prevalence of obesity may reflect the fact the majority of the population has fallen below such a level of physical activity. However, a causal relation between physical inactivity and obesity is still difficult to prove, probably because of the lack of longitudinal models to investigate the physiological consequences of inactivity and because the deleterious consequences of sedentary behaviors are essentially deduced from the benefits of exercise training. By using long term strict bed rest as a unique model of inactivity, we provide evidence that inactivity per se indeed disrupts fuel homeostasis and partitions post-absorptive and post-prandial fat use towards storage, thus promoting weight gain in the long term. More research is needed to investigate mechanisms and to determine the minimal physical activity our body has been engineered for by evolution.     

Streptavidin-biotin binding energetics

The high affinity energetics in the streptavidin-biotin system provide an excellent model system for studying how proteins balance enthalpic and entropic components to generate an impressive overall free energy for ligand binding. We review here concerted site-directed mutagenesis, biophysical, and computational studies of aromatic and hydrogen bonding interaction energetics between streptavidin and biotin. These results also have provided insight into how streptavidin builds a large activation barrier to dissociation by managing the enthalpic and entropic activation components. Finally, we review recent studies of the biotin dissociation pathway that address the fundamental question of how ligands exit protein binding pockets.     

Structural energetics of the molten globule state

Certain partly ordered protein conformations, commonly called “moltenglobule states,” are widely believed to represent protein folding intermediates. Recentstructural studies of molten globule states ofdifferent proteins have revealed features whichappear to be general in scope. The emergingconsensus is that these partly ordered forms exhibit a high content of secondary structure, considerable compactness, nonspecific tertiary structure, and significant structural flexibility. These characteristics may be used to define ageneral state of protein folding called “the molten globule state,” which is structurally andthermodynamically distinct from both the native state and the denatured state. Despite exaatensive knowledge of structural features of afew molten globule states, a cogent thermodynamic argument for their stability has not yetbeen advanced. The prevailing opinion of thelast decade was that there is little or no enthalpy difference or heat capacity differencebetween the molten globule state and the unfolded state. This view, however, appears to beat variance with the existing database of protein structural energetics and with recent estimates of the energetics of denaturation of α-lactalbumin, cytochrome c, apomyoglobin, and T4 lysozyme. We discuss these four proteins at length. The results of structural studies, together with the existing thermodynamic values for fundamental interactions in proteins, provide the foundation for a structural thermodynamic framework which can account for the observed behavior of molten globule states. Within this framework, we analyze the physical basis for both the high stability of several molten globule states and the low probability of other protential folding intermediates. Additionally, we consider, in terms of reduced enthalpy changes and disrupted cooperative interactions, the thermodynamic basis for the apparent absence of a thermally induced, cooperative unfolding transition for some molten globule states. © 1993 Wiley-Liss, Inc.     

Theoretical studies of biliverdin: energetics of the reduction pathways to bilirubin

Geometries and energies of formation of bilirubin formed by reduction of biliverdin via three meso carbon sites, the , and positions, have been calculated using semiempirical methods. It has been shown that -bilirubin with a ridge-tile conformation forms six intramolecular hydrogen bonds and is the most stable of the three above mentioned positions by at least 22 kcal mol^–1. Reduction pathways for -, - and -bilirubin formations from biliverdin are studied in detail. The roles of loss of conjugation and hydrogen bond formations in stability of different conformers have been discussed. -Bilirubin was fully optimized by using ab initio methods. Fine refinements of calculated results show excellent agreement with experimental results. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00894-002-0078-9.Electronic Supplementary Material available.     

Autumn-winter energetics of Holarctic tree squirrels:a review

Similarities in general size, geometry, lifestyle, and environment mean that certain energetic constraints are common and peculiar to Holarctic tree squirrels as a group. Holarctic tree squirrels are relatively small, diurnal mammals which, in association with their food niche, maintain activity throughout the autumn-winter period. Despite this, they exhibit no major morphological or physiological adaptations to minimize energy expenditure at low temperatures; on the contrary, both basal metabolism and conductance are higher than expected on the grounds of physical size. When they are active energy expenditure is therefore strongly influenced by effective ambient temperature for these species when active in their natural autumn-winter environments. Nest use allows near-basal metabolism at most natural ambient temperatures. The balance of economical inactivity against feeding rewards offset by cold exposure must therefore be a crucial aspect of the lifestyle of these squirrels.