The Evolution of Endothermy:Role for Membranes and Molecular Activity By Paul L. Else, N. Turner and A.J. Hulbert
Endothermy in Vertebrates:A Defining Property Characteristics (Compared to Ectotherms) • High Rate of Metabolism/Oxygen Consumption • Tissues have high levels of aerobic enzyme activity • Large mitochondrial volume and membrane densities (Table 1) • Internal organs are larger • Organisms with a “high cost of living” which impact every aspect of their life.
Why is there a higher rate of metabolism/O2 consumption??? There are differences in the overall rates of cellular processes when comparing endotherms to ectotherms. (ex. Sodium pump / Protein synthesis
Three Models of Metabolism Show Recurring Themes • Compared body temp between ectotherms and endotherms (vertebrates) and mass-matched species. • Compared animals of varying body mass within a vertebrate class • Metabolic changes that occur during different phases of mammalian development
The Three Ideas of Membrane Pacemaker Theory of Metabolism • Only a few cellular processed consume most of the energy used by most organisms • Ex. Na+/K+ pump drives other processed (moves other ions across membranes) • What does this say???? • No new or special processes acct for the differences in metabolism between species
Pacemaker Theory (continued) • Energy consuming processes acct for similar proportions/% of metabolism in animals with very different rates of metabolism • What does this say??? • The amt of energy associated with a process may be different among species, but as a percentage of metabolic rate, it is similar.
Pacemaker Theory (cont.) • Most processes are directly or indirectly linked to membranes. • (examples – maintenance of ion gradients, protein synthesis, etc. • What does this mean… • The variations in animal metabolism is due to MEMBRANES setting the pace of a small number of Energy consuming processes.
And thus the Membrane Pacemaker Theory of Metabolism… was born!!!!!
Testing the Theory • “The Sodium Pump” A ubiquitous protein in the cells and tissues of all animals • Endotherms vs. Ectotherms tissues • There are no differences in the number or density of sodium pumps • There are differences in the rate of enzyme activity/ molecular activity. • (Figure 1)
Sodium Pump Study More Findings… • Changes in the Pump molecular activity is common during development
Study of the Membrane Environment • Ectotherms have more monounsaturated membranes and fatty acids with only one C=C. • Endotherms compared to Ectotherms, Smaller compared with Larger –sized mammals and birds, and adult compared with young mammals tend to have higher molecular activities and fatty acids with more C=C double bonds.
Membrane “Crossover” Experiments • Used tissues from rat (with high mol. Activity) • Brain and Kidney • Used same tissues from toad (low molecular activity. • METHOD- detergent to lower activity to similar levels between the two, heat to destroy pump while fatty acid comp remained same.
Cross Over • Enzyme activity was reconstituted with same species membrane • Mol activ. Back to normal level • Membrane reconstituted with membrane of another species. • Mol activ. Shifts to the added membranes orig.activity. • Ex. Toad added to rat…reduced Mol. Act. • Other CO exper- During Development • Similar Findings
Possible Mechanisms • The use for polyunsaturates suggests an improvement in the activity of membrane proteins. • Looked at differences in acyl composition of membrane which suggests an altering of the electrical fields within the bilayer • It is still unknown how they influence lipid protein interactions.
Mechanisms (continued) • Experiments on Physical Properties of Membrane Lipids and influence on Molecular activity • Shows increasing molecular activity with increasing area occupied by lipids. • What does this say??? • MA and area lipids correlate to polyunsaturation and • Electrical field strengths are dispersed by the “kinked” carbon chains of the unsaturated lipids
Summing it up • During evolution, as body mass increased, there was a need for animals to reduce their mass-specific met rates. • (too little surface area to get rid of their heat) • Membrane lipid composition becomes important. • If, during evolution, increased polyunsaturated membranes occurred, then an increase in Molecular activity would occur