Not too hot and not too cold: Thermal stress & how organisms adapt? (part 4)

Seasonal migrations in terrestrial & aquatic animals & daily choice by mammals of the right time of day for hunting illustrate how changes in temperature influence animal behavior. Temperature greatly affects all organisms & imposes limits on where they can survive because it influences every aspect of physiology from the structure of macromolecules to the rate at which essential enzyme reactions occur. Thus different species have evolved different physiological systems allowing them to live in temperatures such as 100oC in thermal springs or -80oC in the Antarctic.

Research into thermal stress provides insight into the physiology of temperature regulation in a vast array of species & helps us understand how these adaptations evolved over millions of years. At the present time scientists are trying to understand how the predicted increasing temperature of our planet will impact the distributions & survival of animals such as polar bears, mussels, & sea coral. Thus the research, which has immense practical importance, varies from field studies to biochemical & molecular laboratory experiment. We need to know a) what the optimal temperatures for different species are & why, b) how much change can they tolerate, & c) how close they are to the limit of their tolerance & how quickly can adaptations can evolve. For example global warming may cause extensive bleaching of coral reefs & disturb the availability of insects when migratory birds return to Europe in summer.

To study these questions in the laboratory scientists are now looking at how temperature influences each component of the cell. This complex biochemical & molecular research looks at cell components such as proteins, enzymes, lipid membranes, & nucleic acids.

Proteins & temperature:-

Temperature markedly affects the rate of chemical reactions e.g. cooking food in a pot or heating chemicals in a test tube speed up changes in the ingredients. The rate of a chemical reaction can also speed up if a catalyst is added e.g. transition metals such as iron & nickel, but the catalyst is not itself changed by the reaction. In our bodies enzymes, which are proteins, act as biological catalysts & speed up metabolic reactions e.g. conversion of glucose to energy  (Figure 1).

Reaction of enzyme with specific substrate to form active complex & subsequent release of product with enzyme left unchanged. (click to enlarge)

 

A useful analogy to describe enzyme activity was first postulated by Emil Fischer in 1894 (Figure 2).  The action of an enzyme is specific for a specific substrate because its amino acids are linked into a certain 3D shape (the lock) and thus only a substance (a key) with a specific shape will fit. When this occurs the enzyme lowers the energy of the substrates chemical bonds, and a new product or products are produced by new chemical bonding.  Note that the configuration of the enzyme changes during the reaction & then returns to its original configuration so it can recycle for new reactions. The enzyme can then repeat the process as it has itself not been changed. (Figure  2).

Lock & key analogy of enzyme activity. Note the temporary change in enzymes configuration, when it couples with substrate, does not involve the active site i.e. the essential lock. (click to enlarge)

Similarly to metal catalysts, enzymes are also influenced by the temperature. For example, the large decrease in metabolic rate occurs as hibernating animals cool down because this decreases the activity of metabolic enzymes involved in many processes. The effect of temperature on enzyme activity has widely been studied in ectotherms e.g. cold & warm water mussels & crabs as there is concern about whether they will be able to adapt to global warming (The physiology of climate change: how potentials for acclimatization and genetic adaptation will determine ‘winners’ and ‘losers’. G. N. Somero J Exp Biol 213, 912; 2010). For example the heart may fail if the enzymes responsible for energy production are less efficient at higher temperatures as their shape changes

The effect of temperature on biological activity can be described by the temperature coefficient or Q10. This describes the effect of a 10o C change in temperature has on the rate of a metabolic process. For many processes e.g. rate of respiration or enzyme activity the Q10 is 2X, within the animals normal temperature range. Outside this range the activity will decrease dramatically as the system in question is gradually damaged by the heat.

All animals use the same enzymes for essential metabolic processes e.g. energy production. The most striking everyday example how heat affects amino acids is the change seen in the egg white when it is boiled. Of course the changes in enzyme configuration with small temperature changes are much more subtle than this dramatic example, but the change in egg white protein does help visualize the effect. Since enzyme activity is affected by temperature adaptations have to occur in different environments. This is not done by altering the cell concentration of enzymes but rather their activity. The configuration of the active pocket (“lock”) cannot change as the substrate (keys) will then not fit, So the changes in configuration occurs in other parts of the amino acid composition. Thus genetic mutations alter single amino acids in the enzyme protein. These amino acid substitutions in the protein scaffolding alters the flexibility of the enzyme & make it more resistant to hot or cold temperature induced changes in its configuration. Thus in summary, fine tuning of amino acid composition in cells allows organisms to conduct the same enzymatic processes in vastly different thermal environments!

The role of chaperones in heat stress:- Despite the evolution of enzymes with resistance to heat damage structural changes will occur under heat stress. Repair mechanisms have been discovered. “Heat stress proteins” (“HSP”) were first discovered in the Drosophila fly but are produced by all organisms in response to  heat damage, & other insults e.g. hypoxia & hyperoxia. Their function took longer to clarify but one role is to refold proteins which have been denatured by heat damage back into their correct 3D structure. The HSP’s are referred to as “chaperones” & deficiencies in their function may also play a role in the pathogenesis of  diseases such as dementia & Parkinson’s disease.

 

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