11. ORGANISMS AND THERMAL ENERGY
505
acterized by a large contribution of the entropy term to the free energy
of activation (6, 7). This offsets the high values for the heat of activation, and makes it possible for these reactions to occur at relatively low
temperatures. The entropy of activation, in turn, depends upon the
change from order to disorder which occurs when the first few bonds
are broken (6, 7). These bonds are considered to be hydrogen bonds
(7), hence a change in the degree of "effectiveness" of hydrogen bonding might be expected to have marked effects on the ease of thermal
denaturation of proteins.
Thermolabile enzymes have been found, however, even in stenothermal thermophilic microorganisms. Gaughran (191-193) studied the
effect of temperature on the activity of the complete respiratory system
and of catalase, cytochrome oxidase, and several dehydrogenases from
five stenothermal thermophils. The curves for the activity of all the enzymes studied indicated appreciable inactivation at temperatures from
37 to 50°, although the bacteria grew well at these temperatures. Militzer
et al. (187) found that the pyruvic oxidase from the same thermophilic
bacterium which had yielded a number of thermostable enzymes was
thermolabile outside the cell, although fairly stable in the intact
organism.
Certain workers (193-198) have emphasized the role played by the
degree of saturation of the protoplasmic lipids in determining the
temperature range for growth of organisms. Measurements have shown
that an organism, whether a bacterium or a homoiothermic animal,
grown at a high temperature has more saturated fatty acids in its lipids
than one grown at a low temperature. This result has been interpreted
in terms of the need for maintaining a certain protoplasmic viscosity if
vital processes are to continue. Its meaning for the functioning of the
organism remains, however, somewhat obscure.
That thermostable proteins play an important role in permitting some
microorganisms to live at unusually high temperatures is evident from
the preceding discussion. The question may be asked, however, whether
other factors do not make equally important contributions to the adjustment of the organisms to these extreme conditions. There is a considerable body of evidence which indicates that this question should be
answered in the affirmative.
The eurythermal thermophilic bacteria do not possess markedly
thermostable proteins (4, 5), yet are able to grow at temperatures up to
60-65°. Both stenothermal and eurythermal thermophils die rapidly in
the absence of nutrients at temperatures which permit good growth
when proper nutritional conditions are maintained (4, 5). Thermophilic
bacteria do not show the stationary phase characteristic of bacteria
505
acterized by a large contribution of the entropy term to the free energy
of activation (6, 7). This offsets the high values for the heat of activation, and makes it possible for these reactions to occur at relatively low
temperatures. The entropy of activation, in turn, depends upon the
change from order to disorder which occurs when the first few bonds
are broken (6, 7). These bonds are considered to be hydrogen bonds
(7), hence a change in the degree of "effectiveness" of hydrogen bonding might be expected to have marked effects on the ease of thermal
denaturation of proteins.
Thermolabile enzymes have been found, however, even in stenothermal thermophilic microorganisms. Gaughran (191-193) studied the
effect of temperature on the activity of the complete respiratory system
and of catalase, cytochrome oxidase, and several dehydrogenases from
five stenothermal thermophils. The curves for the activity of all the enzymes studied indicated appreciable inactivation at temperatures from
37 to 50°, although the bacteria grew well at these temperatures. Militzer
et al. (187) found that the pyruvic oxidase from the same thermophilic
bacterium which had yielded a number of thermostable enzymes was
thermolabile outside the cell, although fairly stable in the intact
organism.
Certain workers (193-198) have emphasized the role played by the
degree of saturation of the protoplasmic lipids in determining the
temperature range for growth of organisms. Measurements have shown
that an organism, whether a bacterium or a homoiothermic animal,
grown at a high temperature has more saturated fatty acids in its lipids
than one grown at a low temperature. This result has been interpreted
in terms of the need for maintaining a certain protoplasmic viscosity if
vital processes are to continue. Its meaning for the functioning of the
organism remains, however, somewhat obscure.
That thermostable proteins play an important role in permitting some
microorganisms to live at unusually high temperatures is evident from
the preceding discussion. The question may be asked, however, whether
other factors do not make equally important contributions to the adjustment of the organisms to these extreme conditions. There is a considerable body of evidence which indicates that this question should be
answered in the affirmative.
The eurythermal thermophilic bacteria do not possess markedly
thermostable proteins (4, 5), yet are able to grow at temperatures up to
60-65°. Both stenothermal and eurythermal thermophils die rapidly in
the absence of nutrients at temperatures which permit good growth
when proper nutritional conditions are maintained (4, 5). Thermophilic
bacteria do not show the stationary phase characteristic of bacteria