#11 How enzymes propel themselves-I

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Enzyme could speed up chemical reaction but could not change the heat of reaction. What does heat of reaction ultimately do? We have always believed that these heat are nothing but some "byproduct" of the reaction, yet the latest evidence suggested otherwise. Previous research showed that the local temperature changes induced by laser pulse could change the structure of enzymes [1]. What if the same effect could be induced by the heat of reaction?

In these series, we are going to talk about the observation of the whole research in episode I, discuss diffusion theory in episode II ~ IV, and finally explain the observed phenomenon with chemoacoustic effect in episode V ~ VI.
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Previous research indicated that the diffusion constant of an enzyme increases if it is catalyzing. There were some hypothesis proposed before but none of them could successfully explain why some enzymes have their diffusion constant increased while others don't. Therefore the authors of our suggested reading proposed that, the "local structural changes" of enzymes caused by heat of reaction give rise to a pressure in certain direction, and the reaction forces from solvent makes enzyme move faster, which therefore increases the diffusion constant.

2 guesses could be made from this hypothesis. The first one is that the effect of increased diffusion constant would not happen if the enzyme catalyzes an endothermic reaction or the heat of reaction released is too little. The second one is that, if we increase the reaction rate by increasing substrate concentration, the diffusion constant should increase more. The authors therefore chose 4 different enzymes. One of them catalyze a slightly exothermic reaction (heat of reaction = -3kJ/mole) while the other 3 catalyze highly exothermic reactions (heat of reaction about -40 ~ -100 kJ/mole). Only the enzymes catalyzing highly exothermic reactions have their diffusion constant increased. One of them even increased its diffusion constant by 80%!

However, we still have to exclude other possible hypothesis. Some may criticized that the effect could be caused by the structural changes induced merely from substrate binding. To exclude that possibility, they replace substrates with its competitive inhibitor, which could bind to enzyme without eliciting any chemical reaction, and they found no increases in diffusion constant. Others may argue that the increase in diffusion constant is caused by local temperature changes due to heat of reaction. However, the authors estimated the rise in local temperature would be no more than 0.15 K.

Finally, the authors proposed their own physical model, which explained the linear relationship between diffusion constant and the reaction rate. They also estimated the pressure that could produced by local structural changes of the enzymes. If this hypothesis is true, it means enzymes could propel themselves! However, the details about above results would be left in the following episodes.

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Suggested reading：
1. C. Riedel, R. Gabizon, C. A. M. Wilson, K. Hamadani, K. Tsekouras, S. Marqusee, S. Presse & C. Bustamante. The heat released during catalytic turnover enhances the diffusion of an enzyme. Nature 517: 227-230. (2015)

Reference:
[1]B. C., Steel, D. R. McKenzie, M. M. M., Bilek, N. J., Nosworthy, C. G., dos Remedios. (2006). Nanosecond responses of proteins to ultra-high temperature pulses. Biophy. J. 91: L66-L68.
#biophysics #3minBiophysics #生物物理 #三分鐘生物物理 #enzyme #heat #diffusion