期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：24
页码：7484-7488
DOI：10.1073/pnas.1507565112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceSystematic relationships have long been recognized between the hydrophobicities of amino acids and (i) their tendencies to be located at the exposed surfaces of globular and membrane proteins and (ii) the composition of their triplets in the genetic code. Here, we show that the same coding relationships are compatible with the high temperatures at which life is widely believed to have originated. An accompanying paper reports that these two properties appear to be encoded separately by bases in the acceptor stem and the anticodon of tRNA. The hydrophobicities of the 20 common amino acids are reflected in their tendencies to appear in interior positions in globular proteins and in deeply buried positions of membrane proteins. To determine whether these relationships might also have been valid in the warm surroundings where life may have originated, we examined the effect of temperature on the hydrophobicities of the amino acids as measured by the equilibrium constants for transfer of their side-chains from neutral solution to cyclohexane (Kw>c). The hydrophobicities of most amino acids were found to increase with increasing temperature. Because that effect is more pronounced for the more polar amino acids, the numerical range of Kw>c values decreases with increasing temperature. There are also modest changes in the ordering of the more polar amino acids. However, those changes are such that they would have tended to minimize the otherwise disruptive effects of a changing thermal environment on the evolution of protein structure. Earlier, the genetic code was found to be organized in such a way that--with a single exception (threonine)--the side-chain dichotomy polar/nonpolar matches the nucleic acid base dichotomy purine/pyrimidine at the second position of each coding triplet at 25 {degrees}C. That dichotomy is preserved at 100 {degrees}C. The accessible surface areas of amino acid side-chains in folded proteins are moderately correlated with hydrophobicity, but when free energies of vapor-to-cyclohexane transfer (corresponding to size) are taken into consideration, a closer relationship becomes apparent.
关键词：hydrophophobicity ; protein folding ; anticodon ; temperature ; genetic code
