蛋白演化只是在緩慢地發(fā)生,，因為大部分氨基酸替換都可能是有害的,，而且選擇性將有利于功能的保持。Inna Povolotskaya 和 Fyodor Kondrashov試圖搞清楚這個過程到底有多慢，他們的辦法是,，提出這樣一個問題：古老的現(xiàn)存蛋白——即那些存在于“最后普遍共同祖先”(LUCA)身上的蛋白——是否在繼續(xù)從先祖序列分化？
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他們所采用的方法是Edwin Hubble在其關(guān)于宇宙中星系退行的研究中所用的方法,，其結(jié)論表明,，現(xiàn)存蛋白序列仍然在從相互之間擴展，因此也在從共同祖先擴展,。分化的速度是非常慢的：35億年左右,，因為LUCA的時間還沒有長到達到序列分化極限的程度,。這種緩慢性是序列空間中的功能性蛋白序列稀少以及蛋白適應(yīng)性非常強所造成的：98%的點不能在一個給定的時刻接受一個氨基酸替換，而當其他互補性變化發(fā)生時,，大多數(shù)點最終會被允許發(fā)生演化,。(生物谷Bioon.net)
生物谷推薦原文出處：
Nature doi:10.1038/nature09105
Sequence space and the ongoing expansion of the protein universe
Inna S. Povolotskaya & Fyodor A. Kondrashov
The need to maintain the structural and functional integrity of an evolving protein severely restricts the repertoire of acceptable amino-acid substitutions1, 2, 3, 4. However, it is not known whether these restrictions impose a global limit on how far homologous protein sequences can diverge from each other. Here we explore the limits of protein evolution using sequence divergence data. We formulate a computational approach to study the rate of divergence of distant protein sequences and measure this rate for ancient proteins, those that were present in the last universal common ancestor. We show that ancient proteins are still diverging from each other, indicating an ongoing expansion of the protein sequence universe. The slow rate of this divergence is imposed by the sparseness of functional protein sequences in sequence space and the ruggedness of the protein fitness landscape: ~98 per cent of sites cannot accept an amino-acid substitution at any given moment but a vast majority of all sites may eventually be permitted to evolve when other, compensatory, changes occur. Thus, ~3.5?×?109?yr has not been enough to reach the limit of divergent evolution of proteins, and for most proteins the limit of sequence similarity imposed by common function may not exceed that of random sequences.
