Exploring the fundamental principles of protein evolution.
Tyler Hampton, Inference-Review
IN 1969, Frank Salisbury, writing in Nature, observed that the fact of protein specificity and the fact of protein evolution are, if not in conflict, then under tension.1 At some moment, argued Salisbury, the prebiotic soup must have disappeared. Early life had to begin fending for itself. Demands were imminent. For each essential chemical that had become either scarce or unavailable, an enzyme had to appear to catalyze its synthesis. Since chemicals had precursors, enzymes would by necessity form pathways and cascades.
Salisbury urged his audience to imagine a mundane protein for such a task, one 300 amino acids in length. Its encoding gene must have been, to use a round figure, one thousand nucleotides.2 Large numbers now accumulate rapidly. Since there are four canonical nucleotides in biology and as many assumed for pre-biology,3 and one thousand positions along its hypothetical chain, there are 41000 ways of arranging this gene. This makes for roughly 10600 possibilities. The number, Salisbury observed, is too large coherently to grasp. Even if trillions of distinct sequences could fulfill a single enzyme function, there is only a 1 in 10500 chance that such an enzyme would arise spontaneously. Neither inflation of resources, bigger oceans, more sloshing chemicals, nor a multiplication of planets would make this probable.