Axe (2004) And The Evolution Of Protein Folds
In my second response to Arthur Hunt on the origin of the T-urf13 gene (which specifies a mitochondrial ligand-gating pore-forming receptor for T-toxin in maise), I briefly mentioned towards the end of my post Arthur Hunt’s comments on the Panda’s Thumb blog regarding the Axe (2004) result concerned with the rarity of catalytic domains within sequence space.
As I noted in my previous post, Axe’s 2004 JMB paper is not an isolated result. I cited a number of papers which attained similar results with respect to the rarity of functional domains within sequence space. In one study, published in Naturein 2001 by Keefe & Szostak, it was documented that more than a million million random sequences were required in order to stumble upon a functioning ATP-binding protein, a protein substantially smaller than the transmembrane protein specified by the gene, T-urf13, discussed by Hunt. In addition, I noted, a similar result was obtained by Taylor et al. in their 2001 PNAS paper. This paper examined the AroQ-type chorismate mutase, and arrived at a similarly low prevalence (giving a value of 1 in 10^24 for the 93 amino acid enzyme, but, when adjusted to reflect a residue of the same length as the 150-amino-acid section analysed from Beta-lactamase, yields a result of 1 in 10^53). Yet another paper by Sauer and Reidhaar-Olson (1990) reported on “the high level of degeneracy in the information that specifies a particular protein fold,” which it gives as 1 in 10^63. In my previous post, I also strongly encouraged Arthur Hunt and others to read Douglas Axe’s excellent review article in Bio-complexity which covers this topic in more detail, as well as to read the recently-published The Nature of Nature — Examining The Role of Naturalism in Science, which is highly accessible for non-specialists.
Yesterday, I posted a short itallicised update to my previous article, having now looked somewhat closer at the article to which Hunt referred me. For those that missed it, allow me to highlight just a few of the points at which Hunt errs.