It may seem intuitive that peptide bond formation should work in a mechanism almost the reverse of peptide bond hydrolysis, but it is nevertheless impressive and unexpected that the same type of mechanism catalyzed by a complex protein can also be achieved by a nucleic acid. All proteins constructed within the ribosome must exit through this 100 Angstrom long gateway, which is built from both RNA and proteins. The answer, thinks Steitz, may be that the nascent proteins must not interact chemically or structurally with the walls of the exit tunnel. Any interactions with the wall would result in a great decrease in entropy and therefore be thermodynamically unfavorable. The vast majority of proteins do not have hydrophilic regions that would be expected to interact with the walls of the tunnel.
Proteins enzyme frequently perform general acid-base catalysis.
Ribosomal RNA can recognize and react with mRNA and tRNA much more precisely than proteins can. Since interactions among RNA molecules are very different than those of proteins and RNA, an RNA catalyst should be better equipped to interact with other RNA molecules. The discovery of the catalytic properties of the ribosome strengthens the argument for an RNA world, an idea first proposed by the evolutionary biologist Carl Woese. In this world, DNA and proteins had not yet emerged so RNA would have functioned as both a carrier of genetic information and a catalyst of biochemical reactions. The first ribosome would have been composed exclusively of RNA, and from this ribosome emerged the world's first protein.