A physical basis for protein secondary structure

A physical theory of protein secondary structure is proposed and tested by performing exceedingly simple Monte Carlo simulations. In essence, secondary structure propensities are predominantly a consequence of two competing local effects, one favoring hydrogen bond formation in helices and turns, the other opposing the attendant reduction in sidechain conformational entropy on helix and turn formation. These sequence specific biases are densely dispersed throughout the unfolded polypeptide chain, where they serve to preorganize the folding process and largely, but imperfectly, anticipate the native secondary structure.

Four primordial modes of tRNA-synthetase recognition, determined by the (G,C) operational code

In distinction to single-stranded anticodons built of G, C, A, and U bases, their presumable double-stranded precursors at the first three positions of the acceptor stem are composed almost invariably of G-C and C-G base pairs. Thus, the “second” operational RNA code responsible for correct aminoacylation seems to be a (G,C) code preceding the classic genetic code. Although historically rooted, the two codes were destined to diverge quite early. However, closer inspection revealed that two complementary catalytic domains of class I and class II aminoacyl-tRNA synthetases (aaRSs) multiplied by two, also complementary, G2-C71 and C2-G71 targets in tRNA acceptors, yield four (2 × 2) different modes of recognition. It appears therefore that the core four-column organization of the genetic code, associated with the most conservative central base of anticodons and codons, was in essence predetermined by these four recognition modes of the (G,C) operational code. The general conclusion follows that the genetic code per se looks like a “frozen accident” but only beyond the “2 × 2 = 4” scope. The four primordial modes of tRNA–aaRS recognition are amenable to direct experimental verification.

Antibodies have the intrinsic capacity to destroy antigens

Research throughout the last century has led to a consensus as to the strategy of the humoral component of the immune system. The essence is that, for killing, the antibody molecule activates additional systems that respond to antibody–antigen union. We now report that the immune system seems to have a previously unrecognized chemical potential intrinsic to the antibody molecule itself. All antibodies studied, regardless of source or antigenic specificity, can convert molecular oxygen into hydrogen peroxide, thereby potentially aligning recognition and killing within the same molecule. Aside from pointing to a new chemical arm for the immune system, these results may be important to the understanding of how antibodies evolved and what role they may play in human diseases.

Oligonucleotide-directed peptide synthesis in a ribosome- and ribozyme-free system

Peptide bond formation by the ribosome requires 23S rRNA and its interaction with the 3′-CCA end of tRNA. To investigate the possible evolutionary development of the peptidyl transfer reaction, we tried to obtain peptide bond formation without the ribosome or rRNA simply by using a piece of tRNA—an aminoacyl-minihelix—mixed with sequence-specific oligonucleotides that contained puromycin. Peptide bond formation was detected by gel electrophoresis, TLC analysis, and mass spectrometry. Peptide synthesis depended on sequence complementarity between the 3′-CCA sequence of the minihelix and the puromycin-bearing oligonucleotide. However, proximity of the reacting species was not by itself sufficient for peptide bond formation. In addition, imidazole as a catalyst was required. Its role may be similar to the recently proposed mechanism, wherein A2451 of 23S rRNA works as a general base. Thus, peptide bond formation can be achieved with a simple, minimized system that captures the essence of an interaction seen in the ribosome.

A medical book collection for physician assistants

Selecting resources for physician assistants is challenging and can be overwhelming. Although several core lists exist for nursing, allied health, and medical libraries, judging the scope and level of these resources in relation to the information needs of the physician assistant is difficult. Medical texts can be highly specialized and very expensive, in essence, “overkill” for the needs of the physician assistant. This bibliography is meant to serve as a guide to appropriate medical texts for physician assistants. Titles were selected from the Brandon/Hill list, Doody's Electronic Journal, and various other reference resources. Resources were evaluated based on the subject and scope, audience, authorship, cost, and currency. The collection includes 195 titles from 33 specialty areas. Standard texts in each area are also included.