THE ASX-PRO TURN AS A LOCAL STRUCTURAL MOTIF STABILIZED BY ALTERNATIVE PATTERNS OF HYDROGEN-BONDS AND A CONSENSUS-DERIVED MODEL OF THE SEQUENCE ASN-PRO-ASN

Analyses of databases derived from the Brookhaven Protein Data Bank ha ve identified a set of related turn structures formed by the sequence Asx-Pro-Xxx(n). In a variety of flanking structural contexts, more tha n 60% of Asx-Pro sequences adopt a turn conformation stabilized by a s et of alternative hydrogen bonds among the side chain O delta and back bone C=O carbonyl oxygens of Asx (residue i) and the backbone NH of re sidues i + 2, i + 3 and in some cases i + 4. In contrast, the structur es adopted by Ser-Pro, His-Pro and other Xxx-Pro sequences reflect mor e heterogeneous hydrogen-bonding patterns, As expected, structures for med by Asx-Pro-Asx are similar to those formed by Asx-Pro-Xxx(n), but in some cases additional hydrogen bonds are formed between the Asx sid e chains, Hydrogen bond patterns within Asx-Pro and Asn-Pro-Asn turns are consistent with published NMR studies of helical (Asn-Pro-Asn-Ala) (n) peptides, indicating that a consensus structure reflecting these h ydrogen bonds can serve as a partial model of the Asn-Pro-Asn-Ala tetr apeptide repeats of Plasmodium falciparum circumsporozoite protein.