THE ROLES OF INDIVIDUAL GAMMA-CARBOXYGLUTAMATE RESIDUES IN THE SOLUTION STRUCTURE AND CATION-DEPENDENT PROPERTIES OF CONANTOKIN-T

The solution structure of the Ca2+-loaded conantokin-T (con-T), a gamm a-carboxyglutamate (Gla)-containing al-residue peptide (NH2-G(1)E gamm a gamma Y(5)QKML gamma(10)NLR gamma A(15)EVKKN(20)A-CONH2, gamma = = G la), has been elucidated by use of distance geometry calculations with experimental distances derived from two-dimensional H-1 NMR spectrosc opy. An end-to-end cc-helix was the dominant conformation in solution, similar to that of apo-con-T, except that reorientation of several si de chains occurred in the Ca2+-coordinated complex. The most notable e xamples of this were those of Gla(10) and Gla(14), which were more opt imally positioned for complexation with Ca2+. In addition to the stabi lization offered to the alpha-helix by Ca2+ binding, hydrophobic clust ering of the side chains of Tyr(5), Met(8), Leu(9), and Leu(12), and i onic interactions between Lys(7) and Gla(3)/Gla(10) and between Arg(13 ) and Gla(14), along with hydrogen bonding between Gln(6) and Gla(10), were among the side chain interactions Likely playing a significant r ole in maintenance of the alpha-helical conformation. Docking of Ca2in the con-T structure was accomplished using genetic algorithm-molecu lar dynamics simulation approaches. The results showed that one Ca2+ i on is most likely coordinated by four side chain oxygen atoms, two eac h from Gla(10) and Gla(14). Another bound Ca2+ ion has as its donor si tes three oxygen atoms, two from Gla(3) and one from Gln(6). To examin e the functional roles of the individual Gla residues, a series of var iant peptides have been synthesized with Ala substituted for each Gla residue, and several properties of the resulting variants have been ex amined. The data obtained demonstrated the importance of Gla(10) and G la(14) in stabilizing binding of the highest affinity Ca2+ site and in governing the conformational change induced by Ca2+. The critical nat ure of Gla(3) and Gla(4) in inhibition of the spermine-induced potenti ation of the binding of MK-801 to open ion channels of the N-methyl-D- aspartate receptor was established, as well as the role of Gla(4) in s tabilizing the apo-con-T alpha-helical conformation.