The existence of two conformational states of concanavalin A (Con A) with different metal ion binding properties has been recently demonstrated (Brown, R. D., Brewer, C. F„ & Koenig, S. H. (1977) Biochemistry 16, 3883). Introduction of Mn2+ to the SI site and Ca2+ to the S2 site of apo-Con A was shown to induce a conformational change in the protein, ascribed to a cis-trans isomerization of a peptide bond in the secondary structure, which results in extremely tight binding of the metal ions. This induced conformation is referred to as ”locked” and the initial conformation as ”unlocked”. The locked ternary complex is identical with the native protein. In the present paper, we report evidence for the formation of a relatively stable, locked, ternary Ca2+-Con A complex that possesses properties similar to those of native Ca2+-Mn2+-Con A. The experimental technique involves measurement of the magnetic field and time dependence of the nuclear magnetic relaxation rate (1 /T1) of so vent water protons in solutions of Ca2+-Con A, after the addition of Mn2+ ions which slowly bind to the protein. The kinetic data can be fit by a model for Ca2+ interactions with Con A which indicates that Ca2+, in the absence of Mn2+, can bind at both the SI and the S2 sites of the protein and, furthermore, can induce the protein to undergo the unlocked to locked conformational transition. In terms of this model, the time-dependent binding of the Mn2+ ions is due to replacement of Ca2+ ions at the S1 sites in the locked protein. The off-rate of Ca2+ from the S2 site of the locked ternary Ca2+-Con A complex is much greater than that from the locked Ca2+-Mn2+-Con A complex. From the effects of added α-methyl D-mannopyranoside on the rate of replacement of Ca2+ by Mn2+ at the SI site of the locked ternary Ca2+-Con A complex, it is concluded that the latter complex binds saccharides as strongly as the locked Ca2+-Mn2+-Con A complex. In addition, analysis of the data indicates that apo-Con A in the locked conformation binds α-methyl d-mannopyranoside with approximately 7% of the affinity of the fully metallized locked form of the protein. This strong saccharide-binding activity of locked apo-Con A, compared with that of the unlocked apo-Con A, was further demonstrated by equilibration of unlocked apo-Con A with α-methyl D-mannopyranoside, which resulted in the formation of the locked apo-Con A-saccharide complex. These results demonstrate that it is the locked conformation of Con A that is primarily responsible for saccharide-binding activity, and that the function of the bound metals is primarily to maintain the protein in the locked conformation. © 1978, American Chemical Society. All rights reserved.