St. Wang et al., STRUCTURAL DETERMINANTS OF CA2-TERMINAL OF CARDIAC TROPONIN-C( EXCHANGE AND AFFINITY IN THE C), Biochemistry (Easton), 37(41), 1998, pp. 14539-14544
The C terminal of cardiac troponin C (TnC) has two Ca2+-Mg2+ sites whi
ch exhibit similar to 20-fold higher Ca2+ affinity than the two C-term
inal Ca2+ specific sites in calmodulin (CaM). Substitution of the thir
d EF-hand of TnC for the corresponding EF-hand of CaM produced a mutan
t (CaM[3TnC]) with a 10-fold higher C-terminal Ca2+ and Mg2+ affinity.
Substitution of loop 3 of TnC for loop 3 of CaM produced a mutant (Ca
M[loop3TnC]) with a 10-fold faster Ca2+ on rate and a 5-fold faster Ca
2+ off rate than CaM. A mutant CaM (CaM[loop3X,Z]) which contained the
identical coordinating amino acids and X and Z acid pairs of TnC loop
3 had a 3-fold higher C-terminal Ca2+ affinity without the increased
Ca2+ exchange rates exhibited by CaM[loop3TnC]. Thus, loop factors oth
er than the acid pairs must be responsible for the rapid Ca2+ exchange
rates of CaM[loop3TnC]. Helix 6 and helix 5 in the third EF-hand of T
nC support the rapid Ca2+ on rate of TnC's loop 3 and produce an simil
ar to 4-fold reduction in its Ca2+ off rate, explaining the high Ca2affinity of the third EF-hand of TnC. Exchanging loop 3 or helix 5 of
TnC into CaM increased the Mg2+ affinity by decreasing the Mg2+ off ra
te. Our results are consistent with the high Ca2+ and Mg2+ affinity of
the third EF-hand of TnC resulting from the two (X and Z) acid pairs
in loop 3, coupled with the greater hydrophobicity of helix 6 and heli
x 5 compared to that of the third EF-hand of CaM.