A. Malmendal et al., SEQUENCE AND CONTEXT DEPENDENCE OF EF-HAND LOOP DYNAMICS - AN N-15 RELAXATION STUDY OF A CALCIUM-BINDING SITE MUTANT OF CALBINDIN D-9K, Biochemistry, 37(8), 1998, pp. 2586-2595
The influence of amino acid sequence and structural context on the bac
kbone dynamics of EF-hand calcium-binding loops was investigated using
N-15 spin relaxation measurements on the calcium-free state of the ca
lbindin D-9k mutant (A14D+A15 Delta+P20 Delta+N21G+P43M), in which the
N-terminal pseudo-EF-hand loop, characteristic of S100 proteins, was
engineered so as to conform with the C-terminal consensus EF-hand loop
. The results were compared to a previous study of the apo state of th
e wild-type-like P43G calbindin D-9k mutant. In the helical regions, t
he agreement with the P43G data is excellent, indicating that the stru
cture and dynamics of the protein core are unaffected by the substitut
ions in the N-terminal loop. In the calcium-binding loops, the flexibi
lity is drastically decreased compared to P43G, with the modified N-te
rminal loop showing a motional restriction comparable to that of the s
urrounding helixes. As in P43G, the motions in the C-terminal loop are
less restricted than in the N-terminal loop. Differences in key hydro
gen-bonding interactions correlate well with differences in dynamics a
nd offer insights into the relationship between structure and dynamics
of these EF-hand loops. It appears that the entire N-terminal EF-hand
is built to form a rigid structure that allows calcium binding with o
nly minor rearrangements and that the structural and dynamical propert
ies of the entire EF-hand-rather than the loop sequence per se-is the
major determinant of loop flexibility in this system.