Ms. Cates et al., Metal-ion affinity and specificity in EF-hand proteins: coordination geometry and domain plasticity in parvalbumin, STRUCT F D, 7(10), 1999, pp. 1269-1278
Background: The EF-hand family is a large set of Ca2+-binding proteins that
contain characteristic helix-loop-helix binding motifs that are highly con
served in sequence. Members of this family include parvalbumin and many pro
minent regulatory proteins such as calmodulin and troponin C. EF-hand prote
ins are involved in a variety of physiological processes including cell-cyc
le regulation, second messenger production, muscle contraction, miorotubule
organization and vision.
Results: We have determined the structures of parvalbumin mutants designed
to explore the role of the last coordinating residue of the Ca2+-binding lo
op. An E101D substitution has been made in the parvalbumin EF site. The sub
stitution decreases the Ca2+-binding affinity 100-fold and increases the Mg
2+-binding affinity 10-fold. Both the Ca2+- and Mg2+-bound structures have
been determined, and a structural basis has been proposed for the metal-ion
-binding properties.
Conclusions: The E101D mutation does not affect the Mg2+ coordination geome
try of the binding loop, but it does pull the F helix 1.1 Angstrom towards
the loop. The E101D-Ca2+ structure reveals that this mutant cannot obtain t
he sevenfold coordination preferred by Ca2+, presumably because of strain l
imits imposed by tertiary structure. Analysis of these results relative to
previously reported structural information supports a model wherein the cha
racteristics of the last coordinating residue and the plasticity of the Ca2
+-binding loop delimit the allowable geometries for the coordinating sphere
.