Calmodulin EF Hand
Helix - Loop - Helix

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I. Introduction

Here is the structure of amino acids 118-148 of human calmodulin. This proteins binds calcium and modulates many intracelluar processes - it is a calcium modulatory protein (or calmodulin). Shown is a helix-loop-helix supersecondary structure motiff. Calcium (shown in spacefill rendering) is bound in the loop and is coordinated to Glu side chains protruding in from the loop

For more information see Biochemistry Online: Chapter 2C - Understanding Protein Conformation

II. General Structure

Backbone and side chains
Ca2+ and side chain interactions - notice the carboxyl Os interacting with the calcium 
Cartoon showing hydrogen bonds
Cartoon and Wireframe
Dot surface and Wireframe