What is Calmodulin? - Definition & Structure

Instructor: Shelly Watkins

Shelly has instructed college biology courses, as well as graduate students in health profession programs. She has a doctorate in physical therapy.

In this lesson, you will learn about the definition of calmodulin, as well as its general functions related to anatomy and physiology, and how those functions relate to its molecular structure.

What is Calmodulin?

Imagine not being able to pump blood through your arteries or digest your food. These functions are regulated, in part, by calmodulin. This widespread calcium-binding protein is able to bind to and regulate many different protein targets, thus impacting many different bodily functions. It is important that cellular levels of calcium are regulated in order to maintain homeostasis, a stable internal environment within the body. Calmodulin serves as a mediator between calcium release and target tissue responses. This mediator relays signals that allow us to contract muscles that move our undigested food along without our conscious control. The function of calmodulin is determined by its structure.

Structure of Calmodulin

Let's examine the molecular structure first. It may come as a surprise that humans share identical calmodulin sequences with other species including fish, frogs, and birds. Calmodulin is made of one peptide chain and 148 amino acid residues. Peptide chains are short chains of amino acids, and amino acids are the building blocks of proteins. When two or more amino acids combine to create a peptide, water is removed, and what is left is called an amino acid residue.

Have you ever lifted free weights such as dumbbells? The shape of dumbbells is similar to calmodulin that is bound to calcium. If you imagine each rounded end of the dumbbell representing two parts, with another part connecting these ends, there are three parts in total. These 'parts' are called domains. A domain is a part of a certain protein sequence and structure that can function independently of the rest of the protein chain. The globular ends of the dumbbell each contain two high-affinity calcium binding sites known as EF-hands. Affinity describes the attraction of these binding sites for calcium. Because of these globular ends, there are a four total calcium binding sites per calmodulin molecule.

Unlike a dumbbell, the middle region of calmodulin is flexible. Research suggests that the flexible, interconnecting alpha helix is responsible for interactions between calmodulin and other proteins, and might play a role in calmodulin's ability to bind to calcium. If we look inside the rounded ends and connector of the dumbbell, we would see several alpha helices. An alpha helix is a coiled polypeptide chain, similar to a winding staircase. Each rounded end of the dumbbell has three alpha helices and two EF-hands, with one interconnecting alpha helix between these ends.

Illustration of Calmodulin
Illustration of calmodulin protein

Functions of Calmodulin

Like guards regulating the capacity of a venue during events, calmodulin functions to regulate the levels of calcium inside our cells. Calcium ions play very important roles in functions such as sending nerve signals and muscle contraction. Calmodulin is a calcium-modulated protein. It is found in many types of cells and is located in the cytoplasm, inside organelles, or within membranes. Calmodulin relays messages like a game of telephone. This molecule detects calcium levels and sends signals to different enzymes, ion channels, or proteins.

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