Proteins have very diverse roles. You’ll learn about the many functions of proteins (Table 3.1) in virtually every chapter of this book. Proteins are polymers made up of 20 amino acids in different proportions and sequences. Proteins range in size, from small ones such as the human hormone insulin, which has 51 amino acids and a molecular weight of 5,733, to huge molecules such as the muscle protein titin, with 26,926 amino acids and a molecular weight of 2,993,451. Proteins consist of one or more polypeptide chains—unbranched (linear) polymers of covalently linked amino acids. Variation in the sequences of amino acids in the polypeptide chains allows for the vast diversity in protein structure and function. Each chain folds into a particular three-
| Category | Function |
|---|---|
| Enzymes | Catalyze (speed up) biochemical reactions |
| Structural proteins | Provide physical stability and movement |
| Defensive proteins | Recognize and respond to nonself substances (e.g., antibodies) |
| Signaling proteins | Control physiological processes (e.g., hormones) |
| Receptor proteins | Receive and respond to chemical signals |
| Membrane transporters | Regulate passage of substances across cellular membranes |
| Storage proteins | Store amino acids for later use |
| Transport proteins | Bind and carry substances within the organism |
| Gene regulatory proteins | Determine the rate of expression of a gene |
| Motor proteins | Cause movement of structures in the cell |
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A protein’s tertiary structure describes its three-
Exposed surface groups on a protein provide both shapes and chemical groups that can interact specifically with other molecules or ions.
The forces that stabilize a protein’s secondary, tertiary, and quaternary structures make the protein vulnerable to disruption by environmental factors.