▲ FIGURE 3-1 Overview of protein structure and function.

(a) The linear sequence of amino acids (primary structure) folds into helices or sheets (secondary structure) which pack into a globular or fibrous domain (tertiary structure). Some individual proteins self-associate into complexes (quaternary structure) that can consist of tens to hundreds of subunits (supramolecular assemblies). (b) Proteins display functions that include catalysis of chemical reactions (enzymes), flow of small molecules and ions (transport), sensing and reaction to the environment (signaling), control of protein activity (regulation), organization of the genome, lipid bilayer membrane, and cytoplasm (structure), and generation of force for movement (motor proteins). These functions and others arise from specific binding interactions and conformational changes in the structure of a properly folded protein.

ing from genetically mutating it, from the biochemistry of the complex to which it belongs, or from all three.

In this chapter, we begin our study of how the structure of a protein gives rise to its function, a theme that recurs throughout this book (Figure 3-1). The first section examines how chains of amino acid building blocks are arranged and the various higher-order folded forms that the chains assume. The next section deals with special proteins that aid in the folding of proteins, modifications that take place after the protein chain has been synthesized, and mechanisms that degrade proteins. The third section focuses on proteins as catalysts and reviews the basic properties exhibited by all enzymes. We then introduce molecular motors, which convert chemical energy into motion. The structure and function of these functional classes of proteins and others are detailed in numerous later chapters. Various mechanisms that cells use to control the activity of proteins are covered next. The chapter concludes with a section on commonly used techniques in the biologist's tool kit for isolating proteins and characterizing their properties.

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