Macromolecules: Structure, Shape, and Information

Protein Structure²⁰

Outline Introduction The Shape of a Protein Molecule Is Determined by Its Amino Acid Sequence Common Folding Patterns Recur in Different Protein Chains Proteins Are Amazingly Versatile Molecules Proteins Have Different Levels of Structural Organization Domains Are Formed from a Polypeptide Chain That Winds Back and Forth, Making Sharp Turns at the Protein Surface Relatively Few of the Many Possible Polypeptide Chains Would Be Useful New Proteins Usually Evolve by Alterations of Old Ones New Proteins Can Evolve by Recombining Preexisting Polypeptide Domains Structural Homologies Can Help Assign Functions to Newly Discovered Proteins Protein Subunits Can Assemble into Large Structures A Single Type of Protein Subunit Can Interact with Itself to Form Geometrically Regular Assemblies Coiled-Coil Proteins Help Build Many Elongated Structures in Cells Proteins Can Assemble into Sheets, Tubes, or Spheres Many Structures in Cells Are Capable of Self-assembly Not All Biological Structures Form by Self-assembly Summary Tables Table 3-1: Approximate Chemical Compositions of a Typical Bacterium and a Typical Mammalian Cell Panels Panel 2-5: The 20 amino acids involved in the synthesis of proteins Section References Branden, C.; Tooze, J.Introduction to Protein Structure. New York: Garland, 1991. Creighton, T.E.Proteins: Structure and Molecular Properties, 2nd ed. New York: W.H. Freeman, 1993. Dickerson, R.E.; Geis, I.The Structure and Action of Proteins. New York: Harper & Row, 1969. Schulz, G.E.; Schirmer, R.H.Principles of Protein Structure. New York: Springer, 1990.

Introduction     To a large extent, cells are made of protein, which constitutes more than half of their dry weight (see Table 3-1). Proteins determine the shape and structure of the cell and also serve as the main instruments of molecular recognition and catalysis. Although DNA stores the information required to make a cell, it has little direct influence on cellular processes. The gene for hemoglobin, for example, cannot carry oxygen; that is a property of the protein specified by the gene.     DNA and RNA are chains of nucleotides that are chemically very similar to one another. In contrast, proteins are made from an assortment of 20 very different amino acids, each with a distinct chemical personality (see Panel 2-5, pp. 56-57). This variety allows for enormous versatility in the chemical properties of different proteins, and it presumably explains why evolution eventually selected proteins rather than RNA molecules to catalyze most cellular reactions.

© 1994 by Bruce Alberts, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, and James D. Watson.