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Biochemistry, 4th Edition P20

Biochemistry, 4th Edition P20. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | 6.4 How Do Polypeptides Fold into Three-Dimensional Protein Structures 153 core of the protein. The extensively H-bonded nature of -helices and -sheets is ideal for this purpose and these structures effectively stabilize the polar groups of the peptide backbone in the protein core. The framework of sheets and helices in the interior of a globular protein is typically constant and conserved in both sequence and structure. The surface of a globular protein is different in several ways. Typically much of the protein surface is composed of the loops and tight turns that connect the helices and sheets of the protein core although helices and sheets may also be found on the surface. The result is that the surface of a globular protein is often a complex landscape of different structural elements. These complex surface structures can interact in certain cases with small molecules or even large proteins that have complementary structure or charge Figure 6.20 . These regions of complementary recognizable structure are formed typically from the peptide segments that connect elements of secondary structure. They are the basis for enzyme-substrate interactions protein-protein associations in cell signaling pathways and antigen-antibody interactions and more. The segments of the protein that are neither helix sheet nor turn have traditionally been referred to as coil or random coil. Both of these terms are misleading. Most of these loop segments are neither coiled nor random in any sense of the words. These structures are every bit as organized and stable as the defined secondary structures. They just don t conform to any frequently recurring pattern. These so-called coil structures are strongly influenced by side-chain interactions with the rest of the protein. Waters on the Protein Surface Stabilize the Structure A globular protein s surface structure also includes water molecules. Many of the polar backbone and side chain groups on the surface of a globular protein make H .