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Biochemistry, 4th Edition P30

Biochemistry, 4th Edition P30. 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. | 9.2 What Are the Structure and Chemistry of Membrane Proteins 253 charged groups amino or guanidinium extending beyond to associate with negatively charged phosphate groups. This behavior with the side chain pointing up out of the membrane core has been termed snorkeling Figure 9.16 . If a Phe residue occurs near the lipid-water interface it is typically arranged with the aromatic ring oriented toward the membrane core. This is termed antisnorkeling. Membrane Protein Structures Show Many Variations on the Classical Themes Although it revealed many insights of membrane protein structure bacteriorhodopsin gave a relatively limited view of the structural landscape. Many membrane protein structures obtained since bacteriorhodopsin and a few others have provided a vastly more complex picture to biochemists. For example the structures of a homodimeric chloride ion transport protein and a glutamate transport protein show several novel structural features Figure 9.17 . In addition to several transmembrane helices that lie perpendicular to the membrane plane like those of bacteriorhodopsin these structures each contain several long severely tilted helices that span the membrane. Both these proteins also contain several reentrant loops consisting of a pair of short a-helices and a connecting loop that together penetrate part way into the membrane core. There are also regions of nonhelical polypeptide deep in the membrane core of these proteins with helical segments on either side that extend to the membrane surface Figure 9.17 . Finally most membrane protein structures are relatively stable that is transmembrane helices do not flip in and out of the membrane and they do not flip across the lipid bilayer inverting their orientation. However a few membrane proteins can in fact change their membrane orientation. Aquaporin-1 is a protein that functions normally with six transmembrane -helices. When this protein is first inserted into its membrane it has only four transmembrane .