tailieunhanh - Biochemistry, 4th Edition P22

Biochemistry, 4th Edition P22. 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. | How Do Protein Subunits Interact at the Quaternary Level of Protein Structure 173 FIGURE The quaternary structure of liver alcohol each subunit is a six-stranded parallel sheet. Between the two subunits is a two-stranded antiparallel sheet pdb id 1CDO . Jane Richardson. How Do Protein Subunits Interact at the Quaternary Level of Protein Structure Many proteins exist in nature as oligomers complexes composed of often symmetric noncovalent assemblies of two or more monomer subunits. In fact subunit association is a common feature of macromolecular organization in biology. Most intracellular enzymes are oligomeric and may be composed either of a single type of monomer subunit homomultimers or of several different kinds of subunits heteromultimers . The simplest case is a protein composed of identical subunits. Liver alcohol dehydrogenase shown in Figure is such a protein. Alcohol consumed in a beer or mixed drink is oxidized in the liver by alcohol dehydrogenase. Hormonal signals modulate blood sugar levels by controlling the activity of glycogen phosphorylase an elegantly regulated homodimeric muscle enzyme. Oxygen is carried in the blood by hemoglobin which contains two each of two different subunits heterotetramer . A counterpoint to these small clusters is made by the proteins that form large polymeric aggregates. Proteins are synthesized on large complexes of many protein units and several RNA molecules called ribosomes. Muscle contraction depends on large polymer clusters of the protein myosin sliding along filamentous polymers of another protein actin. The way in which separate folded monomeric protein subunits associate to form the oligomeric protein constitutes the quaternary structure of that protein. Table lists several proteins and their subunit compositions see also Table . Proteins with two to four subunits predominate in nature but many cases of higher numbers exist. The subunits of an oligomeric protein .