Lecture32 - -1-LECTURE 32 11 November 2009 (P. J....

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Unformatted text preview: -1-LECTURE 32 11 November 2009 (P. J. Hollenbeck) BIOL231Molecular motors & actin: muscle and non-muscle Read: Chap 17: 597-605; DVD 17.8-10, 17.12, 17.14, 15; Probs: 81, exam IV'05, #2I. Myosin Motor Proteins<To understand the structural basis of muscle contraction, we first need to side-track from consideringthe structure of the sarcomere and take a look at the superfamily of actin-based mechanochemicalenzymes, the myosins.>(A) General structure and function(1) Turning chemical energy into mechanical force. The enzymes that convert chemicalenergy into the force for movement are called mechanochemical enzymes, or motorproteins. There are three superfamilies: the myosins, which interact with actin, thekinesins, and thedyneins the latter two interact with MTs. They all bind andhydrolyze ATP, and they all drive translocation along the surface of their respectivecytoskeletal filament. The myosins that we will consider move exclusively toward theplus ends of actin. (2) Structure. The large superfamily of related myosin proteins shares a basicstructure with a tail region, a neck region, and a globular head region. The headregion or motor domain is conserved among myosin families, and contains the siteswhere they bind actin, and bind and hydrolyze ATP see the schematic structure of thisregion below, left. They differ at the other end of the protein: different myosins havedifferent tails, or cargo binding domains, suiting them for very different moving jobsin the cell. [many myosins are in the Protein Data Bank](3) We have now identified at 19 families of myosin proteins! First lets focus on themyosin-IIfamily, the kind that we find in muscle. These myosins form dimers, withtwo globular heads close together, and a long -helical coiled-coil tail (see figs below). These tails interact with the tails of a few hundred other myosin dimers, to form afilament, with half of the dimers having their heads pointing in one direction and halfin the other (see fig, next page). This bipolarfilament is the thick filament that occupiesthe center of the muscle sarcomere.-2-<To see loads of structural info about myosins, go to:http://www.proweb.org/myosin/index.html,which is the source of the myosin evolutionary tree, above. >(B) Force generation by myosin<So, how does myosin couple ATP hydrolysis to the generation of force how does it interact withactin filaments in the sarcomere to give muscle contraction?> (1) Cross bridge cycle. The myosin head goes through a cycle of ATP binding,hydrolysis, and release of ADP and Pi. This is coupled to a cycle of changingconformation, binding the actin filament, changing conformation again, and releasingthe filament, resulting in force generation. We currently think it proceeds as follows: ifwe begin with myosin bound to the actin filament, with no nucleotide in the bindingsite, we have rigor binding , which underlies the muscle stiffness characteristic of-3-death (hence rigor mortis). death (hence rigor mortis)....
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This note was uploaded on 12/18/2009 for the course BIOL 101 taught by Professor Wormer during the Fall '08 term at Purdue University-West Lafayette.

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Lecture32 - -1-LECTURE 32 11 November 2009 (P. J....

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