Lecture 7 - Lecture 7 Protein Structure(continued...

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Lecture 7 Protein Structure (continued) Quaternary structure Multiple subunits held together by the same forces as for tertiary structure Classes: homomultimers—heteromultimers identical subunits—nonidentical subunits isologous interactions—heterologous interactions Symmetries 2-fold closed is common Some open multimer symmetries (rotational symmetry with lateral advancement: tubulin, actin, TMV coat (helices) Non-symmetrical: α 2 ß G-protein Advantages (p. 177) Stability: decreased surface-to-volume -> more hydrophobic interactions New sites: e.g., Ig binding sites Coupled reactions: trp synthetase: InGP <==> G3P + indole indole + ser <==> trp purine synthesis: 10 reactions, 6 enzymes, 1 complex (in purine depleted medium) Cooperativity: e.g., allostery Example of cooperativity: Mb/Hb Shows advantage of quaternary structure Show example of flexibility: low Δ G of shape change Myoglobin MW ca 17,000 daltons O 2 buffer 75% a-helix
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