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lec7 - #$ $/0-1 2"3'4"5$6 Proteins fold spontaneously under...

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2"3 4+-#"5$6 Proteins fold spontaneously under physiological conditions. In the equilibrium between the denatured state (unfolded or partially unfolded) and the native state (folded, biologically functional), under physiological conditions the vast majority of molecules are in the native state. PRIMARY STRUCTURE DETERMINES TERTIARY (AND QUATERNARY) STRUCTURES. demonstrated by the fact that many proteins can refold from a more or less "random coil" set of conformations without "instructions" from any other cellular components All the information for 3-dimensional structure is provided by the amino acid sequence. Proteins can be unfolded (denatured) in vitro by chemical agents like urea, or extremes of heat or pH, and then refolded (renatured) by diluting out the chemical denaturant, changing the pH, etc. Proteins fold on a defined pathway (or a small number of alternative pathways); they don't randomly search all possible conformations until they arrive at the most stable (lowest free energy) structure.
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2"3 4+-#"5$6 Proteins that don't (re)fold on their own, without assistance, don't need other "instructions" -- they just need "molecular chaperones" (which are also proteins) to keep them from slipping off the folding pathway or to help them to get back on it. Some chaperones require "expenditure" of energy currency (hydrolysis of ATP) to carry out their function. Many diseases are the result of defects in protein folding, e.g., the spongiform encephalopathies (human CJD, bovine “mad cow” disease), Alzheimer disease, Parkinson disease, Huntington disease Diseases involving deposits of misfolded proteins (amyloid deposits) result from aggregation of a specific protein, different for different diseases, that
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