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However, the structure must be sufficiently stable to avoid premature denaturation. Membrane proteins must be flexible enough to allow the conformational changes required for their biological function. Although there has been much discussion about the free energy landscape experienced by these proteins, there is no question that the free energy states that such a protein experiences in a particular aqueous environment can be lost and regained in a denaturation/renaturation experiment. Experimental observations of this kind of behavior led to the concept that these proteins in an aqueous environment were at or near their point of lowest overall free energy in their native structure. Some water-soluble proteins, under carefully defined conditions, can be denatured, including loss of function, and subsequently renatured with gain of function. Understanding denaturation of a protein requires an overall understanding of protein stability (denaturation, as it will be used here, is the loss of secondary and tertiary structure concomitant with the loss of function). Degradation is dominated by two phenomena: targeting of the protein for removal, often by proteolysis by a system triggered via the ubiquitin pathway, and denaturation of the protein (which may then lead to targeting and proteolysis). The lifetime of a protein is controlled by the rate of synthesis and the rate of degradation. Membrane proteins, like all proteins, have finite lifetimes. Yeagle, in The Membranes of Cells (Third Edition), 2016 10.11 Membrane Protein Stability
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