Morpheeins – a model for allosteric regulation based on the reversible association-dissociation of a monomeric-oligomeric protein
An allosteric model based on multiple oligomeric states of a protein has been proposed by E. K. Jaffe (2005. Morpheeins: A new structural paradigm for allosteric regulation. Trends in Biochemical Sciences 30:490–497). This model coins the term morpheeins to describe the different forms of a protein that can assume more than one conformation, where each distinct conformation assembles into an oligomeric structure with a fixed number of subunits. For example, conformation A of the protein monomer forms trimers, whereas conformation B of the monomer forms tetramers. If trimers and tetramers have different kinetic properties (Km and kcat values), as in low-activity trimers and high-activity tetramers, then the morpheein ensemble behaves like an allosterically regulated enzyme. Drawing on the traditional MWC model as an analogy, diagram a simple morpheein model in which wedge-shaped protein monomers assemble into trimers but the alternative conformation for the monomer (a square shape) forms tetramers. Further, the substrate, S, or allosteric regulator, A, binds “only” to the square conformation, and its binding prevents the square from adopting the wedge conformation. Describe how your diagram yields allosteric behavior.
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