Cooperative phenomena allosteric

The helical structure of PLL plays an important role in transferring information on oxygenation to the neighboring heme groups. We call this cooperative phenomenon the pseudo-allosteric effect of the PLL-heme complex. 

There are essentially two types of control mechanisms for biochemical switching allosteric cooperative transition and reversible chemical modification. Allosteric cooperativity, which was discussed in Chapter 4, was discovered in 1965 by Jacques Monod, Jefferies Wyman, and Jean-Picrrc Changeux [143], and independently by Daniel Koshland, George Nemethy and David Filmer [116]. The molecular basis of this phenomenon, which is well understood in terms of three-dimensional protein crystal structures and protein-ligand interaction, is covered in every biochemistry textbook [147] as well as special treatises [215],... 

The multifunctional enzyme V2 is (like the enzymes VI and V3) a regulatory (allosteric) and indeed a hysteretic (8) and, more precisely, a mnemonic enzyme. Hysteresis (kinetic cooperativity or chronological cooperativity) (9) is the phenomenon of slow conformational changes of enzymes by isomerization, association-dissociation or by the binding of an effector. 

Enzyme stimulation is an increase in enzyme activity resrrlting directly from the addition of a chemical. This is a somewhat imusual phenomenon in ertzymology, usually relegated to classically allosteric systems [200]. The concept is that a chemical stimulates the catalytic activity of an enzyme. This cooperativity may be considered in two aspects. One is homotropic cooperativity, in which a chemical stimulates its own biotransformation. This is usually manifested in sigmoidal (S-shaped) plots of v versus S. Heterotropic cooperativity is the stimulation of catalytic activity by direct addition of a different compound. 

Not all enzymes show the simple hyperbolic dependence of rate of reaction on substrate concentration shown in Figure 2.8. Some enzymes consist of several separate protein chains, each with an active site. In many such enzymes, the binding of substrate to one active site causes changes in the conformation not only of that active site, but of the whole multi-subunit array. This change in conformation affects the other active sites, altering the ease with which substrate can bind to the other active sites. This is cooperativity — the different subunits of the complete enzyme cooperate with each other. Because there is a change in the conformation (or shape) of the enzyme molecule, the phenomenon is also called allostericity (from the Greek for different shape ), and such enzymes are called allosteric enzymes. 

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