Conformational changes, enzymatic

Enzymatic activities. The hydrolysis of ATP by actin-activated myosin is the characteristic enzymatic activity of muscle, smooth muscle included. All forms of smooth muscle myosin are slower than those of other muscles. The binding site for ATP and a reduced enzymatic activity are still present in monomeric myosin. The enzymatic activity of monomeric myosin is altered by a conformational change, (the 10S-6S transition) and the species of cations present in the reaction mixture. These differences relate to the possible mechanisms of regulation. 

As we have seen above, FRET is a technique that provides precise information about distances between 10 and 100 A which is in the range of the size of biological molecules and proteins. Researchers have taken advantage of this feature and developed different strategies to synthesize FRET sensors that are able to follow in real time and with high sensitivity very diverse processes such as enzymatic activity, conformational change, or molecule-molecule interaction. The design of these FRET sensors is described below. 

When an enzyme combines with a substrate, both the enzyme and substrate undergo conformational changes that increase the sensibility of the substrate to the attack by H" ", OH" or some other specific functional groups of the enzyme. By this process, the substrate is transformed into its products, which separate from the active site. The enzyme recovers then its original conformation, combines with another molecule of substrate and the cycle is repeated. It is worthwhile to emphasise here that enzymatic reactions occur at very mild "physiological conditions", i.e., room temperature and pH near 7, and with rate accelerations of 10 -10 0 greater than the uncatalysed reaction. 

Although conformational changes are essential features of proteins, the conformational basis of protein activity is not yet understood at the molecular and atomic levels. It is generally assumed that the mechanism of enzyme-catalyzed reactions would he defined if all the intermediates and transition states between the initial and final stages, as well as the rate constants, could be characterized. But in spite of constant progress in such characterization, most enzymatic mechanisms are not understood in terms of physical organic chemistry and enzyme activity is still regarded as a miracle as compared to classical catalysis. 

Perturbation of the fundamental thermodynamic variables pressure and temperature can thus be used to obtain the temporal resolution of every kinetically significant step in an enzymatic reaction. Such perturbations, combined with pH-dependence studies and several different spectroscopic tools, will detect conformational changes if they occur dur-... 

Transporters recognize and bind the molecules to be transported and help them to pass through the membrane as a result of a conformational change. These proteins (permeases) are thus comparable with enzymes—although with the difference that they catalyze vectorial transport rather than an enzymatic reaction. Like enzymes, they show a certain affinity for each molecule transported (expressed as the dissociation constant, in mol L ) and a maximum transport capacity (V). 

Ever since it was discovered that enzymes can be catalytically active in neat organic solvents, the question of how to select the correct solvent for a specified enzymatic conversion has been of crucial importance. The solvent can influence an enzymatic reaction both by direct interaction with the enzyme and by influencing the solvation of the substrates and products in the reaction medium. An example of direct interaction between solvent and enzyme is when the solvent acts as an inhibitor of the enzyme. In other cases the solvent causes conformational changes in the enzyme, thereby changing its catalytic properties. The solvent can also influence the amount of water bound to the enzyme, but this effect can largely be avoided by the use of fixed water activity as described above. Direct interaction between solvent and enzyme can influence enzyme stability as well as activity. 

A similar conformational change has been detected for calmodulin a heat-stable Ca2 +-binding protein. The field of Ca2+ research continues to expand noticeably, with articles focusing on calmodulin ll6). Calmodulin regulates the enzymatic activities of various enzymes such as adenylate cyclase and cyclic nucleotide phosphodiesterase. There are four Ca2+-binding sites in calmodulin, and the Ca2+ binding... 

For some enzymatic reactions a transition state will be favored by a medium of very low dielectric constant and a correctly constructed active site can provide just such a surrounding.1893 Hydrophobic groups may be packed around a site where an ion pair or other ionic interaction between enzyme and substrate occurs increasing the strength of that interaction. Conformational changes may enhance such effects. 

---