Protein: characteristic motion

Effect of Temperature and pH. The temperature dependence of enzymes often follows the rule that a 10°C increase in temperature doubles the activity. However, this is only tme as long as the enzyme is not deactivated by the thermal denaturation characteristic for enzymes and other proteins. The three-dimensional stmcture of an enzyme molecule, which is vital for the activity of the molecule, is governed by many forces and interactions such as hydrogen bonding, hydrophobic interactions, and van der Waals forces. At low temperatures the molecule is constrained by these forces as the temperature increases, the thermal motion of the various regions of the enzyme increases until finally the molecule is no longer able to maintain its stmcture or its activity. Most enzymes have temperature optima between 40 and 60°C. However, thermostable enzymes exist with optima near 100°C. 

Table 2.1. Intramolecular Motions in Proteins and the Values of the Parameters that Characterize Them Mass of Structural Element (m), Amplitude (A), Characteristic Time (r), Activation Energy ( ,), and Cross-Correlation (< X1-ZX2 ...

The motional characteristics of interest are typically those governed by the phospholipid fatty acyl chains and head-group region and the neutral lipid or protein components of membranes. Rotational motion can be subdivided into a structural component, the order or degree of orientational constraint,... 

The values of AE and AS (Table 1) suggest that the probe motion in frozen water and solutions of silica is the slipping by the rigid lattice, but in other cases this is the motion in viscous medium of surface water layers [12]. A kind of compensation effect reveals at that with rising the disperse phase concentration. That means a symbate increase of AE and AS, which is characteristic of the spin label motility in water-protein matrix [12]. 

Experimental results from studies of Arrhenius dependence of different characteristics of lysozyme are presented in Fig 4.1. (Alfimova and Likhtenshtein, 1979 Likhtenshtein, 1993 Likhtenshtein et al., 2000). The discontinuities on the curves indicate local conformational transitions and are apparently due to the appearance of a more open conformation of the protein. As can be seen from Fig. 4.1., these methods reveal conformational transitions at a temperature of about 30°C, whereas the temperature dependence of the partial heat capacity decreases monotonically in this temperature region. Recently, the presence of the conformational transition in lysozyme was confirmed independently. It was shown that the segmental motion of Trp 108 is hindered by the local cage structure at T < 30°C, although relieved from restricted motion by thermal agitation or by the formation of a ligand complex. 

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