Activity Residual contribution

An early discovery by Frederick Richards that turned out to be useful was that the protein could be cleaved between residues 20 and 21 by the bacterial serine protease, subtilisin. The resulting two polypeptides were separated and purified. They were enzymatically inactive individually, but regained the activity of the native enzyme when they were recombined. This work shows that strong, nonco-valent interactions occur that can hold protein chains together even when one of the peptide links is cut. It also makes it possible to modify specific amino acid residues of the two polypeptide chains independently and to explore how each residue contributes to the reassembly of the protein and the recovery of enzymatic activity. 

The catalytic glutamine at position 204 in Gail is a conserved feature in all functional Ga proteins, and in most members of the Ras superfamily (residue 61 in Ras). Mutations of this residue in Ga and Ras abolish GTPase activity, are constitutively active, and contribute to cellular transformation (Barbacid, 1987 Bourne et aL, 1991 Gilman, 1987). Further, GTPase activity of the... 

The activity coefficient is described in UNIFAC-FV and several other estimation models as being roughly composed of two or three different components. These components represent combinatorial contributions (yf) which are essentially due to differences in size and shape of the molecules in the mixture, residual contributions (yt) which are essentially due to energy interactions between molecules in the solution, and free volume (y[v) contributions which take into consideration differences between the free volumes of the mixture s components ... 

Mixtures of hydrocarbons are assumed to be athermal by UNIFAC, meaning there is no residual contribution to the activity coefficient. The free volume contribution is considered significant only for mixtures containing polymers and is equal to zero for liquid mixtures. The combinatorial activity coefficient contribution is calculated from the volume and surface area fractions of the molecule or polymer segment. The molecule structural parameters needed to do this are the van der Waals or hard core volumes and surface areas of the molecule relative to those of a standardized polyethylene methylene CH2 segment. UNIFAC for polymers (UNIFAC-FV) calculates in terms of activity (a,-) instead of the activity coefficient and uses weight fractions... 

Calculations using the UNIFAC-FV model are carried out as follows for a binary mixture of solute (a = 1) dissolved in a solid polymer (P = 2) (Goydan et al., 1989) The activity of the solute, aj, is separated into the three components the combinatorial contribution, a, the residual contribution, a, and the free-volume contribution,... 

Step 19 Determine the residual contribution to the activity coefficient, QR, for each component i using Equation (3C-8). 

The combinatorial part of the activity coefficient [Equation (3C-2)1 is known as the Staverman-Guggenheim form. This term is intended to account for size and shape effects. The residual contribution accounts for interactions among groups. The Staverman-Guggenheim term has been shown to predict an exaggerated degree of nonideality for systems where the residual contribution is zero (Kikic et al., 1980). Predictions for such systems are expected to be less accurate. 

Cyclin-Cdks were appropriately named the cell cycle s engines . The cyclin-dependent protein kinases (Cdks) are soluble serine/threonine kinases of 34-40 kDa. The Cdks share with other serine/threonine protein kinases sequence similarities, including a subset of residues that is essential for catalytic activity. Cdks contribute the catalytic subunit, whereas the regulatory subunit is contributed by a cyclin. Cyclins control the kinase activity, determine the substrate specificity and the subcellular location of Cdks. Each of these processes is a potential site of regulation. The major substrates of the Cdks are proteins regulating gene transcription. Cdks can be controlled in three major ways ... 

Genetic engineering of hen egg-white lysozyme has been used by Kirsch el al. (1989) as an approach to studying the structure—function relationships of lysozyme. Thus, they offer evidence from site-directed mutagenesis of cloned lysozyme (expressed in yeast), that Asp-52 and Glu-35 are vital for the expression of lysozyme. However, it is curious that conversion of Asp-52 to the amide resulted in a form of the enzyme that still had 5% of the normal activity. Conversion of Glu-35 to the amide, on the other hand, resulted in a lysozyme that was devoid of all activity. It was demonstrated by mutagenesis of Asp-IOI to Gly that the ionization of this residue contributes thermodynamically to the association of lysozyme with the inhibitor chitotriose. 

As expected, mutation of the active site Asp30 to Asn did not affeet the overall stmcture of the protein. FIV PR(D30N) is a homodimer (117 residues per monomer, including N-terminal alanine). The tertiary structure of the monomer consists mainly of P sheets and turns and is similar to that of FIV PR(wt) (12). The two monomers are related hy a two-fold axis. The active site, characterized by AsnA30, AsnB30 and the inhibitor molecule LP-149 (Fig. 1), is located between the two monomers (To simplify the description of the dimer, letters A and B are arbitrarily used to describe the residues contributed by the two different monomers for residues adopting two different conformations, we denoted their second conformation by primes). The flaps, two P hairpin structures formed by residues A49-A69 and B49-B69, fold over the inhibitor in the active site. Their position and conformation are similar to those observed in the wild type structure (12). Residues A59-A61 (and B59-B61), which form the tip of the flaps adopt two conformations,... 

When using the UNIFAC model one needs to identify the functional subgroups present in each molecule by means of the UNIFAC group table. Next, similar to the UNIQUAC model, the activity coefficient for each species is written as eqn. (2.4.14), except for the the residual term, which is evaluated by a group contribution method in UNIFAC, The residual contribution of the logarithm of the activity coefficient of group k in the mixture. In F., is obtained from... 

In eqn. (2.4.21) ti is the number of k groups present in species i, and In F][ is the residual contribution to the activity coefficient of group k in a pure fluid of species i molecules. The purpose of the last term is to ensure that, in the limit of pure species i (which is still a mixture of groups unless of course the molecules of species i consist of a single functional group), the residual term is zero. 

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