Properties of Individual Enzymes

Control coefficients are properties of a metabolic pathway as a whole. The kinetic properties of individual enzymes that are relevant for control are expressed in terms of their elasticity coefficients. An elasticity coefficient ef quantifies how strongly a metabolite concentration Xj affects an enzyme rate V directly, at constant concentrations of all other metabolite concentrations ... 

Aside from the inordinately dominant light of molecular genetics, the new wave in biochemistry today is, what has come to be called, metabolic control analysis (MCA) (Comish-Bowden and Cardenas, 1990). The impetus behind this wave is the desire to achieve a holistic view of the control of metabolic systems, with emphasis on the notion of system. The classical, singular focus on individual, feedback-modulated (e.g., allosteric), rate-limiting enzymes entails a naive and myopic view of metabolic regulation. It has become increasingly evident that control of metabolic pathways is distributive, rather than localized to one reaction. MCA places a given enzyme reaction into the kinetic context of the network of substrate-product connections, effector relationships, etc., as supposedly exist in situ, it shows that control of fluxes, metabolite concentrations, inter alia, is a systemic function and not an inherent property of individual enzymes. Such... 

Recent studies on proteolysis in parasites have tended to focus more on the enzymes than on the processes in which they participate (51). A great deal of information is available on the in vitro properties of individual enzymes, but the complexity of the proteolytic systems of some parasites, for example cysteine proteinases are often present... 

Studies on the catabolism of PG were initiated by the isolation and identification of catabolites in urine and plasma which established catabolic pathways especially of PGE and PGF [1,4], Each step of the catabolic pathway is catalyzed by an enzyme. In this chapter only papers describing the properties of individual enzymes will be reviewed. 

The reaction conditions can in fact be further optimized according to the specific properties of individual enzymes. Some of the readily adjustable factors are as follows. 

From this point on there are numerous ways of proceeding with the analysis. Crude extracts can be made from different mutant cells. The extract from one mutant can be used to complement the extract from another mutant in tryptophan synthesis, which can lead to an assay for a particular enzyme carried by one mutant that is missing in the other mutant. The goal at this juncture is to purify each enzyme of the pathway so that the properties of the enzymes and the reactions they catalyze can be individually scrutinized. All of the tryptophan enzymes have been isolated from E. coli. Studies of other systems indicate a remarkable similarity for the operation of this pathway in different microorganisms and plants. 

Much uncertainty reigned over the nature of proteins, the best known of which were hemoglobin, the digestive enzymes, and later, insulin. Properties of individual amino acids and the peptide bond were studied early in this century, but it was not until urease was crystallized by Sumner1 in 1926, followed by the isolation of other pure enzymes, that it was finally accepted in the 1930s that enzymes were proteins and that their catalytic properties were not the function of some adsorbed low molecular weight entity. Somewhat later, towards the end of the 1930s, coenzymes were isolated and their roles established. 

The results of site-directed mutagenesis studies are generally consistent with this model.583 Table 4.3 summarizes the kinetic parameters of the mutant enzymes where the five labeled lysyl residues are individually replaced by Gin. The Lys-367/Gin enzyme was almost completely inactive. The Lys-263/ Gin enzyme had slight activity with decrease in affinity to pyrophosphate and glucose 1-phosphate. The Lys-329/Gin enzyme also had considerably lower affinity to the two substrates, but possessed a Kraax value comparable to the wild-type enzyme. The replacements of Lys-409 and Lys-410 by Gin did not change the kinetic properties of the enzyme. The kinetic properties of the Lys-329/Gin enzyme suggest that Lys-329 interacts with pyrophosphate after UDP-glucose binds to the active site. 

Enzyme Y was purified from a tissue sample obtained from a patient. The kinetic properties of this enzyme and those of the same enzyme isolated from a normal individual are shown in the graph below. Which of the following statements is TRUE ... 

In contrast, some parameters are properties of individual solvent molecules. Examples are dipole moment and log P (the octanol-water partition coefficient). These parameters are appropriate where individual solvent molecules are engaged in interactions away from the bulk phase. Thus, log P is used sensibly to describe the tendency of solvents to interact with (and affect the functioning of) the enzyme molecules. However, these parameters are not good choices when bulk solvent behavior is important, such as its ability to solvate water or reactants (and hence affect their availability to the enzyme). Even when such mechanisms are important, it is quite common to see correlations presented against log P. However, any relationship probably reflects the correlation of log P with appropriate scales of bulk solvent behaviour. 

Thermodynamic stability is a global property of the enzyme structure, and contributions of individual amino acids toward the free energy of folding are additive and highly cooperative. Analysis of mutant proteins has defined the contributions of various amino acids toward the overall stability of the protein. Replacements which alter the formation of ion pairs, hydrogen bonds, van der Waals contacts, or hydrophobic interactions each tend to destabilize the folded protein by a qualitatively comparable amount (14). In one approach to this problem, site-specific substitution of amino acids has provided new approaches toward dissecting the kinetic mechanisms of protein folding (27). 

Fatty acid synthetases can be categorized as Type I, Type II, and Type III. The distribution and some of the properties of the Type I and Type II enzymes are summarized in Table 11.1 (cf. Fig. 11.2). The Type I synthetases tend to occur in higher organisms and all of the purified synthetases of eukaryotes, with the exception of plants, are of this type. Two bacterial genera Mycobacterium and Corynebacterium also contain Type I synthetases. These synthetases are large-molecular-weight multifunctional proteins containing covalently bound ACP (cf. Wakil et al, 1983). On the other hand Type II synthetases such as that from E.coli consist of individual enzymes that can be isolated in an active form. The ACP... 

A wild strain Aspergillus terreus isolated from wood drifted ashore in India (Araujo and D Souza, 1980) exhibits an increased production of 3-glucosidase as compared with other known producers of cellulases. Therefore, in our studies of cellulases we concentrated on this organism. We first determined the effect of cultivation conditions on the properties and production of individual enzymes of the cellulolytic complex (D Souza and... 

Allotopy refers to the modification of the properties of some enzymes depending on whether they are in solution or bound to a membrane. One approach to this problem has been to study the modification of the properties of enzymes induced by attaching them to various synthetic membranes or other carriers. Work is under way to reconstitute the respiratory chain of mitochondria from its Individual components this approach should also provide a great deal of information about the factors involved in allotopy. 

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