Effect on Enzymes

The most distinguished feature of reactions that occur in a living cell is the participation of protein catalysts called enzymes. As with any catalyst, the basic function of an enzyme is to increase the rate of a reaction. All protein enzymes are globular, with each enzyme having a specific function because of its specific globular structure. However, the optimum activity of many enzymes depends on the presence of nonprotein substances called cofactors. The molecular partnership of protein-cofactor is termed a holoenzyme and exhibits maximal catalytic activity. The protein component without its cofactor is termed an apoenzyme and exhibits very low activity or none at all. 

Protein + Cofactor Protein-Cofactor (a complex) (apoenzyme (inorganic ion or holoenzyme optimally 

Several ways in which environmental pollutants may inactivate an enzyme system are described below  

A pollutant may combine with the active site or sites of an enzyme thus inactivating it. For example, a heavy metal such as mercury, lead, or cadmium can attach itself to the thiol or sulfhydryl (SH) group on an enzyme molecule, forming a covalent bond with the sulfur atom. This will lead to inactivation of the enzyme if the sulfhydryl group 

Many enzymes require cofactors, often cations, for their activity. These ions provide electrophilic centers in the active site. A pollutant may inhibit an enzyme by inactivating the cofactor involved. For instance, fluoride is known to be a potent inhibitor of enolase, a glycolytic enzyme that requires Mg2+ ions for its activity. In the presence of phosphate, fluoride inactivates the Mg2+ cofactor, presumably by causing the formation of a magnesium fluorophosphate complex. 

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Regulatory or allosteric enzymes like enzyme 1 are, in some instances, regulated by activation. That is, whereas some effector molecules such as F exert negative effects on enzyme activity, other effectors show stimulatory, or positive, influences on activity. 

Glycogen synthase also exists in two distinct forms which can be interconverted by the action of specific enzymes active, dephosphorylated glycogen synthase I (glucose-6-P-independent) and less active phosphorylated glycogen synthase D (glucose-6-P-dependent). The nature of phosphorylation is more complex with glycogen synthase. As many as nine serine residues on the enzyme appear to be subject to phosphorylation, each site s phosphorylation having some effect on enzyme activity. 

In the following text, examples of solvent effects on enzyme selectivity, referred either to systems based (i) on water-miscible organic cosolvents added to aqueous buffers or (ii) on organic media with low water activity, are discussed. 

In addition to its effects on enzymes and ion transport, Ca /calmodulin regulates the activity of many structural elements in cells. These include the actin-myosin complex of smooth muscle, which is under (3-adrenergic control, and various microfilament-medi-ated processes in noncontractile cells, including cell motility, cell conformation changes, mitosis, granule release, and endocytosis. 

If two compounds bind in a mutually exclusive fashion, then their effects on enzyme velocity are additive and the value of y is infinite (i.e., the combination term... 

As an example, consider an early calculation of isotope effects on enzyme kinetics by Hwang and Warshel [31]. This study examines isotope effects on the catalytic reaction of carbonic anhydrase. The expected rate-limiting step is a proton transfer reaction from a zinc-bound water molecule to a neighboring water. The TST expression for the rate constant k is... 

Olea, D. and Faure, C. (2003) Quantitative study of the encapsulation of glucose oxidase into multilamellar vesicles and its effect on enzyme activity. Journal of Chemical Physics, 119, 6111-6118. 

Mn2+ active transport system in Staphylococcus aureus. These metal-microbe interactions result in decrease microbial growth, abnormal morphological changes, and inhibition of biochemical processes in individual (Akmal et al. 2005a,b). The toxic effects of metals can be seen on a community level as well. In response to metal toxicity, overall community numbers and diversity decrease. Soil is a living system where all biochemical activities proceed through enzymatic processes. Heavy metals have also adverse effects on enzyme activities (Fig. 1). 

Cano, M.P., Hernandez, A., and De Ancos, B. 1997. High pressure and temperature effects on enzyme inactivation in strawberry and orange products. J. Food Sci. 62, 85-88. 

Kirsch, J. F. (1977). In Isotope Effects on Enzyme-catalyzed Reactions (ed. W. W. Cleland, M. H. O Leary and D. B. Northrop). University Park Press, Baltimore,... 

In addition to its direct effect on enzyme activity in the cytosol of target cells, cAMP can also initiate gene expression without actually entering the nucleus. PKA may also... 

Carbon kinetic isotope effects on enzyme-catalyzed decarboxylations are among the most intensively studied enzyme reactions. This is because of the central role that carbon dioxide plays in plant metabolism and also because precise kinetic measurements are relatively easy to obtain since the carbon dioxide liberated in the reaction can be immediately analyzed using isotope ratio mass spectrometry. 

Modeling Isotope Effects on Enzyme-Catalyzed Reactions... 

Northrop, D. B. Determining the Absolute Magnitude of Hydrogen Isotope Effects in W.W. Cleland, W. W., O Leary, M., Northrop, D. B. (Eds.) Isotope Effects on Enzyme-Catalyzed Reactions, University Park Press, Baltimore, MD, 1976, pp. 122-152. 

The usual starting point in enzyme kinetics is the Michaelis-Menten equation for the reaction rate v. This also seems a convenient starting point for interpretation of pressure effects on enzyme mechanisms. It will be shown that this formalism may be deceptive if the definitions and interpretations have not been made clear from the beginning. For the mechanism... 

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