Enzymes active centres

In c, d, and e we have the typical case of a bioelectrocatalyst where, through a mediator, there is electron transfer between the electrode and the enzyme active centre where the substrate is in its turn activated and reacts. In c the components are in solution in d and e the mediator or the enzyme are immobilized on the electrode surface, the electron transfer reaction occurring between mediator and electrode. In case/we have the ideal situation direct electron exchange between the electrode and active centre of the enzyme, the mediator being eliminated. It is, nevertheless, very difficult to reconcile the enzyme characteristics and the electrochemical process, and it continues to be important to find adequate mediators and enzyme immobilization procedures. 

The mechanism and theory of bioelectrocatalysis is still under development. Electron transfer and variation of potential in the electrodeenzyme-electrolyte system has therefore to be investigated. Whether the enzyme is soluble and the electron transfer process occurs through a mediator, or whether there is direct enzyme immobilization on the electrode surface, the homogeneous process in the enzyme active centre has to be described by the laws of enzyme catalysis, and the heterogeneous processes on the electrode surface by the laws of electrochemical kinetics. Besides this there are other aspects outside electrochemistry or... 

For enzymes (natural catalysts) researchers proposed joining the quantum and the classical description by making the precision of the description dependent on how far the region of focus is from the enzyme active centre (where the reaction the enzyme facilitates takes place). Th proposed dividing the system (enzyme -I- solvent) into three regions ... 

Figure 2.1 Mechanism for the oxygenation of iipids by iipoxygenase under aerobic conditions. LH, fatty acid LOOM, fatty-acid hydroperoxide Fe, the redox active centre of the enzyme.

As has been mentioned above, integrated signal intensities for molybdenum have always been less than 1 g. atom of Mo(V) per mole of xanthine oxidase. However, there are indications from recently performed integrations (90) that observed intensities of the Inhibited signal, in which Mo(V) is known to be stabilized (81), can be accounted for quantitatively when due allowance is made for the other species present in the samples. If this is confirmed it should make possible final rejection of earlier suggestions (87) that the enzyme contains two interacting molybdenum atoms in a single active centre. It should also help to eliminate possibilities (cf. 78) that only one of the two molybdenum atoms of the molecule is ever detected by EPR spectroscopy. 

The amino acid histidine has important functions in the active centres of several enzymes. Its biosynthesis involves intermediates which are possibly related to a precursor molecule (purine base) of a ribozyme. 

It was suggested,1 on the basis of kinetic measurements, that the phosphorofluoridates inhibit esterases by virtue of a highly specific affinity for the active centres of this group of enzymes. Preliminary experiments by Boursnell and Webb2 with diisopropyl phosphorofluoridate containing 32P gave results which were in accordance with this view. 

The pH optimum of an enzyme will often vary from one substrate to another and must be determined for each substrate. The buffer system used will often affect the overall activity of an enzyme and may alter its pH optimum. In general, the amino buffers such as glycylglycine and tricine, etc. (Table 8.2) result in a greater enzyme activity than do the simple inorganic buffers such as phosphate and carbonate. Buffers are most effective over a narrow pH range (approximately two units) which centres on their pK, value. Those buffers with pKa values similar to the known optimum pH of the enzyme should be tested for their effect on the activity of the enzyme over a limited pH range. 

Oba, R., Igarashi, A., Kamata, M., Nagata, K., Takano, S., Nakagawa, H. (2005) The N-terminal active centre of human angiotensin-converting enzyme degrades Alzheimer amyloid beta-peptide. Eur. J. Neurosci., 21,133-740. 

The treatment of metal poisoning is to administer a compound that binds the metal ion more strongly than does the group in the active centre of the enzyme. These compounds are known as chelating agents. For lead, the compound ethyl-enediaminetetraacetic acid (EDTA) is used. For mercury, dimercaptopropanol (dimercaprol) is used. 

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