Specific nature of enzymes

Enzyme specificity is said to be absolute if the action of the enzyme is limited to one substrate only. An example is urease, which only catalyses the breakdown of urea to carbon dioxide and ammonia  

In most cases enzymes are able to catalyse reactions in more than one group of substrates. In such cases the specificity is said to be relative. Such group specificity may be of a low order, such as in the case of the digestive enzymes trypsin and pepsin, which catalyse the rupture of peptide bonds. In other cases it may be much higher. Chymotrypsin, for example, catalyses the hydrolytic cleavage only of peptide bonds in which the carboxyl residue is derived from an aromatic amino acid. 

Enzyme specificity arises from the need for spatial conjunction of the active groups of the substrate with the active site of the enzyme. This requires an exact fit of 

Although the lock and key model accounts for enzyme specificity, it does not explain certain other aspects, and the induced-fit model is currently pre-eminent. This model predicates that the reacting sites need not be fully preformed but only so 

In addition to distorting their substrates, enzymes frequently position groups in the right position for the catalytic action to take place, especially in the case of acid-base catalysis. In other instances, the enzyme sets a metal ion in just the right position to allow metal ion catalysis. 

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Specific recognition of enzymes by their natural substrates and cosubstrates is a common rule but what about recognition of an enzyme by artificial cosubstrate The cyclic voltammetric investigation of glucose oxidase provides an answer to the question.11 Because the flavin prosthetic group... 

In view of the specific nature of the whole process it appeared interesting to investigate whether the reduction of thioacetaldehyde to mercaptan could also be accomplished with cell-free enzyme solutions. Neuberg and Nord found that yeast maceration juice effects this hydrogenation (see p. 79). 

Enzymes are highly selective of the substrates with which they interact and in the reactions that they catalyze. This selective nature of enzymes collectively known as enzyme specificity can be best illustrated with oxidoreductases (dehydrogenases), which display substrate and bond specificities (e.g., acting on —CHOH—, versus —CHO versus —CH—CH— versus —CHNH2, and cis versus trans for unsaturated substrates), coenzyme specificity (e.g., NAD(H) versus NADP(H)), chiral stereospecificity (d- versus l- or R- versus S-stereoisomers), and prochiral stereospecificity (A versus B corresponding to proR- versus proS isomers and re face versus si face, respectively). The table lists some dehydrogenases and their coenzyme, substrate, product and stereospecificities (You, 1982) ... 

The chemical nature of enzyme was controversial for a long time, until Buchner succeeded in isolating an enzyme system (zymase) from yeast in a cell-free extract in 1897.2) Urease was then crystallized by Sumner in 1926,3) followed by crystallization of several proteolytic enzymes by Northlop and his colleagues. At present the chemical nature of enzyme is defined as a protein with catalytic activity based on the specific activaiton of its substrate. However, this definition has been somewhat open to debate since a catalytic RNA, ribozyme, was discoved in 1982. 

Taking an advantage of the existence of kinetic measurements data for several inhibitors of PAL from potato,111,112 ab initio investigation of the physical nature of enzyme-inhibitor interactions and the existence of correlation between particular energy components and inhibitors activity was conducted.58 Similarly to the above presented LAP study,54-58 the validity of successive approximations was tested to qualify a permissible level of simplification and - first of all - to reveal which constituents of interaction energy contribute the most to the specific inhibitors... 

Several enzymes are involved in nucleic acid synthesis, especially when one considers the varied nature of enzymes in each group. For example, with the polymerases, there are separate enzymes important in biosynthesis of DNA and RNA, some with specificity for size of the chain length (gap) to be completed. The enzymes have different structural properties depending on whether they are from microorganisms, plants or animals. There are multiple forms within a single cell or organism. They vary from a single polypeptide enzyme of 40,000 daltons [mammalian 3-polymerase U)], to a seven-subunit complex of about 500,000 daltons [E. coti DNA polymerase III ( 2) ]. 

In 1926, James Sumner established the protein nature of enzyme. He was responsible for the isolation and crystallization of urease, which provided a breakthrough in studies of the properties of specific enzymes. 

NOS products of neurons, epithelial cells and other cells in the lung have both bronchodilator and inflammatory properties (Gaston etal., 1994b) the specific nature of this bioactivity depends on the chemical characteristics of the functional products in the specific microenvironment under consideration. For example, NO is capable of complexing with and affecting the activity of a variety of metalloproteins and enzymes, such as guanylyl cyclase and ribonucleotide reductase (Nathan, 1992 Stamler et al., 1992). NO can also complex with superoxide anion to form peroxynitrite, which has a cytotoxic immune effector role (Radi etal., 1991 Stamler etal., 1992). Furthermore, NO can form iron nitrosyl complexes, which are the putative intracellular macrophage products of iNOS responsible for lysis of intracellular parasites (Hibbs... 

The Zn can be selectively removed from the azoenzyme derivative without any appreciable change in the absorption and CD spectra confirming coordination of the azophenol to Co(III) (2). The Co(III) modification of AA-CPA-Zn leads to an enzyme derivative which loses all peptidase activity but still maintains esterase activity (1). The modification can be reversed completely by reducing the Co(III) with Fe(II)-EDTA regenerating the original spectral and enzymatic properties observed for AA-CPA-Zn. The return of the original properties confirms the site specific nature of the modification and the lack of damage to the protein (2 ). 

The application of site-specific modification of enzymes and other proteins has become increasingly common, and a wide range of chemical classes is available from which to select or design a reagent for exploring a particular enzyme. Structural similarity to the natural ligand is always desirable to ensure target sped-... 

Biochemistry, a new superorganic chemistry, was in its infancy as the structural elements of proteins and nucleic acids were deciphered and the nature of enzymes explored. The first decade of the 20th century would also witness synthesis of the first silver bullet drug salvarsan, specifically designed to combat syphilis. 

The utilization of enzymes in the natural fiber modification field is rapidly increasing. This trend may be due to the environmentally-friendly nature of enzyme treatments as the catalyzed reactions are very specific and the performance is very focused [3]. Several enzymes have been used in order to enhance the effectiveness of extracting fibers from the agro-residue or fine-tune the properties of extracted fibers such as lowering their diameter. Xylanases, cellulases, pectinases are the enzyme types that have found more use in agro-residual fiber modification [12,13,38]. 

The ribonucleoside triphosphate reductase of L. leichmannii is an allosteric enzyme, the activity of which is modified in a complex manner by deoxyribonucleoside triphosphates 28, 29). Reduction of each of the four substrates is maximally stimulated by a particular deoxyribonucleoside triphosphate, which Beck 29) terms a prime effector. The data of Table 16-III illustrate the specific nature of the effector stimulation prime effectors are indicated by the italicized data. The effector-induced stimulation of reductase activity appears to be countered in particular ways by deoxyribonucleoside triphosphates for example, dTTP inhibits the dATP-acti-vated reduction of CTP. It has been speculated that these complicated positive and negative allosteric effects produced by nucleotides may constitute a mechanism for ensuring that surpluses or shortages in the production of deoxyribonucleotides do not occur in the cell 29). 

Supramolecular bioinorganic chemistry is a natural evolution in biomimetic metallic systems as it constitutes a further degree of complexity in modeling. The traditional approach consisting in modeling the first coordination sphere of metallic sites proved to be very efficient, as valuable data could be extracted from these examples to gain insight in natural systems mechanisms. But they fail to reproduce several specific aspects of enzymes that can be... 

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