Enzymes and functions

The reactant is referred to as a substrate. Alternatively it may be a nutrient for the growth of cells or its main function may require being transformed into some desirable chemical. The cells select reactants that will be combined and molecules that may be decomposed by using enzymes. These are produced only by living organisms, and commercial enzymes are produced by bacteria. Enzymes operate under mild conditions of temperature and pH. A database of the various types of enzymes and functions can be assessed from the following web site http //www.expasy.ch/enzyme/. This site also provides information about enzymatic reactions. 

The number of enzymes and functional proteins that are reportedly regulated by S-nitrosation is on the rise. For example, a search of PUBMED with the key word S-nitrosation revealed some 70 reports of in vitro regulation of enzymes, proteins and cellular processes that are affected by S-nitrosation. Some of these processes that have been well characterized include, nuclear regulatory proteins the NMDA receptor and the ertrocyte anion exchange protein 1 (AE1) (see review by Gaston, 2003). 

The endoplasmic reticulum is composed of a convoluted network of channels and so has a large surface area. Apart from cytochromes P-450, the endoplasmic reticulum has many enzymes and functions, besides the metabolism of foreign compounds. These include the synthesis of proteins and triglycerides and other aspects of lipid metabolism and fatty acid metabolism. Specific enzymes present on the endoplasmic reticulum include cholesterol esterase, azo reductase, glucuronosyl transferase, NADPH cytochromes P-450 reductase and NADH cytochrome b5 reductase and cytochrome b5. A FAD-containing monooxygenase is also found in the endoplasmic reticulum, and this is discussed later in this chapter. 

Hempel J, Kaiser R, Jdmvall H. Mitochondrial aldehyde dehydrogenase from human liver primary structure, differences in relation to the cytosolic enzyme and functional correlations. Eur J Biochem 1985 153 13-28. 

In an Escherichia coli expression system for the aqualysin I precursor, the precursor is processed autoproteolytically into the mature 28-kDa enzyme by treatment at 65 ° C.23) In this case, the N-terminal pro-sequence is required for the production of active enzyme and functions to stabilize the precursor structure.283 The C-terminal pro-sequence is not essential for the production of active aqualysin 1,293 but seems to be involved in the translocation of the precursor across the cytoplasmic membrane.303 In a Thermus thermophilus expression system,313 the C-terminal pro-sequence is required for the production and extracellular secretion of active aqualysin I.323 In an E. coli expression system for the subtilisin E gene, the N-terminal pro-sequence is essential for the production of active enzyme,333 as in the case of aqualysin I. The requirement of the pro-sequence is also shown in vitro for the refolding of the inactive mature protein to an active enzyme.34 353 The functions of the N-terminal pro-sequences of aqualysin I and subtilisin E seem to be similar. 

Cirilli et al.11171 cloned the gene of diaminopimelate epimerase from Haemophilus influenzae, and purified and crystallized the enzyme. The enzyme is monomeric and has a unique protein fold, in which the amino terminal and carboxyl terminal halves of the molecule fold into structurally homologous and superimposable domains (Fig. 17-13). Cys 73 of the amino terminal domain is found in the disulfide linkage, at the domain interface, with Cys 217 of the carboxy terminal domain 117. Thus, it is most conceivable that these two cysteine residues stay in reduced form in the active enzyme and function as the acid and base in the mechanism. Koo and Blanchard 118 explored a number of kinetic and isotope approaches to clarify the mechanism of the enzyme. However, which of the two cysteine residues is responsible for proton abstraction from the two enantiomeric Ca-H bonds is not yet known. 

Many biomolecules are Lewis adducts with central metal ions. Most often, O and N atoms of organic groups, with their lone pairs, serve as the Lewis bases. Chlorophyll is a Lewis adduct of a Mg " ion and four N atoms in an organic ring system. Vitamin B12 has a similar structure with a central Co, and so does heme, but with a central Fe ". Several other metal ions, such as Zn ", Mo ", and Cu ", are bound at the active sites of enzymes and function as Lewis acids in the enzymes catalytic action. 

Globular proteins include most enzymes and function in aqueous enviromnents. About 65% of the mass of most cells, for example, is water. When placed in water, nonpolar materials, including nonpolar amino acid side chains, cause nearby water molecules to adopt a more ordered arrangement, reducing the entropy of water. This is called the hydrophobic effect. The unfavorable negative A5 is moderated if the protein adopts a spherical shape which places nonpolar side chains inside and polar ones on the surface. Of... 

Noncovalent complex formation allows chemical reartions between specific amino adds on the enzyme and functional groups on the substrate to occur in an environment that is kinetically equivalent to a unimolecular process. Transforming what would otherwise be bimolecular reactions into effectively intramolecular conversions is a major reason that enzymes can accelerate reactions by up to 23 orders of magnitude over the background (uncatalyzed) reaction [ 1 ]. 

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