Enzymes general discussion

Turnover numbers of molybdenum-containing enzymes generally tend to be low. A brief discussion of each of the enzymes in Table 1 is given below. 

The general metabolism of sulfur, extensively described in many texts of biological sciences, is not considered in this article some topics (e.g. metallo-enzymes) are discussed elsewhere in this volume (Chapter 11.2). Our focus is on sulfur-containing secondary metabolites in microorganisms and plants. In view of the vast literature, we can only provide an eclectic account citing recent work where possible. 

Type II copper enzymes generally have more positive reduction potentials, weaker electronic absorption signals, and larger EPR hyperfine coupling constants. They adopt trigonal, square-planar, five-coordinate, or tetragonally distorted octahedral geometries. Usually, type II copper enzymes are involved in catalytic oxidations of substrate molecules and may be found in combination with both Type I and Type III copper centers. Laccase and ascorbate oxidase are typical examples. Information on these enzymes is found in Tables 5.1, 5.2, and 5.3. Superoxide dismutase, discussed in more detail below, contains a lone Type II copper center in each of two subunits of its quaternary structure. 

This section emphasizes work done in the last few years. The reader is referred to other sources for reviews of older work236 or more general discussions of nucleophilic reactions at phosphorus.237"245 More general discussions of enzymic phosphoryl and nucleotidyl transfer are available,246 248 and the role of divalent metal ions has been reviewed.249"251... 

In an adult human some 65% of the total iron is found in hemoglobin and myoglobin, and the bulk of the remainder is found in the storage proteins ferritin and hemosiderin. A small amount is utilized in iron enzymes at any one time. An account will be given of ferritin and the transport protein transferrin, prior to a general discussion of iron transport and storage. 

Figure 3-4. The general phenylpropanoid pathway. The enzymes involved in this pathway are (a) phenylalanine ammonia lyase (PAL E.C. 4.3.1.5), (b) cinnamic acid 4-hydroxylase (C4H E.C. 1.14.13.11), and (J) 4-coumaric acid CoA ligase (4CL E.C. 6.2.1.12). (a) depicts tyrosine ammonia lyase activity in PAL of graminaceous species. The grey structures in the box represent an older version of the phenylpropanoid pathway in which the ring substitution reactions were thought to occur at the level of the hydroxycinnamic acids and/or hydroxycinnamoyl esters. The enzymes involved in these conversions are (c) coumarate 3-hydroxylase (C3H E.C. 1.14.14.1), (d) caffeate O-methyltransferase (COMT EC 2.1.1.68), (e) ferulate 5-hydroxylase (F5H EC 1.14.13), and (g) caffeoyl-CoA O-methyltransferase (CCoA-OMT EC 2.1.1.104). These enzymes are discussed in more detail in Section 10.

A brief and general discussion of the industrial use of enzymes, including general enzymology, specific applications, (not limited to foods) suppliers, properties, sources, production methods, and definitions of units. 

Table XVI gives a partial list of native proteins that have been hydrolyzed with proteolytic enzymes. A discussion of the interpretation of each example listed is beyond the scope of this review, but a few comments concerning certain features of proteolysis are ivarranted. The mechanism of enzymatic hydrolysis of native proteins was studied in detail by Tiselius and Eriksson-Quensel (1939), who examined the action of pepsin on ovalbumin. Two mechanisms of proteolysis were considered by these workers. In the first mechanism the enzyme hydrolyzes all susceptible peptide bonds in one substrate molecule before hydrolysis of a second molecule begins. This type of mechanism has been described by Lmderstrpm-Lang (1952) as the all or none type. In the second mechanism, the enzyme hydrolyzes the single, most susceptible bond in all substrate molecules before hydrolysis of other bonds occurs. This mechanism is called the zipper type. Hydrolysis of a protein can proceed by either of the two mechanisms or by a mechanism which has features of both types. General aspects of the problem have been reviewed and theoretical equations which describe the kinetics of ea( h mechanism have been derived (Linderstr0m-Lang, 1952, 1953).

As one of the blue oxidases, laccase contains four copper ions with strong intermetal-lic interactions between one pair. The involvement of a pair of copper ions in the biologically active role of reduction of dioxygen has added to the interest shown in this enzyme for more than ten years. Reviews in 197087, 197388 and 197589,90 contain general discussions of the copper oxidases including laccase. 

Most allosteric enzymes are multisubunit proteins. The binding of substrate to one protomer in an allosteric enzyme affects the binding properties of adjacent protomers. In addition, the activity of allosteric enzymes is affected by effector molecules that bind to additional sites called allosteric or regulatory sites. Allosteric enzymes generally catalyze key regulatory steps in biochemical pathways. (Regulation of allosteric enzymes is discussed on p. 190.)... 

The importance of the general anion binding site, including Arg-47, for this reaction and the results of an X-ray study (ISO) of the carboxy-methylated enzyme are discussed in Section II,H,l,a. 

Many of the enzymic reactions that form the basis of the stereochemical generalizations discussed in this and subsequent sections of this chapter involve the... 

Interest in enzyme stereospecificity and the stereochemistry of prochiral centres, such as the methylene groups of mevalonic acid, has necessitated more precise definitions of the stereochemistry of the various molecules involved and of the enzymological consequences. The use of multiply labelled mevalonic acid in terpenoid and steroid biosynthesis has been reviewed by Hanson. The Proceedings of the 1970 Phytochemical Society symposium have been published. They include a general discussion of terpenoid pathways of biosynthesis by Clayton and specific chapters on monoterpenoids, diterpenoids, eedysones, carotenoids, isoprenoid quinones, and chromanols. Other reviews concerning biosynthesis have appeared on furanocoumarins, indole alkaloids, monoterpenoids, and diterpenoids. ... 

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