Metal cofactors

Several metals, for example Cu, Zn, and Mn, are associated with a group of enzymes called superoxide dismutases. These enzymes scavenge the superoxide anion, Oj, which may be a by-product of various redox reactions or the electron transport system (Chapter 17). The superoxide anion gives rise to the very de- 

Metal Ionic form Daily requirement Some enzyme(s) in which the metal is found Notes 

Zn Zn2+ 16 mg Over 200 e.g., carbonic anhydrase, carboxypeptidases Sickle cell anemia causes a Zn deficiency 

Cu Cu+, Cu2+ 2 mg Cytochrome c oxidase, tyrosinase, lysyl oxidase, superoxide dismutases Cu deficiency has been associated with atherosclerosis in animals 

Se 50-200 /zg Glutathione peroxidase Associated with vitamin E metabolism may be considered to be a nonmetal 

---

There are two distinct groups of aldolases. Type I aldolases, found in higher plants and animals, require no metal cofactor and catalyze aldol addition via Schiff base formation between the lysiae S-amino group of the enzyme and a carbonyl group of the substrate. Class II aldolases are found primarily ia microorganisms and utilize a divalent ziac to activate the electrophilic component of the reaction. The most studied aldolases are fmctose-1,6-diphosphate (FDP) enzymes from rabbit muscle, rabbit muscle adolase (RAMA), and a Zn " -containing aldolase from E. coli. In vivo these enzymes catalyze the reversible reaction of D-glyceraldehyde-3-phosphate [591-57-1] (G-3-P) and dihydroxyacetone phosphate [57-04-5] (DHAP). 

Besides other functions, vitamin Bj2 and fohc acid take part in providing one-carbon residues for DNA synthesis, deficiency resulting in megaloblastic anemia. Vitamin C is a water-soluble antioxidant that maintains vitamin E and many metal cofactors in the reduced state. 

Copper is an essential trace element. It is required in the diet because it is the metal cofactor for a variety of enzymes (see Table 50—5). Copper accepts and donates electrons and is involved in reactions involving dismu-tation, hydroxylation, and oxygenation. However, excess copper can cause problems because it can oxidize proteins and hpids, bind to nucleic acids, and enhance the production of free radicals. It is thus important to have mechanisms that will maintain the amount of copper in the body within normal hmits. The body of the normal adult contains about 100 mg of copper, located mostly in bone, liver, kidney, and muscle. The daily intake of copper is about 2—A mg, with about 50% being absorbed in the stomach and upper small intestine and the remainder excreted in the feces. Copper is carried to the liver bound to albumin, taken up by liver cells, and part of it is excreted in the bile. Copper also leaves the liver attached to ceruloplasmin, which is synthesized in that organ. 

Heavy metals with no known biological function, such as aluminum, arsenic, lead, and mercury, are nonessential metals.4-5 These metals are toxic because they can irreversibly bind to enzymes that require metal cofactors. Toxic metals readily bind to sulfhydryl groups of proteins.6-7 In fact,... 

The above description provides a possible starting background for the description of the beginning of cellular chemotypes, prokaryotes, but even this is less complicated than the only cells for which we have evidence since they have at least two additional groups of more sophisticated chemicals - coenzymes (see Tables 5.3 and 5.4) and certain metal cofactors, which we presume were additions to the most primitive cells. After we have described them, we shall return to the problem of cellular (cytoplasmic) organisation. Note that coenzyme novelty is not in basic pathways but in control of rates and in energy management. 

The above account of selectivity of inorganic plus organic chemistry in synthesis is given rather extensively to stress three points. All the four (Mg, Fe, Co and Ni) porphyrin products came from one source, the synthesis of uroporphyrin. The basis of selection is very different from that in primitive centres which use thermodynamic stability constant selectivity based on different donor atoms for different metal ions. Here, all ion complexes have the same donor atoms, nitrogen, the most constrained being the coordination of Mg2+ by five nitrogens exactly as is seen for Fe in haemoglobin. Hence, there also has to be a new control feedback to ensure that the appropriate quantities of each metal cofactor is produced in a balanced way, that is synthesis from uroporphyrin has to be divided based upon... 

Step by step introduction of more effective ways of handling cytoplasmic chemistry, using coenzymes and special metal cofactors, e.g. haem (Fe), vitamin B12(Co), F-430(Ni), chlorophyll(Mg) and Moco(Mo). 

As the above discussion indicates, assigning mechanisms to simple anation reactions of transition metal complexes is not simple. The situation becomes even more difficult for a complex enzyme system containing a metal cofactor at an active site. Methods developed to study the kinetics of enzymatic reactions according to the Michaelis-Menten model will be discussed in Section 2.2.4. 

Characterization of structural, spectroscopic, and reactivity properties of model compounds—that is, metal cofactor small molecule analogs. 

Cleavage in the presence of histidine releases the oligonucleotide fragment containing the random sequence, and this is amplified by PCR and the cycle repeated. This selection procedure produced deoxyribozymes that require no metal cofactor but have a specific requirement for L-histidine, which is presumed (from pH-rate profiles) to act as a general... 

Probably the most effective use of XRF and TXRF continues to be in the analysis of samples of biological origin. For instance, TXRF has been used without a significant amount of sample preparation to determine the metal cofactors in enzyme complexes [86]. The protein content in a number of enzymes has been deduced through a TXRF of the sulfur content of the component methionine and cysteine [87]. It was found that for enzymes with low molecular weights and minor amounts of buffer components that a reliable determination of sulfur was possible. In other works, TXRF was used to determine trace elements in serum and homogenized brain samples [88], selenium and other trace elements in serum and urine [89], lead in whole human blood [90], and the Zn/Cu ratio in serum as a means to aid cancer diagnosis [91]. 

Cyanide can inhibit enzymatic activity by binding to the metallic cofactor in metalloenzymes. 

In biological systems, H-bond donors and acceptors are predominantly nitrogen and oxygen atoms. However, the n electrons of aromatic systems can also act as acceptors, and H-bonds involving sulfur groups or metallic cofactors are also known. The presence of individual H-bonds in biomacromolecular structures is usually derived from the spatial arrangement of the donor and acceptor groups once the structure of a molecule has been solved by diffractive or NMR techniques. More detailed information about H-bonds... 

He got a Habilitation a diriger les recherches in 2008 and he is now developing his own project that consists of the elaboration of new hybrid metalloprotein catalysts for selective oxidation reactions, by insertion of metal cofactors into xylanases. He then studies their peroxidase, catalase, and monooxygenase activities, in particular in the selective oxidation of sulfides, alkanes, and alkenes. 

Three other proteins with similar domain structure as that of FprA were reported in other bacteria (WasserfaUen et al. 1995 Gomes et al. 1997, 2000). The recombinant CthFprA and CthHrb, overexpressed in E. coli, were purified and characterized. Both FprA and Hrb were found to be present as dimers. Metal/cofactor analysis of the purified proteins revealed the presence of 2 mol each of iron and flavin (FMN) per mole dimer of Hrb and 4 mol of iron and 2 mol FMN per mole dimer of FprA. The EPR spectra of the purified proteins indicated that iron is present in a di-iron center in FprA and as a Fe(Cys)4 cluster in Hrb. 

In mid-2006, a search of the Jena Library for cytochrome b returned 197 entries, including those described in Sections 7.5 (cytochrome bef) and 7.6 (cytochrome bci). Cytochrome b contains the heme b subunit (heme protoporphyrin IX), the same metal cofactor contained in hemoglobin and myoglobin (see Figure 7.1). Other cytochrome b subunits occur in the cytochrome bef complex, also known as plasotquinol iplastocyanin reductase (EC 1.10.22) and... 

TABLE 7.6 Metal Cofactor Distances in cytochrome bci [2Fe-2S] Position and Mobility... 

To develop deoxyribozymes that make use of a non-metal cofactor rather than divalent metal ions for the cleavage of a ribonucleotide residue we performed an in vitro selection under conditions of low magnesium concentration, or... 

Manganese is an essential metal cofactor in enzymes that coyer the entire range of enzyme functionality. It is only possible to include some of the more important and better-characterized... 

Mammalian Cell Protease Inhibitor CocktaiL These should contain AEBSF, pepstatin A, E-64, bestatin, leupeptin, and aprotinin. (Metal chelators can be added to suppress the activity of calcium ion-dependent proteases such as calpain. Again, one must determine whether the protein or enzyme being purified does not require a divalent metal cofactor for stabihty or activity.)... 

---