Enzymes copper metalloenzymes

Copper Metalloenzymes and Metalloprotein The copper enzymes are mostly oxidases, that is enzymes which catalyse oxidations. Examples are ... 

FIGURE 2.40 The major pathway for taurine biosynthesis in the liver- First, cy teine is converted to cysteine sulfinic acid in an oxygen-requiring reaction catalyzed by an iron metalloeozyme- The second step, catalyzed by a vitamin B -requiring enzyme, is a decarboxylation reaction. The final step appears to be catalyzed by a copper metaLloenzyme and to require oxygen. Apparently, about one-fourth of the cysteine in the liver eventually is converted to taurine. 

Tyrosine monooxygenase uses biopterin as a cofactor. Biopterin is made in the body and is not a vitamin. Its structure resembles that of folic acid. Dopa decarboxylase is a vitamin B -requiring enzyme. Dopamine hydroxylase is a copper metalloenzyme. The active form of the enzyme contains copper in the reduced state (cuprous, Cu+). With each catalytic event, the copper is oxidized to the cupric state (Cu ). The enzyme uses ascorbic acid as a cofactor for converting the cupric copper back to cuprous copper. Thus, each catalytic event also results in the conversion of ascorbic acid to semidehydroascorbate. The semidehydroascorbate, perhaps by disproportionation, is converted to ascorbate and dehydroascorbate. The catalytic cycle of dopamine hydroxylase is shown in Figure 9,86. Dopamine hydroxylase, as well as the stored catecholamines, are located in special vesicles... 

Amidating enz5une is a copper metalloenzyme that requires both oxygen and ascorbic acid. One copper atom is boimd to three residues of histidine (his 107,108, and 172). The other copper atom is boimd to two residues of histidine (his 242 and 244), and to a residue of methionine (met 314). The enzyme is actually bifunctional, that is, it consists of two separate enzymes occurring in a single polypeptide chain. The two enzymes work, one after the other, to create the amidated polypeptide (Prigge et ah, 1997). 

The copper metalloenzymes are involved in oxygen-using reactions. These enzymes include cytochrome c oxidase (respiratory chain), lysyl oxidase (collagen synthesis), and dopamine [3-hydroxylase (neurotransmitter synthesis). Lysyl oxidase is a small protein with a molecular weight of 32 kDa. This enzyme contains an unusual modification, namely cross-linking between two different parts of its polypeptide chain. The cross-linked region consists of a structure called lysine tyrosylquinone (Klinman, 1996). Two amino acids are involved in this cross-linked structure, and these are Lys 314 and Tyr 349. Lysine tyrosylquinone is used as a cofactor and is necessary for the catalytic activity of the enzyme. Other copper metalloenzymes contain a related cofactor, namely 2,4,5-tiihydrox5q5henylalanine (topaquinone, TPQ). Serum amino oxidase is a copper metalloenzyme that contains TPQ. TPQ consists of a modified residue of phenylalanine. The copper in the active site of the enzyme occurs immediately adjacent to the TPQ cofactor. 

This short section attempts to bring together the range of metalloenzymes that are encountered in biodegradation and biotransformation. Fe is the most common component of enzymes, and is followed in freqnency by zinc and molybdennm, while some important enzymes contain nickel, copper, manganese, tnngsten, or vanadinm. 

The Franck-Condon principle states that there must be no movement of nuclei during an electronic transition therefore, the geometry of the species before and after electron transfer must be unchanged. Consequently, the active site geometry of a redox metalloenzyme must approach that of the appropriate transition state for the electronic transfer. Every known copper enzyme has multiple possible copper oxidation states at its active site, and these are necessary for the enzyme s function. 

Superoxide dismutase enzymes are functional dimers of molecular weight (Mr) of approximately 32 kDa. The enzymes contain one copper ion and one zinc ion per subunit. Superoxide dismutase (SOD) metalloenzymes function to disproportionate the biologically harmful superoxide ion-radical according to the following reaction ... 

Oxidative coupling polymerization provides great utility for the synthesis of high-performance polymers. Oxidative polymerization is also observed in vivo as important biosynthetic processes that, when catalyzed by metalloenzymes, proceed smoothly under an air atmosphere at room temperature. For example, lignin, which composes 30% of wood tissue, is produced by the oxidative polymerization of coniferyl alcohol catalyzed by laccase, an enzyme containing a copper complex as a reactive center. Tyrosine is an a-amino acid and is oxidatively polymerized by tyrosinase (Cu enzyme) to melanin, the black pigment in animals. These reactions proceed efficiently at room temperature in the presence of 02 by means of catalysis by metalloenzymes. Oxidative polymerization is observed in vivo as an important biosynthetic process that proceeds efficiently by oxidases. 

Active Sites, Copper Proteins Oxidases, Copper Proteins with Type 1 Sites, Copper Proteins with Type 2 Sites, Copper Enzymes in Denitrification, Iron-Sulfur Models of Protein Active Sites, Iron-Sulfur Proteins Nickel Enzymes Cofactors and Nickel Models of Protein Active Sites). However, since many metalloenzymes have been found or postulated to incorporate metal-sulfur bonding, it is appropriate that a very short sununary be included here. 

Cobalt has recently been used as an ESR active substitute in zinc metalloenzymes. Whilst liquid helium temperatures may be needed and theoretical aspects of the spectra are not yet as well understood, cobalt has two important advantages over copper as a metal substitute, namely that many cobalt derivatives show some enzymic activity (e.g. cobalt in carbonic anhydrase, alkaline phosphatase and superoxide dismutase) and that g values and hyperfine splitting are more sensitive to ligand environment, particularly when low spin. ESR data have been reported for cobalt substituted thermolysin, carboxypeptidase A, procarboxypeptidase A and alkaline phosphatase [51]. These are all high spin complexes. Cobalt carbonic anhydrase has been prepared and reacted with cyanide [52]. In... 

Catabolism of tyrosine and tryptophan begins with oxygen-requiring steps. The tyrosine catabolic pathway, shown at the end of this chapter, results in the formation of fumaric acid and acetoaceticacid, Iryptophan catabolism commences with the reaction catalyzed by tryptophan-2,3-dioxygenase. This enzyme catalyzes conversion of the amino acid to N-formyl-kynurenine The enzyme requires iron and copper and thus is a metalloenzyme. The final products of the pathway are acetoacetyl-CoA, acetyl-Co A, formic add, four molecules of carbon dioxide, and two ammonium ions One of the intermediates of tryptophan catabolism, a-amino-P-carboxyrnuconic-6-semialdchydc, can be diverted from complete oxidation, and used for the synthesis of NAD (see Niacin in Chapter 9). 

Zinc is used by a great number of enzymes and proteins, whereas copper seems to be limited to only a few fimctions. The most thoroughly studied zinc metalloenzymes of mammals are carbonic anhydrase, carboxypeptidase A and related pep-... 

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