Hemocyanin function

Much study was devoted to the capacity of hemocyanin to combine reversibly with oxygen (see Redfield, 1934, for early literature). While in cephalopoda many data show that hemocyanin functions as the oxygen carrier in the blood (Henze, 1905 Redfield and Goodkind, 1929 Winterstein, 1909 Wolvekamp, 1938), in gastropods—with the exception of Busy con canaliculatum—and anthropods such a function was not... 

This discussion of copper-containing enzymes has focused on structure and function information for Type I blue copper proteins azurin and plastocyanin, Type III hemocyanin, and Type II superoxide dismutase s structure and mechanism of activity. Information on spectral properties for some metalloproteins and their model compounds has been included in Tables 5.2, 5.3, and 5.7. One model system for Type I copper proteins39 and one for Type II centers40 have been discussed. Many others can be found in the literature. A more complete discussion, including mechanistic detail, about hemocyanin and tyrosinase model systems has been included. Models for the blue copper oxidases laccase and ascorbate oxidases have not been discussed. Students are referred to the references listed in the reference section for discussion of some other model systems. Many more are to be found in literature searches.50... 

Our biomimetic investigations have focused on the metalloproteins hemocyanin (He) (11-17) and tyrosinase (11,12,14,16,18,29), which contain two copper ions in their active center. The function of hemocyanin is to bind and transport dioxygen in the hemolymph of molluscs and arthropods. Studies employing EXAFS spectroscopy have shown that in the deoxy form, two (19-21) or three (13,21) imidazole units fiom protein histidine residues coordinate to each cuprous ion. Upon addition of O2 to give oxy-Hc, considerable changes take place in the coordination sphere giving rise to tetragonally coordinated Cu(II) ions... 

For keyhole limpet hemocyanine (KLH) both antibody responses and delayed type hypersensitivity (DTH) reactions can be determined [43—45]. In addition several infectious models, including bacterial, viral and parasitic infections may be used to challenge the immune system [18,46]. As survival and eradication of the infections is the primary function of the immune system, these models provide direct information on the functional status of the immune system. Direct immunotoxic compounds will induce immunosuppression and thus an increase in infection rate and/or severity of the infection. The number of infectious agents (bacteria, parasites, or viral colonyforming units), increased morbidity and mortality are indications for an immunotoxic effect. Also a reduction in specific antibody levels in animals treated with the test compound compared to nontreated controls indicates immunosuppression. 

Hemocyanin [30,31], tyrosinase [32] and catechol oxidase (2) [33] comprise this class of proteins. Their active sites are very similar and contain a dicopper core in which both Cu ions are ligated by three N-bound histidine residues. All three proteins are capable of binding dioxygen reversibly at ambient conditions. However, whereas hemocyanin is responsible for O2 transport in certain mollusks and arthropods, catechol oxidase and tyrosinase are enzymes that have vital catalytic functions in a variety of natural systems, namely the oxidation of phenolic substrates to catechols (Scheme 1) (tyrosinase) and the oxidation of catechols to o-quinones (tyrosinase and catechol oxidase). Antiferromagnetic coupling of the two Cu ions in the oxy state of these metalloproteins leads to ESR-silent behavior. Structural insight from X-ray crystallography is now available for all three enzymes, but details... 

Where are the following found and what are their functions Gamma globulin, hemocyanin, pepsin, glucagon, ferritin, phosphorylase. 

Tire most studied of all copper-containing oxidases is cytochrome c oxidase of mitochondria. This multisubunit membrane-embedded enzyme accepts four electrons from cytochrome c and uses them to reduce 02 to 2 H20. It is also a proton pump. Its structure and functions are considered in Chapter 18. However, it is appropriate to mention here that the essential catalytic centers consist of two molecules of heme a (a and a3) and three Cu+ ions. In the fully oxidized enzyme two metal centers, one Cu2+ (of the two-copper center CuA) and one Fe3+ (heme a), can be detected by EPR spectroscopy. The other Cu2+ (CuB) and heme a3 exist as an EPR-silent exchange-coupled pair just as do the two copper ions of hemocyanin and of other type 3 binuclear copper centers. 

However, the molecular weight, for example, may vary from 15,000 (hen egg white lysozyme) to 2,000,000 (keyhole limpet hemocyanin) daltons. Protein antigens function as the most potent immunogens, and polysaccharide antigens rank second. For cell-mediated immunity, only proteins serve as immunogens. Certain nucleic acid types such as Z-DNA and other molecules can also stimulate antibody production. 

Takano, Y., Yamaguchi, K., Hybrid density functional study of ligand coordination effects on the magnetic couplings and the Dioxygen binding of the models of hemocyanin. Int. J. Quantum Chem. 2007, 107, 3103-3119. 

As the focus of this review is on copper-dioxygen chemistry, we shall briefly summarize major aspects of the active site chemistry of those proteins involved in 02 processing. The active site structure and chemistry of hemocyanin (He, 02 carrier) and tyrosinase (Tyr, monooxygenase) will be emphasized, since the chemical studies described herein are most relevant to their function. The major classes of these proteins and their origins, primary functions, and leading references are provided in Table 1. Other classes of copper proteins not included here are blue electron carriers [13], copper-thiolate proteins (metallothioneines) [17], and NO reductases (e.g., nitrite [NIR] [18] or nitrous oxide [19]). 

J. V. Bannister, ed., Structure and Function of Hemocyanin. Springer-Verlag, Berlin and New York, 1977. 

Although the information obtained through the investigation of the purple hemocyanin does not necessarily clarify the structure-function relationship in the environment of the copper ions in native hemocyanin, it would more or less help our understanding of the active site of this important copper protein. 

The goal of our research on the multicopper oxidases has been to determine the spectral features of the type 3 (and type 2) centers, to use these spectral features to define geometric and electronic structural differences relative to hemocyanin and tyrosinase, and to understand how these structural differences contribute to their variation in biological function. The hemocyanins and tyrosinases reversibly bind and activate dioxygen whereas the multicopper oxidases catalyze its four-electron reduction to water. 

Dnring the late 1970s, Solomon and collaborators prepared and extensively characterized a series of derivatives leading to models of the copper active-site structure and it function that are snmmarized in several articles. " While decades ago the end-on coordination was favored when interpreting the spectra, now great effort is applied to interpreting the spectra on the basis of the results of the recently resolved X-ray stracture, " which clearly show that dioxygen is bound in a side-on coordination between the two copper atoms. Several other contributions in this book will focus this point see Copper Hemocyanin/Tyrosinase Models). 

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