Chemistry and Biology of Proteins - F. Haurowitz 1953

Conjugated Proteins
Protein-metal compounds

Ferritin, a crystalline protein containing over 20% iron, was isolated by Laufberger from The Liver and Spleen [86, 87]. The iron content of ferritin is such that there is almost one iron atom for every peptide bond. The iron in ferritin is present as a trivalent cation. When ferritin is treated with reducing agents, such as Na₂S₂О₄, its molecule splits into iron-free apoferritin and ferrous iron salts [89]. Apoferritin has been obtained in crystalline form [89], and its molecular weight was found to be 465,000 [90]. The prosthetic group of ferritin is an inorganic ferric iron compound of the following composition: (FeO ∙ OH)₈(FeO ∙ ОРО₃Н₂) [91]. Ferritin presumably acts as an iron store in the Organism. When ferric salts are administered intravenously, they are incorporated into ferritin [92].

Protein-metal complexes have also been isolated from Blood Plasma. When blood plasma is subjected to fractional precipitation with ethyl alcohol, metal-containing pi-globulins can be detected in fraction IV [62] (see p. 175). These Metalloproteins are a mixture of copper-containing Proteins and iron-containing proteins (siderophilins). The iron-containing siderophilins exhibit the same pink color as iron complexes with hydroxamic acids. It is highly probable that in siderophilin, iron is bound to the —N(OH)CO— group present in the protein molecule [93]. The Molecular Weight of siderophilin is 90,000. Each protein molecule in siderophilin is capable of binding two iron atoms.

Hemerythrin, a protein containing 1.01% iron, serves as the respiratory pigment of Sipunculus nudus, Phascolosoma vulgare, Phascolosoma elongatum, and other marine worms [95, 96]. The molecular weight of hemerythrin is 66,000. Despite containing 3 iron atoms per molecule, each molecule binds only 1 oxygen molecule [97]. Hemerythrin is a ferrous iron compound devoid of Porphyrins and hemins. It has a yellowish color that turns brown upon storage due to The oxidation of iron [96]. The prosthetic group of hemerythrin is hemoferrin. The protein moiety of hemerythrin is a globulin insoluble in salt-free Water; its isoelectric point lies at pH 5.85 [97]. As for hemoferrin, it is likely a polypeptide bound to iron.

Hemocyanin [98], the respiratory pigment of many crustaceans and Mollusks [99], is a copper-protein complex. In its properties, hemocyanin resembles globulins. It can be obtained in crystalline form by ammonium sulfate precipitation or dialysis [100, 101]. The isoelectric point of various hemocyanins lies between pH 4.5 and 5.0. Upon binding oxygen, hemocyanin converts into an oxygenated form characterized by a blue color and containing 1 oxygen molecule per 2 copper atoms [102]. As the pressure drops, oxygen is released from hemocyanin, and its blue color shifts to yellow. Copper in hemocyanin is presumably in the cuprous state [103]. Treatment of hemocyanin with hydrocyanic or Hydrochloric acid results in the dissociation of copper [103, 104]. Hemocyanin is a species-specific protein, as evidenced by the fact that hemocyanin crystals obtained from different animal species differ in shape [105]. Hemocyanins isolated from the lobster, crayfish, and other Arthropods contain 0.17–0.18% copper, whereas hemocyanins from the snail Helix pomatia, Loligo, and other mollusks contain 0.24–0.26% copper [106, 107].

To date, all attempts to isolate the prosthetic group of hemocyanins have failed. Enzymatic or alkaline treatment of hemocyanins has yielded copper-containing Polypeptides; however, it remains unclear whether these compounds pre-exist in the hemocyanin molecule or are formed secondarily [108, 109].

The molecular weight of hemocyanins is exceptionally high. Ultracentrifugation measurements have yielded values ranging from 500,000 to 10,000,000—the highest ever recorded for proteins [110]. Values of the same order of magnitude were obtained using osmometry and light scattering Methods (see p. 61). Cytology/cytology/93.html">ELECTRON MICROGRAPHS OF hemocyanin from the horseshoe crab Limulus polyphemus showed that the molecules are nearly spherical particles with an average diameter of about 200 Å [111]. Similar studies on hemocyanin from Busycon caniculatum revealed that its molecule consists of parallel bundles, each containing four rod-like subunits [112].

Under the action of concentrated urea solutions, hemocyanins undergo disaggregation to form smaller subunits [113]. Reversible disaggregation of hemocyanin molecules is also induced by acids and alkalis (see p. 150). This disaggregation occurs under conditions usually considered too mild to cause Denaturation [114]. The dissociation products share the same shape as the hemocyanin molecule and can reassociate to form native hemocyanin. Consequently, whether this process constitutes denaturation is a matter of terminology rather than substance [114].

The equivalent weight of mollusk hemocyanin per copper atom is 25,000, and that of arthropod hemocyanin is 37,000. These data indicate that each hemocyanin molecule contains A large number of copper atoms and is capable of binding numerous oxygen molecules.

Copper has also been found in the Enzymes laccase, tyrosinase, and ascorbic acid oxidase [98], as well as in two proteins isolated from blood plasma and bovine liver: hemocuprein and hepatocuprein. Both proteins contain 0.34% copper [115]. The biological function of these proteins remains unknown.