Porphyrins Enzymes containing

The structure of cobalamin is more complex than that of folic acid (Figure 15.2 and 15.3). At its heart is a porphyrin ring containing the metal ion cobalt at its centre. In catalytic reactions the cobalt ion forms a bond with the one-carbon group, which is then transferred from one compound to another. Vitamin B12 is the prosthetic group of only two enzymes, methylmalonyl-CoAmutase and methionine synthase. The latter enzyme is particularly important, as it is essential for the synthesis of nucleotides which indicates the importance of vitamin B12 in maintenance of good health. 

Nickel porphyrins are of interest because of their occurrance in coal, shale, and petroleum deposits (1) and because of their key role in biological conversion of COg to methane (2--5). The enzyme methylreductase is the nickel-tetrapyrrole-containing enzyme that catalyses the final step, and possibly other steps, in the 6-electron reduction of CO2 to methane (6). The active site of the methylreductase enzyme contains a nickel-sirochlorin derivative called F 3q (6-8). Understanding the involvement of in methane... 

The prostnetic group of these enzymes contains an iron(III)porphyrin complex and the active oxidant can be formally regarded as a high-valent oxoiron(V)-... 

Catalase is a haem-type enzyme containing an iron atom in a porphyrin ring. A two step reaction removes the dangerous reactive oxygen species through the oxidation and subsequent reduction of the central iron atom ... 

Another class of compounds that readily combine reversibly with molecular oxygen are the metal phthalocyanines.188,189 A number of oxidative enzymes contain the metal-porphyrin structure as the prosthetic group. In view of the... 

Metal phthalocyanine complexes are also frequently used as autoxidation catalysts (see Section II.B.2). They have generally been found to be more active than the corresponding stearates or acetylacetonates. Thus, Uri145 compared the catalytic activity of a series of transition metal stearates with the corresponding metal phthalocyanines in the autoxidation of methyl linoleate. The phthalocyanine complexes afforded faster rates of oxidation. In addition, the phthalocyanine ligand is stable and is not easily destroyed under autoxidizing conditions. Interest in metal phthalocyanine catalysts has also been stimulated by their resemblance to the metal-porphyrin structures contained in many oxidative enzymes (see Sections II.B.2 and V). 

This chapter is divided into two sections. Section 6.1 is concerned with applications of Raman spectroscopy to biochemistry. Related topics to this section are found in Section 3.3.3 of Chapter 3 (SER spectra of dipeptides) and Section 4.1.2 of Chapter 4 (Raman (RR) spectra of peptides, proteins, porphyrins, enzymes and nucleic acids), Section 6.2 describes medical applications of Raman spectroscopy as analytical and diagnostic tools. In contrast to biochemical samples discussed in the former section, medical samples in the latter section contain a number of components such as proteins, nucleic acids, carbohydrates and lipids, etc. Thus, Raman spectra of medical samples are much more complex and must be interpreted with caution. 

There are two possible modes of dissociation of the porphyrin from the enzyme active site. If solution samples are used, the dissociated porphyrin is blotted or rinsed away from the surface with the excess analyte solution. In this case, the absorbance intensity due to the porphyrin-enzyme complex is reduced in the waveguide spectrum by the percentage of complexes impacted (Figure 12.5, Panel D). The difference spectrum calculated as pre-exposure minus postexposure shows a loss in absorbance only at the characteristic peak for that particular porphyrin-enzyme interaction (Figure 12.6). Thus, it is possible to discriminate the binding of analyte(s) to different enzymes co-immobilized on a mixed bed surface containing different enzyme-porphyrin complexes. 

Catalase (1.11.1.6) is present (hydrogen peroxide decomposed by an enzyme containing a heme or porphyrin structural group). 

Cyanide is another widely known poison. Cyanide (CN ) usually comes in the form of potassium cyanide (KCN), sodium cyanide (NaCN), or hydrogen cyanide (HCN). The first two are white solid, and hydrogen cyanide is a pungent-smelling gas. Cyanide binds very strongly with a metal ion such as Fe(II), Fe(III), Cu(II), Zn(II), and many others. These ions constitute important portions, that is, the active sites of many enzymes and proteins (Chap. 6). When a cyanide ion binds with a metal ion, the enzyme s function is disrupted. Cyanide that enters into our body almost indiscriminately binds to any metal ions and disrupts their functions, but an especially sensitive place is the last enzyme in the whole series of respiratory chain. The enzyme contains both iron (embedded in the porphyrin as in hemoglobin) and copper. Cyanide binds to both iron and copper in this enzyme, and stops its function, the last step of respiration. What is the result It stops respiration hence it stops the production of bodily energy (ATP). As we talked about in Chap. 3, we need to continuously produce ATP. Otherwise, we would die. 

L-Tryptophan 2,3-dioxygenase (TPO), which catalyzes the ring cleavage of tryptophan to N-formylkynurenine [eq. (14)] [156], is a heme enzyme containing iron porphyrin [157] and plays a key role in the L-tryptophan metabolic pathway. Kotake and Masayama first... 

The abihty of iron to exist in two stable oxidation states, ie, the ferrous, Fe ", and ferric, Fe ", states in aqueous solutions, is important to the role of iron as a biocatalyst (79) (see Iron compounds). Although the cytochromes of the electron-transport chain contain porphyrins like hemoglobin and myoglobin, the iron ions therein are involved in oxidation—reduction reactions (78). Catalase is a tetramer containing four atoms of iron peroxidase is a monomer having one atom of iron. The iron in these enzymes also undergoes oxidation and reduction (80). 

Proteins may consist exclusively of a polymeric chain of amino acids these are the simple proteins. Quite often some other chemical component is covalendy bonded to the amino acid chain. Glycoproteins and Hpoproteins contain sugar and Hpid components, respectively. Porphyrins are frequently associated with proteins, eg, in hemoglobin. Proteins bound to other chemical components are called conjugated proteins. Most enzymes are conjugated proteins. 

A summary of the steps in the biosynthesis of the porphyrin derivatives from PBG is given in Figure 32-8. The last three enzymes in the pathway and ALA synthase are located in the mitochondrion, whereas the other enzymes are cytosolic. Both erythroid and non-erythroid ( housekeeping ) forms of the first four enzymes are found. Heme biosynthesis occurs in most mammalian cells with the exception of mature erythrocytes, which do not contain mitochondria. However,... 

Cytochrome c can easily be extracted from tissue particles by dilute salt solutions. It was isolated by Keilin and Hartree in 1930 and shown to contain a porphyrin ring structure. In 1933 Zeilen and Reuter established that cytochrome c was a heme (iron-porphyrin) protein. Slightly different forms of cytochrome a were distinguished in yeast and bacteria by Keilin in 1934 and the different properties of cytochrome a and a3 by Tamiya et al. in 1937. The identity of cytochrome 03, the enzyme which activates oxygen with Warburg s atmungsferment, was proposed by Keilin in 1939. Cytochrome a/a3 was renamed cytochrome oxidase by Malcolm Dixon (1939). The oxidation route then offered was ... 

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