Histidine ligands tyrosine

Fe atoms. It had been anticipated that the c-type cytochrome center would have His/Met coordination, but His/His is observed. The former is the more usual coordination, especially at the high potential end E° > +200 mV) ofthe typical bacterial electron transfer chain to which the nitrite reductase is connected (Fig. 2) (7). The second curious feature is that the di heme iron is also six-coordinate thus, the enzyme does not offer a substrate-binding site at either heme. In addition to an expected axial histidine ligand there was an axial tyrosine (residue 25) ligand to the d heme (Fig. 4a). Each monomer is organized into two domains. 

As with any metalloprotein, the chemical and physical properties of the metal ion in cytochromes are determined by the both the primary and secondary coordination spheres (58-60). The primary coordination sphere has two components, the heme macrocycle and the axial ligands, which directly affect the bound metal ion. The pyrrole nitrogen donors of the heme macrocycle that are influenced by the substitutents on the heme periphery establish the base heme properties. These properties are directly modulated by the number and type of axial ligands derived from the protein amino acids. Typical heme proteins utilize histidine, methionine, tyrosinate, and cysteinate ligands to affect five or six coordination at the metal center. 

II. REPLACEMENT OF THE PROXIMAL HISTIDINE LIGAND IN Mb INTO CYSTEINE OR TYROSINE PROTEIN MIMICS OF CYTOCHROME... 

The involvement of tyrosine and histidine ligands had been expected from spectroscopy the electronic spectral band at —465 nm, which is responsible for the orange-red color, is characteristic of iron-phenolate interaction (89, 90), and both EPR (91, 92) and NMR (93) studies indicated metal-histidine coordination. The presence of the Asp ligand... 

Vanadium(IV) complexes with two dipeptides glycyl-tyrosine and glycyl-phenylalanine (Gly-Tyr and Gly-Phe, respectively) and their oxovanadium(lV) and (V) complexes have been observed.382, 01 The complex formed with Gly-Tyr showed coordination by the peptide backbone moieties, and no interaction from the distal phenolic hydroxy group.601 A histidine-tyrosine derivatized peptide was shown to complex Viv in a pentadentate manner (120).564 The adduct between VlV and a model of the active-site peptide of protein tyrosine phosphatases has been spectroscopically characterized.602 Similar characterization of in serum suggests that protein complexes form, and that both Viv and Vv can exist as complexes with transferrin and albumin.603 ESEEM has been used to characterize the complexes of apoferritin with V02+ and suggests that the carboxylates, one water molecule, and one histidine ligand fill the coordination sites of the vanadium.604... 

Horseradish peroxidase, which is the most fully characterized of the heme(Fe )-centered enzymes for the activation of HOOH, has an axial histidine ligand this also is the case for myeloperoxidase (uses HOOH and CF to form HOCl). In contrast, the catalase protein (catalyzes the harmless destruction of HOOH into O2 and H2O) has an axial tyrosine ligand in place of the histidine... 

Structure of the active center. The active centers of this dimeric enzyme are so well embedded into its protein structure that they are inaccessible to the solvent. The two centers are situated approximately 30 A apart from each other but connected by /3-strands. The active center consists of a type 2 copper center and a cofactor. Sequence comparisons have established that the residues His 8, His 246, and His 357 coordinate the copper ions in both yeast and plants (e.g., lentil seeds) [120,122]. The participating cofactor is typical for amine oxidases, diamine oxidases, and lysyl oxidases but has not yet been found in any other protein - 2,4,5-trihydroxy-phenylalanine quinone [123, 124] (also known as TOPA-quinone, TPQ or 6-hydroxy-DOPA quinone), an internal cofactor which is created by post-translational modification of the tyrosine in position 387 [120]. The consensus sequence of the amino acids neighboring the TOPA cofactor are conserved in all known amine oxidases - Asn-TOPA-Asp/Glu [113,120, 123,125-127]. The positions of the histidine ligands relative to TOPA quinone are conserved in all known amine oxidases as well. The chain lengths of the amine oxidase monomers vary according to the organism of origin 692 residues in yeast [128], 762 in bovine serum amine oxidase [128,129] and 569 in the enzyme from lentil seeds [120,130]. 

C) Stereoscopic view of the Bchl b molecules along the local twofold axis. The special pair (Djy[, Dg) is in the center with its tetrapyrrole rings almost perpendicular to the plane of the paper the monomeric chlorophylls are labeled Bg and B. The four histidine ligands to the magnesium ions of the bacteriochlorophylls as well as two tyrosines (Ml 95 and M208) and three water molecules (large circles) are also shown. From Deisenhofer et aJ with permission. 

Galactose oxidase (GO) catalyses the two-electron oxidation of primary alcohols to aldehydes. It contains a single type II copper centre. The enzyme employs the metal and a protein radical cofactor to effect the chemistry. The crystal structure shows a square pyramidal five-coordinate copper site with the metal coordinated by two histidines, two tyrosines and a water or acetate ligand. The equatorial tyrosine, Tyr272, has an interesting crosslink to a cysteine group ortho to the tyrosine oxygen. 

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