Heme proteins with cofactors

The heme moiety provides de novo designed heme proteins with an intrinsic and spectroscopically rich probe. The interaction of the amide bonds of the peptide or protein with the heme macrocycle provides for an induced circular dichroism spectrum indicative of protein-cofactor interactions. The strong optical properties of the heme macrocycle also make it suitable for resonance Raman spectroscopy. Aside from the heme macrocycle, the encapsulated metal ion itself provides a spectroscopic probe into its electronic structure via EPR spectroscopy and electrochemistry. These spectroscopic and electrochemical tools provide a strong quantitative base for the detailed evaluation of the relative successes of de novo heme proteins. 

Molybdenum is a component of at least three enzymes aldehyde oxidase, xanthine dehydrogenase, and sulfite oxidase. The first two contain FAD, whereas the last is a heme protein similar to cytochrome c. Xanthine dehydrogenase can also act as an oxidase, that is, it can use 02 as an electron acceptor. Physiologically, however, it uses NAD+ as an electron acceptor when it converts hypoxanthine to xanthine and the latter to uric acid (see Chapter 10). Aldehyde and sulfite oxidases are true oxidases physiologically they both use 02 as an electron acceptor. Molybdenum in all three enzymes is associated with a pterinlike cofactor whose structure is shown in Figure 6.11. The Mo cofactor cannot be... 

How can a simple cofactor, such as heme, give rise to a wide spectrum of protein functionalities While the Fe(III)/Fe(II) couple has a standard redox potential of 0.77 V, when complexed with a protoporphyrin to form free heme, it may decrease to —0.115 V [3-5]. When heme is introduced into a protein matrix, redox potential shows an impressive variation of around 1 V. The electrochemical data for structurally characterized heme proteins involved in electron transfer and redox catalysis has been compiled at the Heme Protein Database (HPD, http //heme.chem. columbia.edu/heme) [6]. The database comprises not only peroxidases but also catalases, oxidases, monooxygenases, and cytochromes. From b-type heme with histidine-tyrosine ligation (E° = 0.55 V) to c-type heme with histidine-methionine... 

FIGURE 2. A Subunit of S. cerevisiae Flavocytochrome bj The protein is shown as a ribbon diagram with the heme and flavin cofactors in a stick representation. The two domains are clearly delineated. Cyt, cytochrome domain Flav, flavin domain H, interdomain hinge peptide C, C-terminal tail. 

Cobalt B Enzymes Coenzymes Cytochrome Oxidase Iron Heme Proteins Electron Transport Iron Proteins with Dinuclear Active Sites Iron Proteins with Mononuclear Active Sites Iron-Sulfur Models of Protein Active Sites Metallocenter Biosynthesis Assembly. Metalloregulation Molybdenum MPT-containing Enzymes Nickel Enzymes Cofactors Nitrogenase Catalysis Assembly Photosynthesis Tungsten Proteins Vanadium in Biology Zinc DNA-binding Proteins. 

Calcium-binding Proteins Copper Enzymes in Denitrification Copper Proteins with Type 1 Sites Copper Proteins with Type 2 Sites Iron Heme Proteins Electron Transport Iron-Sulfin Proteins Metal-mediated Protein Modification Metallochaperones Metal Ion Homeostasis Molybdenum MPT-containing Enzymes Nickel Enzymes Cofactors, Nitrogenase Catalysis Assembly Zinc Enzymes. 

Electrodes functionalized with monolayers of enzyme cofactors (e.g., NAD+ mono-layers) or redox proteins (e.g., heme proteins or heme polypeptides) demonstrate... 

Numerous cellular enzymes and coenzymes require iron, either as an integral part of the molecule or as a cofactor. Notable are the peroxidases and cytochromes, all of which, like Hb, are heme proteins. Other enzymes, such as aconitase and ferredoxin, have iron that is coordinated with sulfur in a so-called iron-sulfiir cluster. Nearly half of the enzymes of the Krehs cycle contain iron. These enzymes and coenzymes, which appear in aU nucleated cells of the body, are referred to collectively as the tissue iron compartment. The tissue iron compartment normally amounts to approximately 8 mg. Although a small compartment, it is metabolicaUy critical. Some iron enzyme activities dimmish early in the course of iron deficiency. ... 

For systems that are naturally and reversibly photosensitive such as the carbon monoxide complexes of heme proteins [18,. 30, 31], initiation by a light pulse is possible. By preparation of inert but photoactivable reactant or cofactor precursors such as caged ATP [32-34], photosensitivity may be conferred on otherwise photoinert systems [21], thus extending the generality of this approach. This approach was first combined with x-ray monitoring by Blasie [35, 36], in studies of oriented multilayers containing the Ca2+-ATPase from sarcoplasmic reticulum. 

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