Heme modulation

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. 

In contrast to the peptide systems, there exists a somewhat more robust literature on the electrochemistry of hemes in the larger protein systems that provides a wealth of insight into how proteins modulate the reduction potential of heme cofactors. A variety of groups have reported the reduction potential of various hemes in four-a-helix bimdle architectures, which is beginning to reveal the fundamental factors that control heme reduction potentials. 

The first general principle is that the type of metalloporphyrin incorporated into a designed protein or natural cytochrome offers a method to adjust the reduction potential of the heme. Sharp et al. (149) have demonstrated that a synthetic heme, l-methyl-2-oxomesoheme XIII, incorporated into a designed heme protein has a reduction potential 90 mV higher than the same protein with heme b. Additionally, Gib-ney et al. (148) have illustrated that heme a has a reduction potential 160 mV higher than heme b in the identical protein scaffold. This heme-dependent effect provides protein designers with a predictable modulation of the heme reduction potential in a synthetic protein. 

Local charge compensation of the formally charged Fe(III) heme, as discussed more fully in a later section, demonstrates a significant modulation (s 210 mV) of the heme reduction potential in a designed heme protein. This scaffold-dependent effect has been shown to be additive to the heme-dependent effect of porphyrin peripheral architecture to demonstrate the modulation of a designed heme protein reduction potential by 450 mV using a single maquette scaffold. 

The development status of these molecules is not known. It will be interesting to note whether any differences emerge from the CAAX competitive versus FPP competitive molecules as more data become available for these compounds. Since FPP itself contributes to the CAAX peptide binding pocket, the interaction of FPP competitive FTIs with CAAX peptide competitive FTIs will be of interest. The selectivity of FPP competitive FTIs for the FTase pathway versus other biochemical pathways utilizing FPP, such as ubiquinone synthesis and the heme farnesyltransferase, has also not been reported. These other FPP reactions have important roles in mitochondrial function, which presents some risk for adverse events or possibly opportimities for modulating early apoptotic events. 

Similarly, this amphiphilic polymer micelle was also used to dismpt the complex between cytochrome c (Cc) and cytochrome c peroxidase (CcP Sandanaraj, Bayraktar et al. 2007). In this case, we found that the polymer modulates the redox properties of the protein upon binding. The polymer binding exposes the heme cofactor of the protein, which is buried in the protein and alters the coordination environment of the metal. The exposure of heme was confirmed by UV-vis, CD spectroscopy, fluorescence spectroscopy, and electrochemical kinetic smdies. The rate constant of electron transfer (fc°) increased by 3 orders of magnimde for the protein-polymer complex compared to protein alone. To establish that the polymer micelle is capable of disrupting the Cc-CcP complex, the polymer micelle was added to the preformed Cc-CcP complex. The observed for this complex was the same as that of the Cc-polymer complex, which confirms that the polymer micelle is indeed capable of disrupting the Cc-CcP complex. 

Craven, P. A., DeRuberts, F. R., and Pratt, D. W. (1979). Electron spin resonance study of the role of NO catalase in the activation of guanylate cyclase by NaN, and NH2OH. Modulation of enzyme responses by heme proteins and their nitrosyl derivatives.). Biol. Chem. 254, 8213-8222. 

As well as induction of the synthesis of the apoprotein portion of cytochrome P-450, there is also induction of the synthesis of the heme portion. Clearly, it is also necessary to have an increased amount of heme if there is an increase in the amount of the enzyme apoprotein being synthesized. Thus, the rate-limiting step in heme synthesis, the enzyme 5-aminolaevulinate synthetase, is inducible by both phenobarbital and TCDD. This is the result of transcriptional activation of the gene, which codes for the S-aminolaevulinate synthetase. It may be that the decrease in the heme pool, which results from incorporation of heme into the newly synthesized apoprotein, leads to derepression of the gene and hence increased mRNA synthesis. The gene repression could be heme-mediated, or heme may modulate P-450 genes. 

Figure 2 shows the results of the IF analysis, revealing strong modulation at 50 on1 and other low frequency modes, similar to observations by Champion et al. [10][11] For comparison, the IF was also calculated using a sliding window FFT method, yielding similar results as shown in Figure 2. The observation of these low frequency modes is perhaps the most important result of the study. The 50 cm 1 mode in particular has been identified with the doming motion of the heme [12] and the lower frequencies can be correlated to the globin...

Figure 23-31 (A) Stereoscopic ribbon drawing of the photosynthetic reaction center proteins of Rhodopseudomonas viridis. Bound chromophores are drawn as wire models. The H subunit is at the bottom the L and M subunits are in the center. The upper globule is the cytochrome c. The view is toward the flat side of the L, M module with the L subunit toward the observer. (B) Stereo view of only the bound chromophores. The four heme groups Hel-He4, the bacteriochlorophylls (Bchl) and bacteriopheophytins (BPh), the quinones QA and QB/ and iron (Fe) are shown. The four hemes of the cytochrome are not shown in...

Fig. 19. Proton. NMR spectrum at 220 He of ferricytochrome c. Different scales were used for the different spectral regions. The high field line at +23.2 ppm was observed as an inversed resonance of the center-band of the spectrum (the HR-220 operates with a 104 cps field modulation, and one usually observes the first upfield side-band. With the occurrence of large hyperfine shifts the center band and the side bands sometimes overlap). Heme c and the axial ligands of the heme iron are shown at the bottom. (Reproduced from ref. (777))...

Another example, which demonstrates the modulation of the peroxidase activity of Mb by modification of the heme-propionate side chains, has been proposed by Casella and co-workers (75). They prepared a Mb reconstituted with peptide-linked hemin such as protohemin-L-arginyl-L-alanine or protohe-min-L-histidine methyl ester as shown in Fig. 20. The peroxidase activity toward small substrate oxidations by the reconstituted Mb slightly increased the fccat... 

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