Cysteine flavins

All the complexes consist of several subunits (Table 2) complex I has a flavin mononucleotide (FMN) prosthetic group and complex II a flavin adenine dinucleotide (FAD) prosthetic group. Complexes I, II, and III contain iron-sulphur (FeS) centers. These centers contain either two, three, or four Fe atoms linked to the sulphydryl groups of peptide cysteine residues and they also contain acid-labile sulphur atoms. Each center can accept or donate reversibly a single electron. 

Formate dehydrogenases are a diverse group of enzymes found in both prokaryotes and eukaryotes, capable of converting formate to CO2. Formate dehydrogenases from anaerobic microorganisms are, in most cases, Mo- or W- containing iron-sulfur proteins and additionally flavin or hemes. Selenium cysteine is a Mo- ligand. 

A still more complicated reaction is the chemiluminescent oxidation of sodium hydrogen sulfide, cysteine, and gluthathione by oxygen in the presence of heavy metal catalysts, especially copper ions 60>. When copper is used in the form of the tetrammin complex Cu(NH3) +, the chemiluminescence is due to excited-singlet oxygen when the catalyst is copper flavin mononucleotide (Cu—FMN), additional emission occurs from excited flavin mononucleotide. From absorption spectroscopic measurements J. Stauff and F. Nimmerfall60> concluded that the first reaction step consists in the addition of oxygen to the copper complex ... 

The first and most extensively examined system was the hydrolytic enzyme papain. A variety of isomeric a-bromoacetylisoalloxazines were used to selectively tether a flavin moiety to the active site cysteine residue. Different isomeric linkages were proposed to allow orientations of the flavin relative to the substrate binding site which would favor reactions with a bound substrate [65]. 

In contrast to the use of self-assembly reactions and metal ion coordination preferences to direct the construction of mixed cofactor systems, the use of SPPS or selective chemical ligation allows for the direct covalent attachment of cofactors for the construction of mixed cofactor systems within de novo design. Figure 11 shows the flavocy-tochrome maquette constructed by Dutton and co-workers (149) using a flavin moiety covalently attached to a unique cysteine residue inside a four helix bundle with bis-histidine binding sites for heme... 

Kay Have you knocked out the cysteine that binds the flavin Is this cysteine conserved ... 

Silverman and coworkers have carried out extensive research on the mechanism of inactivation of MAO by cyclopropylamine analogues. They first reported in the early 1980s that A/-cyclopropyl-A/-arylalkylamines are mechanism-based inactivators of MAO [121,125], The mechanism proposed was enzyme-catalyzed one electron oxidation of A/-cyclopropylamines to give reactive ring-opened products which further react with either flavin and/or a cysteine residue, depending on the structure of the inactivator. According to their reports [120,125, 126], 1-phenylcyclopropylamine (50) attached reversibly to a cysteine residue and irreversibly to the flavin when it activated MAO B, whereas 50 modified only the flavin during inactivation of MAO A. In the case of frans-2-phenylcyclopropy-lamine (8a) and A/-cyclopropyl-a-methylbenzylamine (51), both MAO A and B are inactivated by attachment to a cysteine residue (Fig. 3). 

Monoamine oxidase (MAO) (E.C. 1.4.3.4) is an enzyme found in all tissues and almost all cells, bound to the outer mitochondrial membrane. Its active site contains flavine adenine dinucleotide (FAD), which is bound to the cysteine of a -Ser-Gly-Gly-Cys-Tyr sequence. Ser and Tyr in this sequence suggest a nucleophilic environment, and histidine is necessary for the activity of the enzyme. Thiol reagents inhibit MAO. There are at least two classes of MAO binding sites, either on the same molecule or on different isozymes. They are designated as MAO-A, which is specific for 5-HT (serotonin) as a substrate, and MAO-B, which prefers phenylethylamine. Similarly, MAO inhibitors show a preference for one or the other active site, as discussed below. 

There is, up to now, one exception known to the four kinds of the above mentioned covalent linkages. The prosthetic group of trimethylamine dehydrogenase is linked via the C(6)-atom of the flavin to a cysteinyl residue (Scheme 3, (5)). As mentioned above the less reactivity of C(6) of flavin as compared to that of CH3(8) requires probably some chemical modification of the prosthetic by biologicai means prior to covalent attachment. The C(6)-S-Cysteinyl flavin was synthesized recently starting with 6-nitro flavin which was subsequently reduced to the amino derivative and transformed to the corresponding bromo derivative via diazotation. Reaction of the bromo derivative with cysteine gave the desired 6-S-Cysteinyl derivative... 

A second group of electron carriers in mitochondrial membranes are the iron-sulfur [Fe-S] clusters which are also bound to proteins. Iron-sulfur proteins release Fe3+ or Fe2+ plus H2S when acidified. The "inorganic clusters" bound into the proteins have characteristic compositions such as Fe2S2 and Fe4S4. The sulfur atoms of the clusters can be regarded as sulfide ions bound to the iron ions. The iron atoms are also attached to other sulfur atoms from cysteine side chains from the proteins. The Fe-S proteins are often tightly associated with other components of the electron transport chain. For example, the flavoproteins Flavin 1, Flavin 2, and Flavin 3 shown in Fig. 10-5 all contain Fe-S clusters as does the Q-cytochrome b complex. All of these Fe-S clusters seem to be one-electron carriers. 

These reactions, which have provided a means of inhibiting the flavin-linked monoamine oxidases, enable us to end on a clinical note. The monoamine oxidases are responsible for the deamination of monoamines such as adrenaline, noradrenaline, dopamine, and serotonin, which act as neurotransmitters. Imbalances in the levels of monoamines cause various psychiatric and neurological disorders Parkinson s disease is associated with lowered levels of dopamine, and low levels of other monoamines are associated with depression. Inhibitors of monoamine oxidases may consequently be used to treat Parkinson s disease and depression. The flavin moiety is covalently bound to the enzyme by the thiol group of a cysteine residue (equation 9.17). The acetylenic suicide inhibitor N,N-dimethyl-propargylamine inactivates monoamine oxidases by alkylating the flavin on N-5.25 A likely mechanism for the reaction is the Michael addition of the N-5 of the reduced flavin to the acetylenic carbon 2... 

Flavin-containing mitochondrial MAO-A and MAO-B catalyze the oxidative deamination of neurotransmitters, such as dopamine, serotonin, and norepinephrine in the central nervous system and peripheral tissues. The enzymes share 73% sequence homology and follow the same kinetic and chemical mechanism but have different substrate and inhibitor specificities. Chemical modification experiments provide evidence that a histidine residue is essential for the catalysis. There is also strong evidence that two cysteine residues are present in the active site of MAO. 

Cysteine (sulfhydryl) cystine glycosyl- dehydroalanyl heme flavin seleno- 7... 

Iron-sulfur proteins contain non-heme iron and inorganic (acid-labile) sulfur in their active centers as 4Fe-4S or 2Fe-2S or, in the case of rubredoxin, as one iron alone. The iron is always bonded to cysteine sulfur. They catalyze redox reactions between +350 and —600 mV (hydrogen electrode = —420 mV). They are usually of low molecular weight (6000-15,000 Daltons) but can form complex enzymes with molybdenum and flavin. They occur as soluble or membrane-bound proteins and catalyze key reactions in photosynthesis, oxidative phosphorylation, nitrogen fixation, H2 metabolism, steroid hydroxylation, carbon and sulfur metabolism, etc. They occur in all organisms so far investigated and may... 

Abbreviations used in this table MPT designates molybdopterin MGD designates MPT guanine dinucleotide MCD designates MPT cytosine dinucleotide N-His, O-Ser, S-Cys, Se-Cys designate attachments from protein residues histidine, serine, cysteine and selenocysteine. b Flavin adenine dinucleotide — FAD. c Dimethyl sulfoxide — DMSO. 

---