Cofactor thiol binding

Some compounds of this type may have a high affinity for proteins that is not due to their binding to two thiol groups (35). In particular, arsenite also reacts with the molybdenum-pterin cofactor of many enzymes (35a-d). This usually inhibits the enzyme, but in particular cases (35e) the arsenite may be oxidized indeed the enzyme arsenite oxidase contains such a center (35f). 

Lysine mutase is the first of a group of AdoCbl-dependent enzymes that catalyses the 1,2-migration of an amino group (Fig. 26). It has been isolated from Clostridium sticklandii [39] and consists of a cobalamin-binding orange protein and a smaller yellow protein. Apart from AdoCbl, several other essential cofactors have been identified, such as pyridoxal phosphate, ATP, FAD, thiols, Mg2+ and K+ [38]. The function of the yellow protein and some of these cofactors is to renew continuously... 

Fig. 23. Proposed mechanisms for Tyr-Cys cofactor biogenesis. (A) Initial Cu(II) binding leads to the appearance of resonance contributions from a reactive phenoxyl that electrophilically attacks the neighboring Cys-228 thiol. (B) Cu(I) binding produces an oxygen-reactive complex that drives hydrogen abstraction from Cys-228 to form a thiyl free radical which subsequently attacks the neighboring Tyr-272 ring with formation of a carbon-sulfur covalent bond.

F430, and cofactor F430 forms the base of a narrow well accommodating the two substrates.The phosphate group of HS-CoB binds at the upper lip of the well with its thiol group located 6 A from the central Ni atom of F430 Synthesis of methane and the heterodisulfide... 

As seen in the previous work, arsenic will bind to biological molecules and hinder enzymic activity. The main mode of action seems to be to bind to systems that contain thiol as a functional group. For proteins, this can be either cysteine or cofactors that contain thiol groups. Our recent research efforts have elucidated the interaction with some of the target molecules. The arsenical that we have used is phenyldichloroarsine, an organic trivalent species that can penetrate cells easily. 

Thiol extrusion of the FeS centres from Azobacter vinelandii and Clostridium pasteurianum with alkyl fluorothiols allows identification of at least two Fe4S4 and one FegSj cluster in the (Fe-Mo) protein. A cluster model in which two Fe4S4 units are bridged through a comer iron on each cube by a Mo S4 unit is proposed for the active site of the (Fe-Mo) cofactor. Molecular nitrogen binds axially to the central molybdenum and is Ji-bonded to the iron in the cubes. This weakens and activates the N=N bond and electrons are injected stepwise via the two iron-sulphur cubes with successive protonation at the terminal N. This latter point is consistent with a report that N—NHg is important in nitrogen fixation. 

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