Ligand at the active site

Similar structural features are present in the [FeFe]-hydrogenases (FeS electron relay centers, CO and CN-ligands at the active site, and channels for gas access). 

From HNMR studies protonation and dissociation of the two histidine ligands at the active site of spinach PCu(I) has been reported [47], The acid dissociation pK value for the reversible process (1),... 

There is general agreement that carbonic anhydrase activity is linked to the zinc ion and its ligands. At the active site water is bound to the zinc but the state and involvement of the water in the actual catalysis is a matter of controversy. According to a widely accepted opinion the zinc-bound water is ionized and the activity is a function of pH1S6. ... 

B. Coupling of Protein Fluctuations to the CO Ligand at the Active Site... 

Studies of the infrared spectra carbon monoxide inhibited forms of [FeNi]-hydrogenase revealed that the native protein exhibits absorbances at energies not normally associated with protein samples. These absorbances in the 1900-2100 cm region are also observed in [Fe]-hydrogenase samples. This absorption is attributed to the cyanide and carbonyl ligands at the active site metal centers. Cyanide and carbon monoxide are not commonly found as ligands to metal centers in metal-loproteins. The different redox states of the enzymes give rise to different absorption spectra. 

Presence of multiple binding sites strategically located in a ligand at the active site invariably poses an important subject for due cognizance. 

Metallocene (Section 14 14) A transition metal complex that bears a cyclopentadienyl ligand Metalloenzyme (Section 27 20) An enzyme in which a metal ion at the active site contributes in a chemically significant way to the catalytic activity... 

The active-site Fe ion also eluded detection hy either EPR (9) (and references therein) or Mdsshauer (88, 89) spectroscopy, suggesting that it is low spin and diamagnetic at least in those states that have been studied. This is consistent with the presence of one CO and two CN s, which are strong ligands, in the active-site Fe coordination... 

The crystal structure of a CODH/ACS enzyme was reported only in 2002.43,44 It reveals a trio of Fe, Ni, and Cu at the active site (6). The Cu is linked to the Ni atom through two cysteine-S, the Ni being square planar with two terminal amide ligands. Planarity and amide coordination bear some resemblance to the Ni porphinoid in MCR. A two-metal ion mechanism is likely for acetyl CoA synthesis, in which a Ni-bound methyl group attacks an adjacent Cu—CO fragment with formation of a Cu-acyl intermediate. A methylnickel species in CODH/ACS has been identified by resonance Raman spectroscopy.45... 

The lines in blue show shifts at the active site residues while the imidazole signal is not shifted. The effects on the NMR signals in black (medium effects) and magenta (strong effects) are caused by changes in the pH (as indicated by the shifted imidazole line) induced by the ligands. 

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