Bioinorganic Chemistry

Transition-metal ions play a number of extremely critical roles in biology. However, the size and complexity of the metalloproteins which contain them makes it difficult to determine the properties which are responsible for their function. Although information regarding the probable structure of the active site of a metalloenzyme can be revealed by an X-ray crystal structure, in many cases the resolution of these structures does not allow essential molecular level details to be deciphered. It's also difficult to rule out the possibility that the structure of the crystallized protein has been subtly altered relative to the catalytically active form. By modeling the metal ion’s local environment (ie, its primary coordination sphere), and making systematic changes to this environment, one can determine if there is a correlation between structure, physical properties, and function. This approach to understanding metalloenzymes is generally referred to as the synthetic analogue approach.

To the right is an example of a helical ligand, designed by our group, which ligates both Fe(III) and Co(III). The resulting complex is reactive and will bind a number of biologically—relevant

substrates. The properties (spin—state, epr and electronic spectrum) of the iron complex are remarkably similar to those of the active site of the metalloenzyme nitrile hydratase (NHase). Nitrile hydratase is a non—heme iron bacterial enzyme which catalyzes the hydration of nitriles to amides. The iron is ligated by three cysteinates, two peptide amides and a water. Two of the three cysteinates appear to be oxidized, one to a sulfenate, the other to a sufinate. Our work suggests that at least two of the three sulfurs remain unmodified, however. By removing one of the methylenes from the backbone of the ligand shown, reactivity increases by two orders of magnitude. Quantitative evidence for this comes from temperature—dependent substrate binding studies which have afforded the thermodynmic parameters( delta)H and (delta)S. The reason for this increased reactivity was found to be structural in nature: removal of a single methylene causes an angle to open up by ~10 degrees, making the metal ion more accessible to substrate. This illustrates how a protein might hold a metal ion in a near transition state (ie, an entatic state), and shows how subtle changes to the secondary coordination sphere can have a dramatic effect on a metalloenzyme's active site properties.

 

 

 

 

 

page authored by J. Kovacs