Pearson's hard soft acid base theory in bioinorganic - Term Paper Example

The theory implies that soft acids tend to bind to soft bases and hard acids to hard bases. An increase in the electronegativity of an element or ligand causes an increase in the polarizability; this in turn increases hardness. The theory is useful in predicting the pathways of chemical reactions. The chemical conditions in which a hard or soft base or acid in put in can cause the hardness (or softness) characteristics of the acid or base to change. Therefore, borderline elements and ligands might increase or decrease in hardness or softness depending on the chemical conditions.

Because of this reason, the metals in various metalloenzymes may be subjected to chemical conditions that might alter their hardness or softness properties. Enzymes with metals that exhibit Pearson’s hard and soft acids and bases theory include: 1) Urease This is an enzyme with nickel at its active site found in many species of bacteria, algae, plants (such as Jack Bean) and invertebrates. It plays a key role in the catalytic hydrolysis of urea to form ammonia and carbon dioxide as pre the equation below: Urease in Jack Bean has a single catalytic unit made up of an ?

-subunit that has the active site with a dimeric nickel center. One of the two Ni atoms (Ni-1) coordinates to histidine via the nitrogen atoms and a water molecule. The second Ni atom (Ni-2) is similarly coordinated to histidine via the N atoms, two water molecules and to aspartic acid via the O atom. Mechanism: There are several mechanisms that explain how urease works. These include: a) Zerner mechanism In this, a carbonyl oxygen in urea attacks one of the water ligands attached to Ni-1. A nitrogen atom in the urea molecule donates its lone pair electrons to a carbon atom forming an N=C bond (Dixon, Riddles and Blakeley).

This then reacts with a carboxylate ion. A base-catalyzed deprotonation of one –OH ligand on Ni then occurs. The resultant electronegative O attacks the carbonyl carbon. The N=C bond initially formed donates two electrons to the nitrogen, cancelling out the charge on it. The intermediate carbon formed with a coordination of 4 is then broken down by a sulfhydryl group. Ammonia is released when the C-N bond is broken after an H atom bonds to the N. This occurs alongside the breaking of the bond between the octahedral nickel and oxygen.

A carbamate ion coordinated to the Ni is then formed. Water displaces the carbamate. The resultant carbamate then degenerates to yield carbonic acid and urea. b) Mangani mechanism This mechanism stipulates that both Ni-1 and Ni-2 take part in the reaction. The first atom, Ni-1, binds to urea, causing its activation. The second, Ni-2 binds to a water molecule, causing its activation (Benini, Rypniewski and Wilson). Ni-1 is in a five-coordinate formation, bound to urea via a carbonyl O atom. The distance between the two Ni atoms is reduced by the movement of the urea molecule towards Ni-2.

The relatively low Lewis base property of NH2 in urea makes it a poor chelating ligand. Its high basicity however, enables the binding to Ni to occur. 2) Carbon monoxide dehydrogenase This is a nickel-based enzyme found in various bacteria. The enzyme plays a role in the catalytic oxidation of carbon monoxide to form carbon dioxide as per the equation below: There are two classes of carbon monoxide hydrogenase enzymes: one has a Mo-[Fe2-S2] active site and the other a Ni-[Fe3-S4]