“Applications of the Pearson’s hard soft acid base theory in inorganic biochemistry
Iron and its role in enzyme catalyzed activities
Iron plays a very crucial role in a number of reactions in the body. One of the enzymes that heavily relies on iron is an enzyme known as catalase. This enzyme is found essentially in all living organisms. This enzyme catalyzes a reaction that involves the breakdown of hydrogen peroxide to give oxygen and water. It is a very crucial enzyme especially to reactions which are reproductive. It also has among the highest turnover figures.
Structure of catalase
This enzyme is a tetramer. It is made up of four polypeptide chains. Each of these polypeptides is made up of more than five hundred amino acids. It has four porphyrin iron (heme) groups. These are the ones which allow that enzyme to react with hydrogen peroxide. The optimum PH at which it acts is 7but can range from 6.8 to 7.5 but this varies from species to species and can go as low as 4 and as high as 11. The optimum temperature at which the enzyme also acts varies from species to species.
How this enzyme reacts
It catalyses the reaction of decomposition of hydrogen peroxide
2H2O2 → 2H20 + 02
The presence of this enzyme in a tissue or a microorganism can be tested by the addition of a known volume of hydrogen peroxide and then observing how the reaction proceeds. There will be a formation of bubbles and this will indicate production of iron, an indicator of the reaction taking place. This is the simplest assay of its activity since it can be observed by the naked eyes. This occurs because this enzyme has high specificity.
This reaction occurs in two stages.
H2O2 + Fe (iii)-E → H2O + O=Fe (IV)-E (+)
H2O2 + O=Fe (IV)-E (+) → H2O + Fe (iii)-E + O2
Fe ()-E represents the center of iron of a heme group which is usually attached to the enzyme. Fe (IV)-E (+) act as a mesomeric form of the Fe (IV)-E. This means that iron does not undergo complete oxidation to the +V, but in the process, it receives some electrons from the ligand which is heme. As the hydrogen peroxide goes into the active centre, it interacts with the amino acids in the active site leading to a proton transfer between two oxygen atoms. The free oxygen atoms then facilitate the formation of a new water molecule and Fe (IV) =O. Fe (IV)=O then reacts with another hydrogen atom leading to the formation of Fe (III)-E; hence oxygen and water are formed. The reactivity of this center of iron may be made better by the addition of a phenolate ligand of tyrosine at position 357. This assists in oxidation of iron III to iron IV. This enzyme can catalyze oxidation of hydrogen peroxide of many toxins and metabolites such as formic acid, phenols, formaldehyde and alcohols. Any ion of a heavy metal ion can be a non competitive inhibitor of this enzyme. Competitive inhibitors usually bind the heme moieties strongly hence inhibiting the action of the enzyme.
This enzyme is used a lot in the food industry like in the removal of hydrogen peroxide before manufacturing cheese. It is also used to prevent oxidation of foods. It also used in the textile industry in removing hydrogen peroxide from fabrics. It also finds a minor use in contact lenses.
This metal is a very crucial metal especially to some enzymes like laccases. These enzymes are copper containing enzymes which carry out oxidation. They occur widely especially in fungi, plants and microorganisms. Copper metal is usually bound to several sites within the active site. Type 1 copper acts on solvents such as water. This can be displaced by metals such as mercury, be substituted by metals such as cobalt or be removed by complexones of copper. Cyanide may act and remove it all from the enzyme.
These enzymes usually act on phenols or related molecules. They carry out oxidations usually involving one electron. Some theories suggest that these enzymes play a role in the in lignin formation. This is through the coupling of monolignols. These enzymes can be polymeric, trimer and or dimer.
These enzymes usually do not give toxic products or intermediates. The four copper centers on the active site usually act as oxidation centers. They are mono electronic especially for type 1 copper. From this type of copper, the electrons are then transferred to type 2 and type 3 copper tri nuclear clusters. This is the actual place where oxygen reduction occurs to give rise to water. This occurs because these many copper centers allow substrate reduction to oxygen to occur without formation of intermediates which are toxic. The products formed then can undergo various reactions such as polymer degradation, cross linking of monomers.
Application of laccases
These enzymes have undergone examination as cathodes in enzymatic biofuels. They can undergo pairing with electron mediator to facilitate transfer of electrons. They are also sold as industrial catalysts. They can also be employed in the dyeing of textiles, teeth whitening, wine work making, synthetic, diagnostic and environmental uses. They can also be applied in bioremediation. It’s also has activity in wheat dough.
In the food industry, the enzyme is used especially in the beer industry in the removal of polyphenols. It is also used in the fruit juices such as grape and apple juices to prevent excess phenolic oxidation which might lead to changes to taste, odour, color changes and even mouth feel. These increase the shelf lives of these juices.
Application of the concept in toxicology
Many of the chemical toxins together with their metabolites are electrophiles. They cause cellular injury through the formation of covalent bonds with some nucleophilic targets found on macromolecules. These covalent reactions between electrophilic and nucleophilic reagents are somehow discriminatory because there is a degree in selectivity that is associated with these reactions. Bisswanger notes that these covalent bonds then impair the functions of various proteins, macromolecules, enzymes and DNA material (95). Known chemical compounds like acrolein and acrylaide lead to cellular cytotoxicity by formation of covalent adducts, which cause broad organ toxicity.
Thus, this concept has been successfully applied in toxicity that has been induced in biological systems. According to the above principle of hard and soft acids, the toxic chemical electrophile reacts preferably with a biological target of similar softness or hardness. This hard/soft classification of xenobiotic electrophiles helps in proper understanding of various mechanisms by which toxicity takes place in biological systems and also it helps in understanding the biological targets that exist. This formation of covalent bonds can be best described by use of the properties of frontier orbitals. Since the energies of these orbitals can be estimated through calculation by use of quantum mechanical models, then quantification of the relative softness and hardness of the nucleophiles and electrophiles can be possible. This will, in turn, help in the determination of the indices of reactivity. This information can help a lot and provide crucial insight especially to the research and investigation in toxicology. This, in turn, will help researchers to understand the molecular sites properly and also help them come up with proper mechanisms to counter the effects of these toxins…”
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