Bio-lubricant from the olive mill waste through a new biocatalyst immobilized on ZnFe2O4

The IUBMB (International Union of Biochemistry and Molecular Biology) introduced a classification of enzymes according to the type of reaction catalyzed [37]:
• Oxidoreductase: catalyzes oxidation-reduction reactions in which electron transfers, e.g. hydrogen atoms, occur. The common name is hydrogenase when the substrate oxidizes and oxidase when oxygen is reduced.
• Transferase: catalyzing reactions in which there are shifts of groups of atoms.
• Hydrolase: catalyzes hydrolysis reactions. These involve transfers of functional groups to the water. Hydrolases could be seen as a subclass of transferases because they transfer a group to an acceptor, which in this case is water.
• Liase: they catalyze reactions in which the functional groups are transferred to break double bonds or to form them as a result of their removal.
• Isomerase: catalyzes reactions that transfer functional groups to form isomers.
• Ligases: their function is to join two substrates through condensation reactions. Examples are carbon-carbon, carbon-nitrogen, etc. bonds.

Reaction mechanism
Catalysis occurs at the active site of the enzyme where the residues involved in the rupture and formation of chemical bonds are present. These residues are called catalytic groups. Although enzymes differ in structure and specificity, they have certain common characteristics:
• The active site is a three-dimensional fissure formed by groups coming from different parts of an amino acid sequence;
• The active site occupies a minimum part of the total volume of the enzyme;
• Active sites are unique microenvironments;
• Substrates are bound to enzymes by weak interactions;
• The specificity is due to the precise arrangement of the atoms in the active site [38].

Factors influencing the catalytic activity of enzymes
There are several factors that influence enzyme activity [39]:

• Temperature: As the temperature increases, the kinetic energy in the reacting molecules increases. This increase causes an increase in collisions and, as a result, the reaction rate increases. Enzyme activity increases to an optimal temperature value of 37.5°C for human cells. Above this temperature, the enzyme undergoes a degradation process known as denaturation (destruction of the tertiary structure)[38].
• pH: Enzymes work in a very small range of pH. There is an optimal pH value at which enzyme activity is at its highest, beyond this value the bonds present within the enzyme could be broken, leading to its deactivation.
• Enzyme and substrate concentration: The kinetic constant is a function of both enzyme and substrate concentration; as both increases, the reaction speed will also increase. For a given enzyme concentration, the speed will increase as the substrate concentration increases to a maximum that coincides with the saturation of the active sites [38].
• Inhibition: Some substances are able to reduce or even block the catalytic activity of enzymes; these substances are called inhibitors. Inhibitors can occupy the active site thus preventing the substrate from binding or occupy other parts of the enzyme. In the first case, we talk about competitive inhibition and in the second case we talk about non-competitive inhibition [38].