Enzyme, principles underlying specificity substrate

Enzyme, diagram of postulated union of substrate with, V, 56 Enzyme specificity, principles underlying, in the domain of carbohydrates, V, 49-78 Enzyme value, definition, V, 62 comparative values from rates of hydrolysis of phenyl hexosides and pentosides, V, 62... 

For enzymes another point can be important Not all pure enzymes are stable in solution. In particular in organic solvents enzymes can be very unstable (4). The stability of enzymes can be improved by genetic modifications, by chemical modification, and most easily and without specialist equipment by immobilization. Furthermore, immobibzation opens up the possibility of improving other properties of the enzyme, such as its activity, substrate specificity, and enantioselectivity. Although the underlying principles are not always understood enzyme immobilization is a powerful tool for the improvement of activity and enzyme stability, specificity, and selectivity. 

In principle, the amount of a biocatalyst may be expressed as a number of elementary entities, an amount of substance, a mass, or by a catalytic effect. In many cases, due to the lack of information or for practical reasons, the later route is used. Then, the generally accepted term for the expression of enzyme activity is the Unit (U) defined in terms of reaction rate. One unit (U), accordingly to the International System of Units, is defined as the amount of enzyme that catalyzes the conversion of 1 pmol of substrate (or the formation of 1 pmol of product) in 1 min under standard conditions. The specific activity is the number of units per milligram of protein. If the relative molecular mass of the enzyme is accurately known it is then possible to express the activity as the molar catalytic activity, defined as the number of units per micromole of enzyme. This is, the number of moles of product formed, or substrate consumed, per mole of enzyme per minute. This may not correspond to the number of moles of substrate converted per enzyme active site since the enzyme may contain more than one active site. If the number of active sites per mole is known, then the activity may be expressed correspondingly as the catalytic center activity. 

---