P1 + P2is replaced by at least two:
CATALYSIS
For carrying out chemical reactions in a cell ( metabolism) certain energy of activation is necessary. The value of this energy depends on the type of the reaction. Catalysts (E) for a short time join the substance, which undergo changes, substrate (S), decrease the value of the energy of activation, and speed up the reaction, that is, formation of the product (P). One reaction
S P1 + P2
is replaced by at least two:
E + S 
ES 
E + P1 + P2.
ES – is the compound of the catalyst and the substrate. The activity of the catalyst is determined by the fact that the sum of the activation energies for reactions 2 and 3 is less than the activation energy for reaction 1.
Enzymes can serve as examples of biocatalysts of protein nature.