For a reaction A + B → C + D
A and B are the reactants and C and D are the products. Energetically favorable reactions proceed spontaneously from reactants to products. In such a case the energy state of reactants is lower than the energy state of products, meaning that the reaction proceeds in a downhill direction in terms of chemical energy. However, to reach this downhill state, the reactants must pass over a 'speed bump', an energetic barrier.
Catalysts lower the energetic barrier between substrates and products yet catalysts emerge unchanged from the reaction. Biological catalysts are called enzymes – these are usually proteins, but may be molecules of RNA (ribozymes).
Although enzymes alter k, the rate of a reaction, they do not alter Keq, the actual equilibrium point. However, if the products of a reaction are removed by a second reaction, then the product side of the reaction equation will be favored.
Enzymes increase the rate of reactions by virtue of a transient binding of substrate (A and/or B) to the active site of the enzyme. This binding of substrate to enzyme occurs at the active site where it is stabilized by numerous weak interactions (hydrogen bonds, electrostatic interactions, hydrophobic contacts, and van der Waals forces). The enzyme-substrate complex dissociates into enzyme (in original state) and products (C,D).
Endothermic reactions, such as the steps in biosynthesis of macromolecules, require heat input, while exothermic reactions release heat.