Enzyme

Enzymes act as specific catalysts. That is to say each enzyme accelerates one or more specific chemical reactions without affecting the final equilibrium concentrations of reactants and products. In thermodynamic language, enzymes reduce the activation energy of a reaction but do not affect the free energy change of the overall reaction. Many enzymes are so effective that they will catalyse intracellular reactions which are too slow to be observed at all under comparable conditions in the absence of enzyme catalysis. Enzymes are often highly specific, both for the molecules they will accept as substrates and for the precise chemical changes that they will catalyse, and the presence of active enzymes is essential to form most of the molecules in the cell.

Enzyme reactions can be either anabolic or catabolic in nature .  

Two important enzyme parameters in a simple enzyme catalysed reaction are the Michaelis-Menten constant (K_m) and the maximum reaction velocity (V_max). 

There are many enzymes used in labs, one of which is the restriction enzyme.

Restriction endonucleases are used naturally in a wide range of prokaryotes as a self-defence mechanism against foreign DNA molecules.  The prokaryotes own DNA is methylated so it will not be cut by the enzyme.They recognise a specific 4-8 base pair palindromic sequence and by carrying out a hydrolysis reaction cut at that specific point. They may cut to form a blunt end or a sticky end. A blunt end is when the enzyme cut the DNA symmetrically. Asymmetrical cleavage leaves sticky end, these are unpaired bases. These sticky end can anneal to complementary bases on another strand.

References

Berg J., Tymoczko J and Stryer L. (2007) Biochemistry, 6th edition, New York: WH Freeman.