Michaelis-Menten hypothesis states that

Michaelis and Menten have proposed a hypothesis for enzyme action, which is most acceptable. According to their hypothesis, the enzyme molecule (E) first combines with a substrate molecule (S) to form an enzyme-substrate (ES) complex which fuher dissociates to form product (P) and enzyme (E) back. Enzyme once dissociated from the complex is free to combine with another molecule of substrate and form product in a similar way. The ES complex is an intermediate or transient complex and the bonds involved are weak non-covalent bonds, such as H-bonds, Van der Waals forces, hydrophobic interactions. Sometimes two substrates can bind to an enzyme molecule and such reactions are called as bisubstrate reactions. The site to which a substrate can bind to the enzyme molecule is extremely specific and is called as active site or catalytic site. Normally the molecular size and shape of the substrate molecule is extremely small compared to that of an enzyme molecule. The active site is made up of several amino acid residues that come together as a result of foldings of secondary and teiary structures of the enzyme. So, the active site possesses a complex three-dimensional form and shape, provides a predominantly non-polar cleft or crevice to accept and bind the substrate. Few groups of active site amino acids are bound to substrate while few groups bring about change in the substrate molecule.Ref: Textbook of medical biochemistry, MN Chatterji, 8th edition, page no: 124