Di-sulfide bonds can stabilize this 3-D shape that forms particular environmental pockets or active sites that can stabilize the transition state of the reactant molecules that fit. The cell's molecular crowding and enzyme colocalization in isolated functional pathways promotes the speed of reactants binding.
Cysteine residues have a thiol sulfur bearing chemical group. Two cysteins, once bonded together in a di-sulfide bridge, become a single a dimeric amino acid called a cystine couple or just cystine. This bond is 40% weaker than C-C or C-H bonds and can be disrupted by heat or by being reduced (gaining electrons) from a polar reagent when the pH is unsuitable.
http://en.wikipedia.org/wiki/Disulfide_b…
Quaternary sub unit structure is when multiple protein subunits make up the functional protein. They can be held together by electrostatic bonds between carboxyl, imidazole or amino groups; Hydrogen bonds between hydroxyl, amide and phenol groups or hydrophobic attractions between aromatic groups hiding from the aqueous cytosol; or covalent di-sulfide bonds.
This 3-D shape can be altered by cleavage to an active form or by moving to a location with a different pH or temperature. Alternately it can bind a substrate molecule that produces a shift in the thermodynamic potential in the protein. This can cause it to rearrange into a more energetically favorable shape.