There are five known families of carbonic anhydrases (a, b, g, d and z) that catalyze the reaction,

CO₂ + H₂O <-> HCO₃^- + H⁺

a. Carbonic anhydrase is found in humans and all other animals.

b. Carbonic anhydrase is found in higher plants and certain bacteria. Unlike the a form, the b form is an allosteric enzyme that is regulated by a bicarbonate ion (also a product of the reaction).  

Most b-carbonic anhydrases are composed of a dimer-of-dimers structure, i.e. a tetramer made up of two dimers. In one specific type of tetrameric b-carbonic anhydrase (Rv3588), a large number of basic and acidic amino acid residues interact on the interface between the dimers.

i. Which type of protein structure is a dimer of dimers: primary, secondary, tertiary, or quaternary? 

ii. Which type of noncovalent interactions are responsible for the two dimers binding to form a tetramer? Explain your answer.