Protein ubiquitination is a post-translational modification that controls essential biological processes through its regulation of protein concentration, function, and cellular location. RING E3 ligases are a critical component of a three-enzyme cascade that facilitates the ubiquitination of proteins. RING-type E3 ligases represent one class of E3 ligases that function by binding the substrate protein and ubiquitin-conjugating enzymes (E2s). Proteins exhibiting RING-type E3 ligase activities do so via a domain that adopts a ββα-RING fold and coordinates two zinc ions. To date, structural studies show that the RING domain interacts with the catalytic domain of the E2 enzyme. The catalytic domain is approximately 150 amino acids and adopts a canonical structure consisting of four α-helices and 3–4 β-strands. Structural analyses of RING–E2 complexes reveal that RING domains interact on a similar surface of the E2 enzyme. We postulate that the mechanism of interaction between an E2 enzyme and its cognate RING E3 domain may contribute to the extent of substrate modification. In this review, we compare the primary and secondary structures of human E2 enzymes and examine their quaternary structure with RING domains. Our analyses reveal the interactions appear to be relatively conserved with similar types of amino acids involved.