A model of the quaternary structure of enolases, based on structural and evolutionary analysis of the octameric enolase from Bacillus subtilis

Purified enolase from Bacillus subtilis has a native mass of approximately 370 kDa. Since B. subtilis enolase was found to have a subunit mass of 46.5 8 kDa, the quaternary structure of B. subtilis is octameric. The pH for B. subtilis enolase is 6.1, the pH optimum (pH(o)) for activity is 8.1-8.2, an d the K-m for 2-PGA is approximately 0.67 mM. Using the dimeric C alpha str ucture of yeast dimeric enolase as a guide, these dimers were arranged as a tetramer of dimers to simulate the electron microscopy image processing ob tained for the octameric enolase purified from Thermotoga maritima. This ar rangement allowed identification of helix J of one dimer (residues 86-96) a nd the loop between helix L and strand 1 (HL-S1 loop) of another dimer as p ossible subunit interaction regions. Alignment of available enolase amino a cid sequences revealed that in 16 there are two tandem glycines at the C-te rminal end of helix L and the HL-S1 loop is truncated by 4-6 residues relat ive to the yeast polypeptide, two structural features absent in enolases kn own to be dimers. From these arrangements and alignments it is proposed tha t the GG tandem at the C-terminal end of helix L and truncation of the HL-S 1 loop may play a critical role in octamer formation of enolases. Interesti ngly, the sequence features associated with dimeric quaternary structure ar e found in three phylogenetically disparate groups, suggesting that the anc estral enolase was an octamer and that the dimeric structure has arisen ind ependently multiple times through evolutionary history.