Conformation of pepsin and pepsinogen: some aspects of the role of tyrosine residues and the 1-44 segment of pepsinogen on conformational stability

Conformational changes induced in pepsin and pepsinogen by iodination of tyrosine residues and the possible role of lysine residues on conformational stability of pepsinogen are investigated by circular dichroism (CD) studies in solution. At low degrees of iodination (6 I/molecule) the pepsin molecule denatured, with complete loss of β-structure at pH 5.5. Pepsinogen showed greater resistance to conformational change on iodination (10 I/molecule) and about 30% of its ordered structure is retained. In the aromatic region, the tyrosyl CD bands of iodinated pepsin decreased in intensity, indicating a change in the environment of tyrosine residues. A comparison with the CD spectra of expanded structures of pepsin in 6 m guanidine hydrochloride or alkaline solutions (pH 9.75) indicated retention of a significant amount of tertiary structure in iodinated pepsin. Changes in tertiary structures were marginal on iodination of pepsinogen. Less than 1% (residue moles) of poly-l-lysine, a known inhibitor, was found to destabilize the secondary and tertiary structure of pepsin at pH 6.75, although the lysine-rich 1–44 segment of pepsinogen tends to stabilize the conformation of the pepsin chain. This seems to suggest that the inhibitory effects of polylysine on pepsin occur by a mechanism different from that of the activity-limiting effect of the lysine-rich 1–44 segment of pepsinogen.