Molecular mechanisms for the conversion of zymogens to active proteolytic enzymes

Abstract

Proteolytic enzymes are synthesized as inactive precursors, or "zymogens," to prevent unwanted protein degradation, and to enable spatial and temporal regulation of proteolytic activity. Upon sorting or appropriate compartmentalization, zymogen conversion to the active enzyme typically involves limited proteolysis and removal of an "activation segment." The sizes of activation segments range from dipeptide units to independently folding domains comprising more than 100 residues. A common form of the activation segment is an N-terminal extension of the mature enzyme, or "prosegment," that sterically blocks the active site, and thereby prevents binding of substrates. In addition to their inhibitory role, prosegments are frequently important for the folding, stability, and/or intracellular sorting of the zymogen. The mechanisms of conversion to active enzymes are diverse in nature, ranging from enzymatic or nonenzymatic cofactors that trigger activation, to a simple change in pH that results in conversion by an autocatalytic mechanism. Recent X-ray crystallographic studies of zymogens and comparisons with their active counterparts have identified the structural changes that accompany conversion. This review will focus upon the structural basis for inhibition by activation segments, as well as the molecular events that lead to the conversion of zymogens to active enzymes.

Structure, function and latency regulation of a bacterial enterotoxin potentially derived from a mammalian adamalysin/ADAM xenolog

“…S2D). Taken together, all these results indicate that directly expressed fragilysin-3 is unstable and strongly point to a role for the PD, in addition to latency maintenance, as a chaperone that assists in the folding and stabilization of CD, as previously reported for other MP zymogens (14,15). In the absence of this chaperone, fragilysin-3 CD is not able to fold correctly in the environment provided by the bacterial expression host.…”

Factor B structure provides insights into activation of the central protease of the complement system

“…factor B locked in its proenzyme state. The structure adds this locked conformation to the open and closed conformations described in past studies of the VWA and I domains 8,9 and provides insights into a previously uncharacterized mechanism for regulating serine protease activity 21 . Assembly of an active protease complex probably proceeds through a number of steps.…”

“…In addition to the interface loops, loop 2 of the SP domain is markedly different in factor B and fragment Bb. Putatively, Arg705 of this loop (like Arg696 in C2a 11 ) induces formation of the oxyanion hole, similar to the N terminus in trypsin after proteolytic activation of trypsinogen 21 . Consistent with this model, the guanidinium group of the arginine interacts with an aspartate of the oxyanion-hole loop in Bb and C2a ( Supplementary Fig.…”

Proenzyme Structure and Activation of Astacin Metallopeptidase

“…Another regulatory mechanism is zymogenic latency, which is provided by mostly N-terminal prosegments. These block access of substrates to the active-site cleft, and they are removed by limited proteolysis during maturation (2,3). Such prosegments often fold independently and guide on their part the folding process of the cognate protease domains.…”

“…Such prosegments often fold independently and guide on their part the folding process of the cognate protease domains. They may also act as intramolecular chaperones or inhibitors of the mature enzymes in trans and in intracellular sorting of the zymogen (2). Therefore, the study of the molecular mechanisms by which MPs maintain latency is indispensable to an understanding of their basic mode of action.…”