Molecular mechanisms for the conversion of zymogens to active proteolytic enzymes.

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.     

The pro-dependent folding of microbial proteolytic enzymes

Modern published data on the folding of microbial proteolytic enzymes mediated through their pro-parts, or on a so-called pro-dependent folding, are surveyed. Various aspects related with the functioning of pro-sequences are under discussion. The possibility to build up, on the basis of one and the same amino-acid sequence, the special protein conformers involving different pro-peptides is in the focus of attention.     

A modified procedure for the detection of microbial producers of extracellular proteolytic enzymes

Summary A simple procedure for the detection of microbial producers of proteolytic enzymes using dyed gelatin microcarrier particles incorporated into appropriate nutrient agar is described. Extracellular proteinases produced by the tested microbial strains hydrolyzed the substrate and clear dyed zones around and under the colonies were formed.     

Molecular mechanisms of autoimmunity triggered by microbial infection

Autoimmunity can be triggered by microbial infection. In this context, the discovery of Toll-like receptors (TLRs) provides new insights and research perspectives. TLRs induce innate and adaptive antimicrobial immune responses upon exposure to common pathogen-associated molecules, including lipopeptides, lipopolysaccharides, and nucleic acids. They also have the potential, however, to trigger autoimmune disease, as has been revealed by an increasing number of experimental reports. This review summarizes important facts about TLR biology, available data on their role in autoimmunity, and potential consequences for the management of patients with autoimmune disease.     

The versatility of proteolytic enzymes

The growing realization of their physiological importance has generated renewed interest in the study of proteolytic enzymes. Modern methods of protein chemistry and molecular biology have revealed new insights into the protein and gene structure of a variety of protein precursors and their processing by limited proteolysis. Examples are given in this review for transmembrane processes and the role of signal peptidases of both eukaryotic and prokaryotic origin, the processing of prohormones and precursors of growth factors, protein components of blood coagulation, fibrinolysis, and of the complement system, and a group of granulocyte proteases, including the mast cell serine proteases. The relationship of homologous domains found in many of these proteases and their zymogens to protein evolution is a recurrent theme of this discussion.     

Excretion of proteolytic enzymes by Aedes aegypti after a blood meal.

Excretion of active proteolytic enzymes during the period of blood digestion in a mosquito has been demonstrated for the first time. The rate of excretion has been determined for both proteases and uric acid; each appears in a distinct peak. During the first half of the digestion period, when protease activity in the midgut is increasing, uric acid excretion predominates. During the second half of the digestion period, after the protease has reached its maximum in the midgut, there is considerable excretion of active protease, mainly trypsin.By sealing the anus after feeding (blood enema), it has been demonstrated that secretion of the proteolytic enzymes in the midgut actually stops when maximum activity is reached. Sealing the anus did not interfere with egg development.A model for protease secretion is suggested in which the proteolytic enzymes are induced by their substrate (globular proteins), and secretion stops when 80 per cent of the protein is digested, or the inducer is removed.