Metabolic pathways are enzyme-mediated biochemical reactions that lead to biosynthesis (anabolism) or breakdown (catabolism) of natural product small molecules within a cell or tissue. In each pathway, enzymes catalyze the conversion of substrates into structurally similar products. Metabolic processes typically transform small molecules, but also include macromolecular processes such as DNA repair and replication, and protein synthesis and degradation. Metabolism maintains the living state of the cells and the organism.
Proteases are used throughout an organism for various metabolic processes. Proteases control a great variety of physiological processes that are critical for life, including the immune response, cell cycle, cell death, wound healing, food digestion, and protein and organelle recycling. On the basis of the type of the key amino acid in the active site of the protease and the mechanism of peptide bond cleavage, proteases can be classified into six groups: cysteine, serine, threonine, glutamic acid, aspartate proteases, as well as matrix metalloproteases. Proteases can not only activate proteins such as cytokines, or inactivate them such as numerous repair proteins during apoptosis, but also expose cryptic sites, such as occurs with β-secretase during amyloid precursor protein processing, shed various transmembrane proteins such as occurs with metalloproteases and cysteine proteases, or convert receptor agonists into antagonists and vice versa such as chemokine conversions carried out by metalloproteases, dipeptidyl peptidase IV and some cathepsins. In addition to the catalytic domains, a great number of proteases contain numerous additional domains or modules that substantially increase the complexity of their functions.
Imbalances in metabolic activities have been found to be critical in a number of pathologies, such as cardiovascular diseases, inflammation, cancer, and neurodegenerative diseases.