Tissue, pathway, proteinase, peptidase, protease ,acrosin, lipoprotein, activator, caspase, trypsin, papain, esterase inhibitors are proteins or receptor ligands or receptor antagonists that bind to an enzyme receptor and decreases its activity. Since blocking an enzyme's activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors. Not all receptor antagonist that bind to enzymes are inhibitors; enzyme activator ligands or agonists bind to enzymes and increase their enzymatic activity, while enzyme substrates bind and are converted to products in the normal catalytic cycle of the enzyme.Serine protease, D- or L-serine arginine rich enzyme of serine threonine kinase with serine that is encoded by the codons UCU, UCC, UCA, UCG, AGU and AGC is an ɑ-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH+ 3 form under biological conditions), a carboxyl group. It is non-essential in humans, meaning the body can synthesize it.