Enzymes are complex proteinous substances which increase the rate of reaction. According to lock and key model, enzyme works as a lock and substrate works as a key. A perticular enzyme reacts with a perticular or specific substrate just like a lock is opens by a specific key. Lock and key model of enzyme action was proposed by Fischer. Koshland proposed Induced-Fit theory for enzyme action. Kuhne coined term enzyme and Buchner extracts first enzyme zymase from yeast cells.