INT. J. BIOL. BIOTECH., 16 (1): 9-21, 2019. BIOSYNTHESIS OF C 10 H 16 (LIMONENE) BY MEVALONATE PATHWAYS AND ITS PHARMACOLOGICAL APPLICATIONS Mehmood Khan 1 , Fozia Karam Khan 1 , Sadaf Moneeba 4 , Mehru Nisa 1 , Robina Manzoor 1 , Kaleem Imdad 3 *, Kaleem Ullah 2 , Zahid Mehmood 1 , Ashif Sajijad 1 , Muhammad Ayub 1 , Noor Hassan 1 and Amir Rasool 1,2 1 Institiute of Biochemistry, University of Balochistan, Quetta 87300, Pakistan 2 School of Life Science, Beijing Institute of Technology, Bejing 100081, Pr China 3 Department of Bioscience, Comsats University, Islamabad 45550, Pakistan 4 Department of Biotechnology and Bioinformatics, International Islamic University, Islamabad, Pakistan *Corresponding author: Email: kaleem.imdad@comsats.edu.pk; Cell# +92-334-4448241 ABSTRACT Limonene is a transparent, colourless compound (chemical) has D and L isomer; which is found in oil of citrus fruits peels. It is a monoterpene (monocyclic) hydrocarbon and used in the production of several commodity chemicals, biodiesel products, medicinal compounds and pharmaceutical and it is also an important precursor of many flavouring. Limonene is having low toxicity and can be used for several purposes. Overproduction of Limonene can be achieved by genetic engineering of Mevalonate pathway and 2-C-methyl-D-erythritol-4-phosphate pathway (MEP and MVA) in microorganisms by introducing limonene synthase. Limonene has several applications in industrial products and also can be used in clinical or medical field. Moreover the metabolic engineering of the pathways is potential production platform for any valuable limonene derivative. Limonene is flammable, function as biofuel. Limonene is naturally occurring in plants but genetically engineered microbes could produce sufficient quantity of limonene to cover the need but yet requires more efforts in labs to engineer more effective and active microbes for overproduction of limonene by developing Geranyl Pyrophosphate (GPP) pool in microbes. Keywords: MEP (Mevalonate pathway), MVA (2-C-methyl-D-erythritol-4-phosphate pathway), BUN (Blood Urea Nitrogen), IDP (Isopentenyl diphosphate) and DMADP (dimethyl allyl diphosphate), HMGR (hydroxymethylglutaryl_CoA reductase), GPPS (Guanosine pyrophosphohydrolase/synthetase). INTRODUCTION Limonene is a liquid hydrocarbon belongs to monoterpene family of terpenoids, colourless organic compound with D and L isomers. D-isomer smells like an orange (Fahlbusch et al., 2003) and L-isomer has fragrance similar to a resin of pine tree or wood turpentine (Fig. 1). Fig 1. structures Limonene takes its name from the peel of lemon. Biological sources produce only one enantiomer such as citrus fruits because limonene is a chiral organic molecule. Racemic mixture of limonene both D and L forms are called as dipentene (Simonsen, 1947). Using techniques such as centrifugal separation or steam distillation D-limonene can be obtained from citrus fruits.