Protein folding is the process by which a polypeptide chain acquires its functional, native 3D
structure. Protein misfolding, on the other hand, is a process in which protein fails to fold into its native
functional conformation. This misfolding of proteins may lead to precipitation of a number of serious
diseases such as Cystic Fibrosis (CF), Alzheimer’s Disease (AD), Parkinson’s Disease (PD), and
Amyotrophic Lateral Sclerosis (ALS) etc. Protein Quality-control (PQC) systems, consisting of molecular
chaperones, proteases and regulatory factors, help in protein folding and prevent its aggregation. At
the same time, PQC systems also do sorting and removal of improperly folded polypeptides. Among the
major types of PQC systems involved in protein homeostasis are cytosolic, Endoplasmic Reticulum
(ER) and mitochondrial ones. The cytosol PQC system includes a large number of component chaperones,
such as Nascent-polypeptide-associated Complex (NAC), Hsp40, Hsp70, prefoldin and T Complex
Protein-1 (TCP-1) Ring Complex (TRiC). Protein misfolding diseases caused due to defective cytosolic
PQC system include diseases involving keratin/collagen proteins, cardiomyopathies, phenylketonuria,
PD and ALS. The components of PQC system of Endoplasmic Reticulum (ER) include Binding
immunoglobulin Protein (BiP), Calnexin (CNX), Calreticulin (CRT), Glucose-regulated Protein GRP94,
the thiol-disulphide oxidoreductases, Protein Disulphide Isomerase (PDI) and ERp57. ER-linked misfolding
diseases include CF and Familial Neurohypophyseal Diabetes Insipidus (FNDI). The components
of mitochondrial PQC system include mitochondrial chaperones such as the Hsp70, the
Hsp60/Hsp10 and a set of proteases having AAA+ domains similar to the proteasome that are situated in
the matrix or the inner membrane. Protein misfolding diseases caused due to defective mitochondrial
PQC system include medium-chain acyl-CoA dehydrogenase (MCAD)/Short-chain Acyl-CoA Dehydrogenase
(SCAD) deficiency diseases, hereditary spastic paraplegia. Among therapeutic approaches
towards the treatment of various protein misfolding diseases, chaperones have been suggested as potential
therapeutic molecules for target based treatment. Chaperones have been advantageous because of
their efficient entry and distribution inside the cells, including specific cellular compartments, in therapeutic
concentrations. Based on the chemical nature of the chaperones used for therapeutic purposes,
molecular, chemical and pharmacological classes of chaperones have been discussed.