Classification G protein–coupled receptor

classification scheme of gpcrs. class (rhodopsin-like), class b (secretin-like), class c (glutamate receptor-like), others (adhesion (33), frizzled (11), taste type-2 (25), unclassified (23)).

the exact size of gpcr superfamily unknown, 800 different human genes (or ~ 4% of entire protein-coding genome) have been predicted code them genome sequence analysis. although numerous classification schemes have been proposed, superfamily classically divided 3 main classes (a, b , c) no detectable shared sequence homology between classes.

the largest class far class a, accounts 85% of gpcr genes. of class gpcrs, on half of these predicted encode olfactory receptors, while remaining receptors liganded known endogenous compounds or classified orphan receptors. despite lack of sequence homology between classes, gpcrs have common structure , mechanism of signal transduction. large rhodopsin group has been further subdivided 19 subgroups (a1-a19).

more recently, alternative classification system called grafs (glutamate, rhodopsin, adhesion, frizzled/taste2, secretin) has been proposed. according classical a-f system, gpcrs can grouped 6 classes based on sequence homology , functional similarity:

class (or 1) (rhodopsin-like)
class b (or 2) (secretin receptor family)
class c (or 3) (metabotropic glutamate/pheromone)
class d (or 4) (fungal mating pheromone receptors)
class e (or 5) (cyclic amp receptors)
class f (or 6) (frizzled/smoothened)

an study based on available dna sequence suggested human genome encodes 750 g protein–coupled receptors, 350 of detect hormones, growth factors, , other endogenous ligands. approximately 150 of gpcrs found in human genome have unknown functions.

some web-servers , bioinformatics prediction methods have been used predicting classification of gpcrs according amino acid sequence alone, means of pseudo amino acid composition approach.