Cell surface receptors with direct ligand effect

Many hormones cannot pass through the
plasma membrane; instead, they interact with
cell surface receptors. Their effects are direct
and very rapid. With ligand-activated (or ligand-
gated) ion channels (1), binding of the ligand
to the receptor changes the conformation of
the receptor protein. This causes an ion-specific
channel in the receptor protein to open. The resulting
flow of ions changes the electric charge
of the cell membrane. Receptors with ligandactivated
protein kinase (2) further activate a
substrate protein. Most protein kinases
phosphorylate tyrosine (tyrosine kinase),
serine, or threonine by transferring a phosphate
residue from adenosine triphosphate (ATP),
which is then converted to adenosine diphosphate
(ADP). Other receptors mediate the
removal of phosphate from a phosphorylated
tyrosine side chain by means of their
phosphatase activity (3). With one important
type of receptor, ligand binding activates
guanylate cyclase (4), which catalyzes the formation
of cyclic guanosine monophosphate
(cGMP) from guanosine triphosphate (GTP).
The cGMP functions as a second messenger and
brings about a rapid change of activity of
enzymes or nonenzymatic proteins. Removal or
degradation of the ligand reduces the concentration
of the second messenger and ends the
reaction.