The shapes and actions of<unk>  are commonly determined by X-ray crystallography and computer modelling. These methods have increased the current understanding of drug action at binding sites on the<unk> .<unk>  exist in different types, dependent on their ligand and function:
Some<unk> proteins are peripheral membrane proteins;
Many hormone<unk>  and neurotransmitter<unk>  are transmembrane proteins: transmembrane<unk>  are embedded in the phospholipid bilayer of cell membranes, that allow the activation of signal transduction pathways in response to the activation by the binding molecule, or ligand.
Metabotropic<unk>  are coupled to G proteins and affect the cell indirectly through enzymes which control ion channels.
Ionotropic<unk>  contain a central pore which functions as a ligand-gated ion channel.
Another major class of<unk>  are intracellular proteins such as those for steroid and intracrine peptide hormone<unk> .  These<unk>  often can enter the cell nucleus and modulate gene expression in response to the activation by the ligand.
Metabotropic<unk>  are coupled to G proteins and affect the cell indirectly through enzymes which control ion channels.
Ionotropic<unk>  contain a central pore which functions as a ligand-gated ion channel.
Receptor 
Sensory receptor
Ion transporter
Enzyme
Ion channel
Caspase
Disintegrin
Isozyme