Acetylcholine
Acetylcholinesterase (EC 3.1.1.7, e.g. from Torpedo californica, TcAChE) hydrolysizes the neurotransmitter acetylcholine , producing group. ACh directly binds (via its nucleophilic Oγ atom) within the catalytic triad of (ACh/TcAChE structure 2ace). The residues are also important in the ligand recognition. After this binding acetylcholinesterase ACh.
Acetylcholine Receptors
Nicotinic acetylcholine receptors
The receptor is a transmembrane pentameric glycoprotein. It cylindrical in appearance by electron microscopy approximately 16nm in length and 8nm in diameter. The main ion channel is composed of a water pore that runs through the entire length of the protein. If viewed from the synaptic cleft, the protein will look like a pseudo-symmetrical rosette shown in the picture below composed of 10 different alpha and 4 different beta subunits.
When cobra venom is introduced into the body is moves along the bloodstream to a diaphragm muscle. It works as a postsynaptic neurotoxin binding to the receptor as an extracellular ligand by interacting with OH group leaving the acetylcholine channel open which releases ions used in creating an action potential. There must be 5 molecules of cobra toxin (red) to block the receptor (blue) as each molecule binds with an individual alpha chain on the acetylcholine receptor. The 2nd image depicts an individual toxin binding with one chain on the receptor, both in the same color. . This representation shows each molecule of the .
Muscarinic acetylcholine receptors
M1, M3, M5 receptors are coupled with Gq proteins, while M2 and M4 receptors are coupled with Gi/o proteins. They belong to GPCRs Subfamily A18.
Acetylcholinesterase
Adenosine
at Human A2A receptor.
Adenosine receptors
Gs → cAMP up
Agonists:
- N6-3-methoxyl-4-hydroxybenzyl adenine riboside (B2)
- ATL-146e
- CGS-21680
- Regadenoson
- Adenosine
Antagonists:
- Caffeine
- aminophylline
- theophylline
- istradefylline
- SCH-58261
- SCH-442,416
- ZM-241,385
Adrenaline (Epinephrine)/Noradrenaline (Norepinephrine)
- .
- .
- Phenylethanolamine N-methyltransferase (Noradrenaline N-Methyltransferase) catalyzes the conversion of norepinephrine (noradrenaline) to epinephrine (adrenaline). This is the last step in the conversion of tyrosine to adrenaline[1].
The adrenergic receptors are metabolic G protein-coupled receptors. They are the targets of catecholamines. The binding of an agonist to them causes a sympathetic response.
- The α-2 adrenergic receptor (A2AR) inhibits insulin or glucagons release.
- The β-1 adrenergic receptor (B1AR) increases cardiac output and secretion of rennin and ghrelin.[2]
- The β-2 adrenergic receptor (B2AR) triggers many relaxation reactions.
β1 adrenergic receptor
- β1-adrenergic agonists:
- Beta blockers:
- Metoprolol
- Atenolol
- Bisoprolol
- Propranolol
- Timolol
- Nebivolol
- Vortioxetine
β2 adrenergic receptor
β2-adrenergic agonists:
- Salbutamol (Albuterol in USA)
- Bitolterol mesylate
- Formoterol
- Isoprenaline
- Levalbuterol
- Metaproterenol
- Salmeterol
- Terbutaline
- Ritodrine
- Beta blockers:
- Butoxamine
- Timolol
- Propranolol
- ICI-118,551
- Paroxetine
Monoamine oxidases (MAO)
Monoamine oxidases (MAO) (EC 1.4.3.4) are a family of enzymes that catalyze the oxidation of monoamines including adrenaline, noradrenaline, serotonin and dopamine.
Dopamine
Dopamine Receptors
Dopamine receptors are a class of metabotropic G protein-coupled receptors that are important in the central nervous system. Dopamine receptors are involved in many neurological processes that comprise motivation, pleasure, cognition, memory, learning, and fine motor skills. There are five subtype dopamine receptors, D1, D2, D3, D4, and D5. The D3 receptor is a part of the D2-like family.[3]
Agonists
- Amphetamine[4]
- Methamphetamine[5]
Antagonists
- Clebopride[6]
- Nafadotride[7]
- Eticlopride.
(3pbl).
.
Parkinson's disease
DOPA decarboxylase is responsible for the synthesis of dopamine and serotonin from L-DOPA and L-5-hydroxytryptophan, respectively. It is highly stereospecific, yet relatively nonspecific in terms of substrate, making it a somewhat uninteresting enzyme to study. Although it is not typically a rate-determining step of dopamine synthesis, the decarboxylation of L-DOPA to dopamine by DDC is the controlling step for individuals with Parkinson's disease[8], the second most common neurodegenerative disorder, occuring in 1% of the population over the age of 65. The loss of dopaminergic neurons is the main cause of cognitive impairment and tremors observed in patients with the disease. The hallmark of the disease is the formation of alpha-synuclein containing Lewy bodies.
Currently, treatment for the disease is aimed at DOPA decarboxylase inhibition. Since dopamine cannot cross the blood-brain barrier, it cannot be used to directly treat Parkinson's disease. Thus, exogenously administered L-DOPA is the primary treatment for patients suffering from this neurodegenerative disease. Unfortunately, DOPA decarboxylase rapidly converts L-DOPA to dopamine in the blood stream, with only a small percentage reaching the brain. By inhibiting the enzyme, greater amounts of exogenously administered L-DOPA can reach the brain, where it can then be converted to dopamine. [9]. Unfortunately, with continued L-Dopa treatment, up to 80% of patients experience 'wearing-off' symptoms, dyskinesias and other motor complications (referred to as the "on-off phenomenon". [10]. Clearly, a better understanding of the catalytic mechanism and enzymatic activity of DDC in both healthy and PD individuals is critical to drug design and treatment of the disease.
GABA
GABA receptors
GABA (i.e. gamma-aminobutyric acid) is the primary inhibitory neurotransmitter of the vertebrate central nervous system. GABA can bind one of two different receptor proteins, each using a discrete mechanism to elicit a cellular response. Upon binding with GABA, GABAB receptors (metabotropic) utilize a second messenger amplification pathway that ultimately results in an inhibitory signal for neuronal transmission. This pathway for signal transmission differs from GABAA receptors (ionotropic), which are considered ligand-gated ion channels as the binding of GABA results in the opening of ion channels leading to the inhibition of a neuronal signal.
(PDB code 4ms3).
Glutamate
Glutamate receptors
Ionotropic Glutamate Receptors
Ionotropic Glutamate Receptors (IGluRs) are a family of ligand-gated ion channels that are responsible for fast excitatory neurotransmission.[11] Primarily localized to nerve synapses in mammals, IGluRs are implicated in nearly all aspects of nervous system development and function.[12] Synapses form the connection between two neuronal cells. Within synapses, neurotransmitters are released from vesicles in presynaptic cells and interact with receptors in postsynaptic cells to allow for communication between nerve cells.[11] GluR domains include the amino terminal domain (ATD), transmembrane domain (TMD) and ligand-binding domain (LBD). Glutamate is the predominant neurotransmitter of excitatory synapses and interacts specifically with AMPA and NMDA IGluRs.
Full view of the glutamate receptor shows the overall structure (N-terminal, ligand-binding and transmembrane domains) in and models. is a part of the extracellular domain. This domain is implicated in receptor assembly, trafficking, and localization.
- .
- . This domain widens in response to glutamate binding allowing for positive ions to pass through the post-synaptic membrane.
- .
- .
- Glutamate receptor (GluA2)
The homomeric rat GluA2 receptor arranged in a 'Y'-shape with the . This structure is a functional homotetramer of the AMPA-subtype; native ionotropic glutamate receptors are almost exclusively heterotetramers. .
Domains
The subunits themselves are modular [13]and the major domains are found in layers in the tetrameric structure.
- The 'top' layer is composed of the
- This .
- participates directly in agonist/competitive antagonist binding, affects activation gating, and is the portion that forms the 'middle' layer.
- in the structure.
- The [14], was studied as a treatment for stroke because it had demonstrated neuroprotective efficacy in experimental models of stroke and tolerability in healthy volunteers; however, in a multicenter, double-blind, randomized, placebo-controlled phase II trial, it was found to have significant sedative effects in patients with acute stroke which precludes its further development as a neuroprotective agent[15].
- is the portion that forms the membrane-spanning on the 'bottom' of the solved structure.
- To help give a better idea of how the glutamate receptor is oriented on the cell surface in the membrane lipid bilayer, as calculated by the Orientations of Proteins in Membranes database (University of Michigan, USA) is shown with the red patch of spheres indicating the boundary of the hydrophobic core closest to the outside of the cell and the dark blue patch of spheres indicating the boundary closest to the inside of the cell.
- The carboxy-terminal domain that plays a role in both receptor localization and regulation is not seen in the structure but would be below the transmembrane domain as it is cytoplasmic.
Domain swapping between the subunits and symmetry mismatch between the domains
- Unanticipated is the domain swapping and crossover that occurs between the subunits interactions. In order to discuss the remarkable swapping, it is best to :
A B C D
- Considering each chain, there is crossover as the pairs of subunits seen in the ATD are swapped in the LBD.
- In the ATD domain -
- .
- And the .
- While that is going on, in the ATD there is also inter-pair interactions mediated between . Note this view really highlights the two-fold symmetry between the A-B and C-D pairs at the level of the ATD.
- In the LBD domain -
- Whereas in the ATD domain A and B paired up, in the LBD.
- And the .
- While that is going on, in the LBD there is also extensive inter-pair interactions mediated between . Note this view highlights the two-fold symmetry between the A-D and B-C pairs at the level of the LBD. .
- The domain swapping can be observed from the side following the backbone of each chain as well: , , , and . And .
- The . Thus, remarkably, the symmetry switches from an overall two-fold symmetry for the ATD and LBD to four-fold for the TMD.
As a result of the swapping and symmetry mismatch, there is subunit non-equivalence; even though all the chains are the same chemically, there are 2 distinct conformations of the subunits. This means there are 2 matching pairs of subunits.
-
-
- Subunit A is equivalent to Subunit C (in the small structure window in this section). In the main window, a .
- Subunit B is equivalent to Subunit D (in the small structure window in this section). In the main window, a .
However, each of the subunit A/C group though is distinct from those of the B/D group. Having established the two equivalent groups we can simplify the discussion of the relationship between the two pairs by focusing solely on comparing Subunit A' and Subunit B.
The domains themselves stay relatively static between the two conformational forms, with the linkers in between and the resulting arrangement changing. This is best illustrated by superposition of the individual domains of Subunit A and Subunit B:
between the two conformational forms.
- The linkers are key; besides playing roles in domain swapping and resolving the symmetry mismatch, they are also responsible for relaying the modulation signals from the ATD to the other domains and signaling the conformational change of the LBD to control the opening and closing of the pore. Beyond the two conformations seen here though this particular structure (3kg2) of the receptor does not shed light on the transduction process.
Transmembrane domain architecture and the occluded pore
-
- The segments shown again, .
- There is consistent with the channel being in a closed state with the antagonist ZK200775 bound to the LBD.
- It is that occludes the channel. [BE PATIENT as a small surface is generated.]
- Note . This is in part is why the symmetry is only approximately four-fold and is one of the several intriguing observations in regard to symmetry for this macromolecule. In fact, the location of 2-fold symmetry at the ends of M3 is just above the portion that spans the membrane and is close to the last region of the structure that doesn't show four-fold symmetry as abruptly below this point everything is 4-fold symmetric.
- To better observe the contributions of each of the membrane segments to the subunit-subunit interactions, . [Note: this scene generates a substantial surface which may take about a minute to calculate. Be patient.]
- Note that the M4 segment associates with the ion-channel core of an adjacent subunit.
- .
- The TMD domain of the GluA2 receptor shares structural and sequence similarity with the pore region of the potassium (K+), as hinted at by earlier work[16][17][18]. Here the pore region of Streptomyces lividans potassium channel (1bl8), specifically the . The of the K+ channel even though these portions weren't even included in the calculation of the alignment seen here.
Metabotropic Glutamate Receptors
Metabotropic glutamate receptors are glutamate receptors that activate ion channels indirectly through a signaling cascade involving G proteins[19]. They are members of the large class of seven-transmembrane domain receptors, the G protein-coupled receptors. Glutamate receptors are classified into 3 groups based on their homology, mechanism and pharmacological properties.
Histamine
.
Histamine receptors
Allergy symptoms are mostly caused by the release of histamine in response to allergens. The binding of histamine to the extracellular portion of the H1 receptor triggers a structural change of the transmembrane portion, leading to a change in the C terminal area. This c terminal region interacts with G proteins, leading to the activation of the Gq signalling pathway, which triggers allergy symptoms like itchy eyes and runny noses. Many allergy drugs are anti-histamines, in that they bind to the histamine receptor but do not cause the conformational change that leads to a response.
Neurotensin
Neurotensin receptors
The neurotensin receptor (NTSR1) belongs to the superfamily of proteins known as G protein-coupled receptors (GPCRs). Currently around 800 G protein-coupled receptors have been identified and are hypothesized to be responsible for roughly 80% of signal transduction.[20] GPCRs are involved in a vast array of physiological processes within the body that range from interactions with dopamine to effects on secretion of bile in the intestines.[21] [22] Due to the vast array of functions that these proteins serve and their high abundance within the body, these proteins have become major drug targets.[23]
The ligand for NTSR1 is the 13 amino acid peptide, neurotensin (NTS)[24], and the majority of the effects of NTS are mediated through NTSR1[24]. NTS has a variety of biological activities including a role in the leptin signaling pathways [25], tumor growth [26], and dopamine regulation [27]. NTSR1 was crystallized bound with a C-terminal portion of its tridecapeptide ligand, . The shortened ligand was used because of oits higher potency and efficacy than its full-length counterpart[24].
Neurotensin receptor
Serotonin
Serotonin receptors
5-hydroxytryptamine (5-HT), Serotonin receptors are found on the membrane of neurons in the central nervous system and peripheral nervous system. These receptors allow for the body to respond to serotonin and regulate many biological pathways. Serotonin, also known as 5 hydroxytryptamine, is an endogenous neurotransmitter made from tryptophan and is largely found in the gastrointestinal tract. It is known to regulate mood, appetite, digestion, circadian rhythm, learning and internal temperature regulation. It is can be an inhibitory or excitatory neurotransmitter that is released into the synaptic space and can bind to receptors on the postsynaptic neuron or be taken back up into the presynaptic neuron via Serotonin re-uptake transporters.[28] 5-HT receptors are classified into 7 different subfamilies (5-HT1, 5-HT2, 5-HT3, etc.) by signaling mechanisms and homology of structure. All 5-HT receptors are known to have G-protein linked pathways except for the 5-HT3 receptor which acts as an ion channel. [29]
