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Neurotransmitters and hormones Tissue specialization
of amino acid metabolism The metabolism of amino acids serves as stage for the formation of many small signaling molecules used as hormones and neurotransmitters. Neurotransmitters are synthesized in neurons and act locally. They are released into a synaptic cleft providing a diffusible chemical signal from one cell to the next, cells which are not in direct contact with each other. These synapses are therefore referred to as chemical synapses, as opposed to electrical synapses, which use a gap junction mediated coupling of action potentials without a mediating chemical signaling mechanism. The signal transmission of the latter is much faster than that found in chemical synapses. Neurotransmitters bind to receptor proteins and channels and initiate signaling events mainly related to electrical transmissions such as action potentials, or stimulating calcium waves in the cytoplasm. Some signaling molecules are not only used locally and thus serve as hormones having a larger range of activity, like epinephrine, which affects carbohydrate and lipid metabolism in various target organs, far away from the site of synthesis. Other types include thyroxin, an amino acid derivative produced in the thyroid gland and important for organ differentiation, growth, and metamorphosis, and nitric oxide, NO, a short lived free radical gas that acts as hormone and neurotransmitter. Nitric oxide's role in mammalian physiology had been discovered in the 1980s. Tissue specialization is an important aspect of amino acid metabolism. While neurotransmitters and hormones are synthesized in neurons and specialized glands, it is the liver and skeletal muscle that can utilize amino acid degradation for energy production, while excess aromatic acids like phenylalanine are disposed by the kidney. Tissue specialization for tryptophan produces glucose, ketone bodies, and nicotinamide (for NAD synthesis) in the liver, serotonin in neurons (mood control) and mast cells (allergy, inflammation), while gastro intestinal (GI) bacteria break down tryptophan to provide the host organism with indole ring precursors.
Neurotransmitters Neurotransmitters fall into two broad classes. The first group consists of the amino acids aspartate, glutamate, its decarboxylated form g -amino-butyric acid (GABA), and glycine. The second group contains the biogenic amines acetylcholine, serotonin, histamine, and the catecholamines epinephrine, nor-epinephrine, and dopamine. Except for acetylcholine, all biogenic amines are derived from aromatic amino acids tryptophan and tyrosine. To properly understand the metabolism of neurotransmitters, one has to study their 'life cycle' - (i) biosynthesis, (ii) storage, (iii) release, and (iv) degradation or re-uptake. Acetylcholine Acetylcholine is a neurotransmitter found in cholinergic synapses that provide a stimulatory transmission in the nervous system. Acetylcholine (C01996) is synthesized from choline and acetyl-CoA by the enzyme choline acetyl transferase (EC 2.3.1.6) to form acetylcholine, which is immediately stored in small vesicular compartments closely attached to the cytoplasmic side of presynaptic membranes. Acetylcholine synthesis is a process that occurs only in the specialized region of neurons called synapses. The choline used for the synthesis of acetylcholine is derived from the phospholipid phosphatidylcholine. Enzymes called phospholipases catalyze the degradation of phosphatidylcholine (PC). Two independent pathways have been established for the release of choline. First, phospholipase D (EC 3.1.4.4) cleaves the phosphoester bond towards the choline headgroup forming free choline and the membrane bound phosphatidic acid (PA). Phospholipase D is predominantly localized to late endosomes and lysosomes (FEBS Lett 1999; 442(2-3):221-5). Choline is then covalently linked with an activated acetyl unit from acetyl-CoA to form acetylcholine. This reaction is performed by choline-acetyl transferase (EC 2.3.1.6). Second, PC is degraded into its glycerol backbone and fatty acid components by the sequential action of phospholipases A and B. Phospholipase A1 (EC 3.1.1.32) or Phospholipase A2 (EC 3.1.1.4) removes the acyl chain from the C1 position forming a free fatty acid and lysophosphatidylcholine (lysolecithin). The second phospholipase B (EC 3.1.1.5) removes the C2 acyl residue to form glycerol-3-phospocholine and a fatty acid. Glycerol-3-phospocholine is hydrolyzed to glycerol-3-phosphate (G3P) and choline. Acetylcholine is then accumulated and stored in synaptic vesicles via a vesicular acetylcholine transporter. These vesicles are closely associated with the cytoplasmic surface of the presynaptic membrane. Phospholipase D may play a role in guiding these acetylcholine loaded vesicle to the membrane. This is based on evidence that phospholipase D activity is associated with control of membrane vesicular transport. This lipase may thus provide a double mechanism for choline synthesis and the cytoplasmic transport of acetylcholine containing vesicles. Upon a stimulus from an action potential and mediated by calcium induced membrane fusion (exocytosis), up to 300 vesicles per synapse release their contents (acetylcholine) into the synaptic cleft, instantly providing a high concentration of neurotransmitters. Acetylcholine concentration temporarily raises from 10nM to 0.5mM, a 50,000 fold increase occurring in about a millisecond. Acetylcholine rapidly diffuses across the cleft (20 to 50 nm) binding to nicotinic acetylcholine receptors located in the post-synaptic membranes found at neuro-muscular junctions. These receptors initiate an action potential event in the muscle cell membrane causing a massive influx of extra-cellular calcium, thereby triggering muscle contractions. The exocytotic process of acetylcholine release can be inhibited by botulinum toxin. This potent toxin prevents the membrane fusion process and causes muscle paralysis. Botulinum toxin is actually a mixture containing eight distinct proteins produced by the anaerobic bacteria Clostridium botulinum. So called botulism is a major cause of food poisoning related to unrefrigerated meat. Acetylcholine esterase (AChE) Acetylcholine is rapidly removed by acetylcholine esterase (EC 3.1.1.7; AChE). The turnover rate of hydrolysis is 2.5x104 molecules per second. This hydrolytic degradation ensures that the signal does not overstimulate the post-synaptic membrane. A molecular dynamics simulation published in 1994 in the Journal 'Science' suggested that electrostatic fields funnel acetylcholine into the active site channel and release the hydrolysis products through a 'back door' (Science, 1994, 263(6161)1276). The products choline and acetate are inactive molecules and are reabsorbed by the synapse and recycled to replenish acetylcholine containing vesicles for subsequent chemical transmission. AChE is a serine esterase and has a catalytic triad similar to that found in serine proteases trypsin and chymotrypsin. The triad consists of Ser200, His440, and Glu237 (note that serine proteases contain a aspartate and not glutamate). The enzyme is located on the surface of the post-synaptic membrane and linked by a GPI anchor. AChE can be inhibited resulting in the overstimulation of neuromuscular junctions. This leads to spasms and death by suffocation because the heart muscles experience severe arrhythmia. AChE inhibitors have been used for a long time by the military as nerve gas. The most prominent are tabun and sarin, which are specific for the human acetylcholine esterase. An insect specific AChE inhibitor, malathion, which does not bind to the human isoform of the enzyme, is used by citrus crop producers to fight against Mediterranean fruit fly (Medfly; Ceratitis capitata) infestations. Glutamate and GABA Glutamate (C00025) and its decarboxylated derivative gamma aminobutyric acid (GABA; C00334) are the major excitatory and inhibitory neurotransmitters in the central nervous system, respectively. Glutamate decarboxylase (EC 4.1.1.15) is a pyridoxal-phosphate dependent protein removing the alpha-carboxyl group from glutamate. GABA, if not used as neurotransmitter, can be deaminated to form succinate by an aminotransferase reaction involving the conversion of a-ketoglutarate to glutamate. Thus, succinate can be generated from alpha-ketoglutarate in a citric acid cycle by-pass reaction located in the cytoplasm of neurons. This process is known as GABA shunt. (KEGG pathway MAP00251 glutamate metabolism) GABA is one of the brain's major inhibitory neurotransmitter (in the CNS, the other is glycine, which is predominantly found in the spinal cord) by activating a chloride selective receptor ion channel (GABA receptor). The active phase of these chloride conducting channels stabilizes the negative resting potential of the post-synaptic membrane, counteracting any stimulatory activity by glutamate or acetylcholine. Serotonin The neurotransmitter serotonin (C00780) is found in the brain, lung, and gastro intestinal (GI) system. It is a major control substance of smooth muscle contractions. In the central nervous system, serotonin is involved in fear and flight responses, an activity which is opposed by the aggression stimulating hormone adrenaline (epinephrine) and neurotransmitter dopamine. Serotonin is derived from tryptophan in two reaction steps. First, tryptophan is hydroxylated by tryptophan 5-monooxygenase (EC 1.14.16.4) to form 5-hydroxy-tryptophan (C00643). The net reaction of monooxygenase includes the coenzyme tetrahydrobiopterine: L-Tryptophan + Tetrahydrobiopterin + O2 = 5-hydroxytryptophan is decarboxylated by tryptophan decarboxylase (EC 4.1.1.28) in a reaction analogous to glutamate decarboxylase. Many decarboxylases are more or less substrate specific. Tryptophan decarboxylase, a pyridoxal-phosphate protein, acts on three different substrates including L-tryptophan, 5-hydroxy-L-tryptophan and dihydroxy-L-phenylalanine. Serotonin, after being secreted into the synaptic cleft, is removed from this extra-cellular location by an active (energy consuming) reuptake mechanism, which pumps its back into the synapse. Thus, serotonin is not degraded outside the cell, but by the mitochondrial enzyme monoamine oxidase (MAO; EC 1.4.3.4). The end product of this degradative pathway is 5-hydroxyindolacetate (C05634), which is not metabolized any further, but instead secreted in the urine. Lack of serotonin is often associated with depression. Restoring the normal or enhanced level of this neurotransmitter acts as mood enhancer. Prozac is a mood enhancing drug, which acts in the central nervous system by inhibiting the reuptake mechanism of serotonin into the synapse. Since serotonin is not degraded in the synaptic cleft, Prozac promotes a prolonged presence of serotonin keeping the post-synaptic membrane active. Inhibition of MAO also leads to a prolonged activation of serotonergic synapses. Although the molecular mechanism of reuptake inhibition is fairly well known, the relationship between serotonin, or better its receptors (eight different subtypes known thus far), and mental stability is not understood. Melatonin Melatonin (C01598), synthesized by the pineal gland and retina, is the chemical messenger which allows seasonal animals including man to perceive day length changes. It is derived from serotonin by an acetylation catalyzed by serotonin N-acetyltransferase (EC 2.3.1.87) to form acetylserotonin (C00978). The latter is methylated to melatonin. The methyl group is donated from S-adenosylmethionine and the reaction catalyzed by N-methyltransferase (EC 2.1.1.4). Melatonin production is regulated by light through the retino-hypothalamic tract. The N-acetyltransferase levels are under hormonal control. It is the stimulatory effect of norepinephrine that activates gene expression. Besides controlling sleep patterns, melatonin is also involved in the modulation of mood, sexual behavior, reproductive alterations, and immunological functions. It is also studied as an anti-oxidant molecule in the blood. The critical (rate limiting) step in its synthesis depends on N-acetyltransferase. It has been found that the circadian rhythm is controlled by melatonin blood plasma concentrations. The concentration at night is about 3x higher than during the day. Melatonin uses its own receptors, Mel1 and Mel2. The short description reproduced below the current findings on the structure and function of melatonin receptors (by Jocker et al., C R Seances Soc Biol Fil 1998;192(4):659-67):
The neurotransmitter dopamine (C03758) is a stimulatory neurotransmitter and often functions as a physiological antagonist of serotonin, using its own receptors. Dopamine is also a precursor for the synthesis of the hormones nor-epinephrine (nor-adrenaline) and epinephrine (adrenaline). See KEGG pathway MAP00350 for details. Tyrosine is hydroxylated by monophenol monooxygenase (EC 1.14.18.1) in a reaction similar to tryptophan hydroxylase forming L-DOPA (C00355). This enzyme also catalyzes the formation of melanin (see albinism). Here we have a neuron specific isoform of the monooxygenase. The decarboxylation of L-DOPA is catalyzed by EC 4.1.1.25 or tyrosine decarboxylase. Dopamine is hydroxylated by dopamine beta-monooxygenase (EC 1.14.17.1) to nor-adrenaline (C00547), which is methylated in the presence of S-Adenosyl-L-methionine by noradrenaline N-methyltransferase (EC 2.1.1.28) to form adrenaline (C00788). Note that while dopamine is a neurotransmitter of the central and peripheral nervous system and thus acts as a local chemical messenger, both adrenaline and nor-adrenaline are hormones and are produced in the adrenal medulla containing the corresponding tissue specific enzymes. This is the reason why tyrosine monooxygenase (EC 1.14.18.1) can form either the catecholamines in neurons or adrenal medulla, or melanin in skin epithelia. It is important to keep in mind that the complicated pathways presented in KEGG like the one for tryptophan metabolism does not exist in all cells due to cell type specific activation or inhibition of gene expression. Thus an intermediate like L-DOPA does not accumulate in the adrenal medulla. Thyroxine The thyroid gland is a specialized organ that releases the growth hormone thyroxine. Thyroxine (C01829) is synthesized from the aromatic amino acid tyrosine by multiple iodination, rearrangement and hydrolysis steps. The process occurs on the specialized protein thyroglobin using the protein's tyrosine residues as substrate. Thyroglobin contains about 140 tyrosine residues. Tyrosine residues are iodinated at position 3 and 5 of the benzene ring to form the intermediates 3-iodo-L-tyrosine and 3,5-diiodo-L-tyrosine. The enzyme catalyzing the reaction is called iodoperoxidase (EC 1.11.1.8). Two diiodo-tyrosine residues are linked through an ether bond forming thyroxine, which at this point is still part of thyroglobin. During the rearrangement, the mechanism of which is not fully elucidated, epoxide intermediates are likely to form. Note that all reaction steps are catalyzed by the thyroid specific peroxidase. Tyrosine residues next to each other on the surface of thyroglobin are first iodinated and then covalently linked through a ether bridge. The rearrangement reaction involves the formation of possibly epoxide intermediates. The exact reaction mechanism is not fully understood. Research on non-protein bound tyrosine yields reactive epoxide intermediates with structures that may resemble the intermediates found during thyroxine synthesis on thyroglobin. Thyroglobin linked thyroxine units are not physiologically active. Thyroglobin acts as a thyroxine storage device and the proteolysis acts as control mechanism of hormone activation. Thyroglobin is proteolytically degraded (peptide bond hydrolysis) and free thyroxine units are released. This free thyroxine is subsequently secreted into the blood plasma where it is bound to plasma proteins like globins, pre-albumin and albumin. Thyroxine enters target cells where it controls gene expression by binding to nuclear receptor proteins (transcription factors in eukaryotic cell nuclei). It acts as a morphogen by controlling the growth and differentiation of cells. For example, it has been identified as the hormone controlling metamorphosis of tadpoles to frogs. |
