L10 Metabolism of Nitrogenous Compounds II Amino Acids, Porphyrins, and Neurotransmitters
Metabolism of Nitrogenous Compounds II: Amino Acids, Porphyrins, and Neurotransmitters
- 氮化合物代谢II：氨基酸，卟啉，神经递质
一、Pathways of Amino Acid Degradation
Glucogenic amino acids (生糖氨基酸) are amino acids whose carbon skeletons are degraded to one of these five intermediates:
- Pyruvate 丙酮酸
- Ananine
- Glycine
- Cystenine
- Serine
- Threonine
- Tryptophan
- α-Ketoglutarate α-酮戊二酸
- Proline
- Arginine
- Histidine
- Glutamine
- Glutamate (Glutamic acid)
- Succinyl-CoA 琥珀酰-CoA
- Methionine
- Threonine
- Isoleucine
- Valine
- Fumarate 延胡索酸
- Aspartate (Aspartic acid)
- Tyrosine
- Phenylalanine
- Oxaloacetate 草酰乙酸
- Asparagine
- Aspartate (Aspartic acid)
Ketogenic amino acids (生酮氨基酸) are amino acids whose carbon skeletons are degraded to:
- Acetyl-CoA
- Threonine
- Tryptophan
- Leucine
- Isoleucine
- Acetoacetate 乙酰乙酸
- Tryptophan
- Leucine
- Lycine
- Phenylalanine
- Tyrosine
- Pyruvate 丙酮酸
Some amino acids are both glucogenic and ketogenic.
Fates of the amino acid carbon skeletons
- Orange: glucogenic
- Blue: ketogenic
- Purple: glucogenic and ketogenic
Pathways of Amino Acid Degradation
Like other gasotransmitter（气体递质） molecule, nitric oxide (NO), hydrogen sulfide (H2S), derived from cysteine, is a powerful gaseous signaling molecule involved in the regulation of vascular blood flow and blood pressure.
- The cardioprotective and antihypertensive effects of dietary garlic（大蒜） are mediated in large part by the production of H2S from organic polysulfides (多硫化物) that are abundant in garlic.
1. Glucogenic Amino Acids
Oxaloacetate Family of Glucogenic Amino Acids
- Asparaginase: acute lymphoblastic leukemia（急性淋巴细胞白血病）; remission
Asparaginase (门冬酰胺酶) catalyzes hydrolytic cleavage of the asparagine amide to aspartate and ammonium.
Aspartate is then transaminated directly to oxaloacetate by aspartate transaminase. (谷草转氨酶)
α-Ketoglutarate Family of Glucogenic Amino Acids
- Arg, His, Pro, Glu, Gln
(1) Histidine and histamine
Histidine also undergoes decarboxylation to generate histamine (组胺), a substance with multiple biological actions.
When secreted in the stomach, histamine promotes the secretion of HCl and pepsin (胃蛋白酶).
- It is a potent vasodilator (血管扩张剂), released locally in sites of trauma, inflammation, or allergic reaction.
Antihistamines (抗组胺药)are in use to treat allergies and other inflammations.
- Typically, these drugs prevent the binding of histamine to its receptors.
Succinyl-CoA Family of Glucogenic Amino Acids
Isoleucine, valine, threonine, and methionine are degraded to the citric acid cycle intermediate, succinyl-CoA, by way of propionyl-CoA (丙酰辅酶A)
Branched-chain amino acid (支链氨基酸) oxidation, fatty acid β-oxidation, and the citric acid cycle share a common chemical strategy:
- A deficiency of branched-chain α-keto acid dehydrogenase complex, which metabolizes valine, leucine, and isoleucine in humans, leads a severe mental developmental defect called maple syrup urine disease (枫糖尿症).
Maple syrup urine disease is caused by deficiency of branched-chain α-keto acid dehydrogenase complex.
Therefore, the isoleucine, valine and leucine cannot undergo transamination and the three amino acids and their α-keto acid derivatives cannot be efficiently used.
Thus, they accumulate in the urine which gives the urine an sweet-smell odor. This disease will cause severe mental retardation and nervous developmental defect.
Pyruvate Family of Glucogenic Amino Acids
Mechanism of serine dehydratase (脱水酶) reaction:
- PLP;
- Schiff base
- Alanine, cysteine, glycine, serine and threonine are degraded to pyruvate.
- PLP
Fumarate Family of Glucogenic Amino Acids
2. Ketogenic amino acids
Lysine Degradation
Although lysine is degraded by several pathways, the major route in mammals, the saccharopine pathway (酵母氨酸途径), does indeed follow this predicted strategy.
Saccharopine is the intermediate in the two-step process that removes the $\varepsilon$-amino group from lysine
- These two reactions are catalyzed by a bifunctional enzyme named α-aminoadipic semialdehyde synthase（α-氨基己二酸半醛合成酶）.
The mammalian enzyme is a single protein composed of separate domains for the two catalytic activities.
Although this process uses α-ketoglutarate as the amino acceptor, and produces glutamate, it is more similar to the argininosuccinate synthetase–argininosuccinase reactions of the urea cycle than to a transamination
Tryptophan Degradation
Metabolic fates of tryptophan:
- The major catabolic pathway degrades most of the tryptophan molecule to acetoacetate and alanine.
- The synthetic pathway to the nicotinamide nucleotides (烟酰胺核苷酸).
Pellagra (nicotinamide deficiency) (糙皮病（烟酰胺缺乏）)
The pellagra is caused by nicotinamide deficiency and this deficiency can be caused by low diet niacin (烟酸) or low tryptophan uptake and low level niacin absorption (caused by diet disorder, alcoholism（酒精中毒）, some kinds of drug treatment or addiction, etc.).
- Because the tryptophan can finally be converted into nicotinamide, the tryptophan uptake can influence the nicotinamide level.
The pellagra will cause abnormal excitation, dermatitis（皮炎）, dementia（痴呆）, and diarrhea（腹泻、痢疾）, because nicotinamide plays important roles in normal biological processes.
The amide nitrogen of glutamine is used in many amidotransferase reactions leading to purine and pyrimidine nucleotides, amino sugars, and nicotinamide nucleotides.
Phenylalanine and Tyrosine Degradation
- Catabolism of phenylalanine and tyrosine to fumarate and acetoacetate
The phenylalanine hydroxylation system:
Conversion of phenylalanine to tyrosine is catalyzed by phenylalanine hydroxylase (苯丙氨酸羟化酶).
Aromatic amino acid hydroxylase:
- Phe hydroxylase;
- Tyr hydroxylase;
- Trp hydroxylase.
Cofactor: Tetrahydrobiopterin (BH4)
A hereditary deficiency of phenylalanine hydroxylase is responsible for phenylketonuria (PKU) (苯丙酮尿症)
- In PKU, phenylalanine accumulates to very high levels (hyperphenylalaninemia (高苯丙氨酸血症)) because of the block in conversion to tyrosine, and much of this phenylalanine is metabolized via pathways that are normally little used—particularly transamination to phenylpyruvate (a phenylketone), and also subsequent conversion of phenylpyruvate to phenyllactate and phenylacetate.
(1) Phenylketonuria (PKU): a hereditary deficiency of phenylalanine hydroxylase
1/10,000;
An autosomal recessive trait, meaning that two parents heterozygous for the trait have 1 chance in 4 of having a phenylketonuric child;
- 2% of the population are carriers;
Phenylalanine accumulates to very high levels (hyperphenylalanine);
- Phenylpyruvate, phenyllactate, phenylacetate in urine in enormous quantities (1-2 grams per day);
If undetected and untreated, PKU leads to profound mental retardation;
- Phenylalanine itself is the neurotoxic molecule;
Fortunately, PKU can readily be detected at birth, and many hospitals carry out routine screening of newborns;
- If the condition is detected early, the onset of retardation can be prevented by feeding for several years a synthetic diet low in phenylalanine and rich in tyrosine, to allow normal development of the nervous system.
(2) Alkaptonuria - “dark urine disease”
Alkaptonuria is caused by deficiency of homogentisate dioxygenase in humans.
The tyrosine can be converted into p-hydroxyphenylpyruvate, the p-hydroxyphenylpyruvate can then be converted into homogentisate catalyzed by homogentisate dioxygenase.
Therefore, the deficiency of homogentisate dioxygenase makes the homogentisate accumulation inside of the cells and accumulation in urine, the oxidation of homogentisate during emiction will turn the urine into a dark color.
p-Hydroxyphenylpyruvate is converted to homogentisate by p-hydroxyphenylpyruvate dioxygenase (对羟基苯丙酮酸), an unusual iron-containing enzyme, which catalyzes decarboxylation, a ring hydroxylation, and side chain migration, using ascorbate as a cofactor.
Procollagen prolyl hydroxylase catalyzes the same chemistry.
This reaction involves a mechanism called the NIH shift, after scientists at the NIH, who described a ring hydroxylation that proceeds via formation of an epoxide (环氧) intermediate and migration of the alkyl side chain.
The NIH shift is the intramolecular movement of a hydrogen atom in an aromatic ring during a hydroxylation reaction.
Alkaptonuria (“dark urine disease”, 黑尿症): homogentisate (尿黑酸) accumulates and is excreted in large amounts in the urine; its oxidation on standing causes the urine to become dark.
A hereditary deficiency of homogentisate dioxygenase.
二、Amino Acids as Biosynthetic Precursors
S-Adenosylmethionine （AdoMet） and biological methylation
AdoMet is an active form of methionine, which can be generated from methionine and ATP.
AdoMet always involves in transmethylations (such as biosynthesis of creatine and phosphatidylcholine) as an methyl group donor and then forms an S-adenosylhomocysteine (AdoHcy). AdoMet plays an important roles in DNA and protein methylation.
Most, though not all, of the group transfer reactions involving AdoMet are transmethylations (转甲基化), in which the methyl group is transferred to an acceptor, with the other product being S-adenosylhomocysteine (AdoHcy, S-腺苷同型半胱氨酸).
- Epigenetics
There are indications that protein methylation protects proteins, in at least two ways:
- By blocking sites of ubiquitination, methylation evidently helps protect proteins from turnover.
- Spontaneous damage of protein molecules during aging causes deamidation, isomerization, and racemization (脱酰胺，异构化，外消旋) of their asparagine and aspartate residues.
- A methylation reaction can initiate the repair of these damaged residues.
Methyl group metabolism and homocystinuria (同型胱氨酸尿症)
Serine hydroxymethyltransferase (丝氨酸羟甲基转移酶) catalyzes the entry of one-carbon units into the tetrahydrofolate (THF) pool.
A deficiency of methylenetetrahydrofolate reductase (MTHFR, 亚甲基四氢叶酸还原酶) or methionine synthase blocks the conversion of homocysteine to methionine.
- Serine hydroxymethyltransferase;
- Methylenetetrahydrofolate reductase (MTHFR);
- Methionine synthase;
- Cystathionine β-synthase.
Severe mental retardation（智力迟钝,智力缺陷）, damage to vessels, dislocation of the lens of the eyes
Whereas the one-carbon and methyl cycles are found in virtually all cells, liver and kidney possess an additional route for methionine synthesis that involves a different kind of transmethylation.
Homocysteine can be remethylated using betaine (甜菜碱) as the methyl donor in a reaction catalyzed by the cytosolic betaine-homocysteine methyltransferase.
- Betaine, also known as trimethylglycine, is formed from the mitochondrial oxidation of choline（胆碱）.
Homocysteinemia (同型半胱氨酸血症) can result from:
Genetic defects in:
- Cystathionine (胱硫醚) β-synthase
Methionine synthase
- 5,10-Methylenetetrahydrofolate reductase （MTHFR）
Dietary deficiency of:
- Folic acid
- Vitamin B6
- Vitamin B12
Glutamate And Polyamines
- Polyamines (多胺) are required for cell proliferation because of their roles in stabilizing duplex DNA structures
Polyamines are special compounds including spermidine (亚精胺), spermine（精胺,精素） which are synthesized from glutamate.
Because the amino groups of polyamines are always positively charged, the polyamines can stabilize conformations of negatively charged nucleotides, therefore they play important roles in duplex DNA structure stabilization during cell proliferation.
In addition to serving as a precursor for ornithine and the polyamines, glutamate is one of several amino acids serving as precursors to compounds that function in transmission of nerve impulses.
- Decarboxylation of glutamate yields –aminobutyric acid, or GABA (氨基丁酸).
In addition, glutamate itself is a neurotransmitter.
Glutathione (谷胱甘肽) and the γ-glutamyl cycle (γ-谷氨酰循环)
- Transport amino acids into cells
The enzymes involved are:
- Glutamylcysteine (谷氨酰半胱氨酸) synthase
- Glutathione synthase
- Glutamyl transpeptidase (转肽酶)
- Glutamyl cyclotransferase (环转移酶)
- 5-Oxoprolinase
- Dipeptidase (二肽酶)
Glutathione reacts with such a compound (denoted RX) as shown here, followed by cleavage of the γ-glutamyl and glycyl residues and then acetylation by acetyl-CoA to give a mercapturic acid.
This more soluble, less toxic derivative of the original compound can then be excreted in the urine.
Biosynthesis of nitric oxide and creatine phosphate from arginine
- In kidney
- In NO synthesis, the Nw-hydroxy-L-arginine intermediate remains tightly
- bound to NO synthase (NOS).
Biosynthesis of thyroid hormones as residues in the protein thyroglobulin （甲状腺球蛋白）
Biosynthesis of thyroid hormones (甲状腺激素) as residues in the protein thyroglobulin (甲状腺球蛋白)
- The iodinated forms of tyrosine—triiodothyronine (T3) and thyroxine (T4) —are released from these proteins by proteolytic degradation.
One result of iodine deficiency is goiter (甲状腺肿), a condition in which the thyroid gland grows abnormally large as it attempts to scavenge all available iodine.
- Thyroglobulin：
- 2750 aa
- 140 Tyr
- 20% Tyr: iodinated
Tyrosin:
- The synthesis of melanins (黑色素) occurs in pigment-producing cells, the melanocytes.
Albinism (白化病): genetic deficiency of tyrosinase;
Albinism is caused by deficiency of tyrosinase, which catalyzes the conversion from tyrosine into dopaquinone (DQ, 多巴醌) and DoPa (多巴,二羟基苯丙氨酸, dihydroxyphenylalanine) into DQ.
- The DQs can subsequently be converted into important molecules involving pigments such as melanin.
The albinism will cause an individual to lose pigmentation therefore lose colors. One example is the albino mice.
- The albino (white) mice and rats commonly used in research have an inherited defect in tyrosinase
- Dark vs. white
- SNP: single nucleotide polymorphism
- SLC24A5: Ala111Thr
- Melanin reduction
三、Porphyrin and Heme Metabolism
Biosynthetic pathways to tetrapyrroles (四吡咯)
Tetrapyrroles include:
- Heme
- Chlorophylls 叶绿素
- Phycobilins 藻胆色素
- Cobalamins 钴胺素
All are synthesized from δ-aminolevulinic acid （ALA）, which is formed differently in plants from the way it is formed in bacterial and animal cells.
The heme biosynthetic pathway
All of the nitrogen of heme is derived from glycine, and all of the carbon is derived from succinate and glycine.
- Hence, this synthesis is often called the succinate-glycine pathway.
Congenital erythropoietic porphyria (先天性卟啉症): uroporphyrinogen III synthase defective, the symmetrical (and metabolically useless) type I prorphyrins accumulate beyond the capacity of the body to excrete them. Urine turn to red, skin is photosensitive (like darkness), teeth become fluorescent, RBC destroyed, anemia, ….. Vampires
The congenital erythropoietic porphyria is caused by deficiency of uroporphyrinogen III synthase, therefore the useless type Ⅰ porphyrins (uroporphyrinogen Ⅰ) can be produced instead of the production of uroporphyrinogen III .
The uroporphyrinogen Ⅰcan be further converted into coproporphyrinogen Ⅰ and causes accumulation of these two molecules.
Because the uroporphyrinogen III can be used for the biosynthesis of some important molecules including heme, cobalamins (in bacteria), chloroplasts (in plants), etc. . Therefore, the congenital erythropoietic porphyria will cause the urine turned to red because of excess coproporphyrinogen I production, making the skin more susceptible to light and making the teeth to be fluorescent.
ALA synthetase: major control point: heme and related compound feedback-inhibit the enzyme
Porphyrin biosynthesis involves:
- Formation of a pyrrole (吡咯) ring.
- Condensation of four pyrrole moieties, giving a cyclic tetrapyrrole (环四吡咯).
- Side chain modifications and ring oxidations.
Porphyrias (卟啉症) involve abnormal accumulations of heme precursors, either from overproduction of the unnatural type I porphyrins or from abnormally high flux through d-ALA synthetase.
Catabolism of heme
- Billiverdin: 胆绿素
- Billrubin: 胆红素
Most of the heme comes from breakdown of aged erythrocytes, but some comes from cytochromes and other heme proteins.
Heme protein degradation in animals releases amino acids and iron, which are reused, and bilirubin, which must be solubilized for excretion.
Jaundice（黄疸）: skin and the whites of the eyes
Liver diseases, bile duct obstruction (gallstones), Rh incompability reactions of infants, or in premature infants
- Jaundiced infants are often placed under intense fluorescent light, which rearranges the structure of circulating bilirubin to more soluble products.
四、Amino Acids and Their Metabolites as Neurotransmitters and Biological Regulators
Glutamate, tyrosine, glycine, and tryptophan serve as neurotransmitters or precursors to neurotransmitters.
The pathway to serotonin (血清素) begins with hydroxylation of tryptophan (四氢生物蝶呤) by a tetrahydrobiopterin-dependent aromatic amino acid hydroxylase, similar to phenylalanine hydroxylase.
This reaction is followed by a PLP-dependent decarboxylation to yield serotonin.
Tryptophan hydroxylase and tyrosine hydroxylase are both tetrahydrobiopterin-dependent monooxygenases and are mechanistically and structurally related to phenylalanine hydroxylase.
The mechanism of all three members of the aromatic amino acid hydroxylase family involves an NIH shift, with migration of a hydride accompanying the hydroxylation.
Biosynthesis of the catecholamines（儿茶酚胺）—dopamine, norepinephrine, and epinephrine
- 儿茶酚胺:-多巴胺，去甲肾上腺素，肾上腺素
Tyrosine hydroxylase catalyzes the rate-limiting step of catecholamine synthesis, and it is feedback inhibited by the end products of the pathway—dopamine, norepinephrine, and epinephrine.
1. The Toxicity of Catecholamins
The catecholamines involves epinephrine, dopamine, and norepinephrine, which are all important signal molecules and act especially as neurotransmitters and hormones.
Under normal conditions, they regulate normal cellular activity including heart rate, blood pressure, blood glucose and controls specific signal pathways inside of the cells. The extreme level especially high level of catecholamines are toxic because it will cause cell necrosis, if this happens in brain, an irreversible damage will be caused
五、Amino Acid Biosynthesis
