Glycine Synthesis

Glycine

• Glycine is a non-essential amino acid that can be synthesized in the body
• It is a neutral and glucogenic amino acid
• Glycine lacks an asymmetric carbon atom and is therefore not optically active

Synthesis (Anabolism) of Glycine

• Glycine can be synthesized from:
  • Serine
  • Threonine
  • CO2 + NH3
  • Glyoxylic acid
  • Choline

Glycine Synthesis from Serine

• Glycine can be synthesized or converted from serine because the reaction is reversible
• The reaction requires serine hydroxymethyl transferase enzyme + FH4 + PLP (B6)

Glycine Synthesis From Threonine

• Cleavage of threonine by threonine aldolase enzyme produces Glycine + Acetaldehyde

Glycine Synthesis From CO2 and NH3

• This reaction is catalyzed by glycine synthase enzyme
• This is a reversible reaction of the glycine cleavage system
• The reaction requires the co-enzymes THF + PLP + NAD + lipoamide.

Glycine Synthesis From Glyoxalic Acid

• Glycine is synthesized from glyoxalic acid via a transamination reaction, where the amino group of glutamic acid is transferred to glyoxalic acid, forming Glycine + α-Ketoglutaric acid
• This reversible reaction requires transaminase enzyme + PLP

Glycine Synthesis From Choline

• Glycine can be obtained from choline; Choline can also be synthesized from glycine
• Choline is utilized for synthesis of:
  • Acetyl-choline
  • Phospholipids, such as lecithin and plasmalogens

Catabolism of Glycine: Conversion to Serine

• Glycine converts to Serine and then to Pyruvate, and then to Glucose through Gluconeogenesis (a glucogenic pathway)
• This is the major pathway for glycine and serine catabolism in the human body

Cleavage (oxidation) of Glycine

• Glycine is oxidized by Glycine Synthase, which is a reversible reaction that produces CO2 + NH3
• This reaction requires co-enzymes: THF + PLP + NAD + lipoamide

Synthesis of Formic Acid

• Formic acid is synthesized in 2 steps:
  • Glycine becomes Glyoxalic acid
  • Glyoxalic acid becomes Formic acid

Glycine to Glyoxalic Acid

• By oxidative deamination of glycine:
  • By glycine oxidase enzyme
  • Glycine is oxidized to imino acid then to glyoxalic acid
• By transamination with α-Ketoglutaric acid:
  • Glycine is catabolized by transamination, where the amino group of glycine gets transferred to α-Ketoglutaric acid to form Glyoxalic + Glutamic acid

Glyoxalic acid to Formic Acid

• It is converted again to glycine by transamination reaction
• It is converted to formic acid by Oxidative Decarboxylation

Glyoxalic Acid Accumulation

• Failure to catabolize Glyoxalic acid can lead to accumulation, leading to its conversion to oxalate via oxidation reaction (Primary Hyperoxaluria)

Synthesis of Other Compounds from Glycine

Formation of Serine:

• Glycine reacts with 5, 10-methylene THF to form serine
• The enzyme is hydroxyl methyl-transferase

Formation of Hippuric Acid:

• It is a detoxication reaction (by conjugation) in the liver
• Glycine conjugates with toxic benzoic acid to produce non-toxic hippuric acid, which is then excreted in urine
• Glycine + Benzoic acid becomes Hippuric acid

Synthesis of Bile Salts from Bile Acids

• Glycine conjugates with bile acids such as Cholic acid in the liver to form bile salts like Glyco-cholic acid

Synthesis of Heme

• Heme is the red pigment that combines with globin protein to form hemoglobin
• The first step of heme synthesis involves the condensation of Glycine + Succinyl CoA to form α-amino β-keto-adipic acid, then Aminolevulinic acid (γ-ALA), which produces heme then hemoglobin

Formation of Purine Ring

• Glycine contributes to the formation of the purine ring by contributing:
  • Carbon number 4
  • Carbon number 5
  • Nitrogen number 7

Formation of Collagen

• Collagen is the main protein of connective tissue of skin and hair
• Glycine shares in synthesis of Collagen
• 1/3 of the amount of amino acids in collagen are glycine

Formation of Glutathione

• Glutathione = (γ-Glutamyl-Cysteinyl-Glycine)
• It is an intracellular tripeptide consisting of Glutamic acid, Cysteine, and Glycine
• It acts as hydrogen carrier and antioxidant

Synthesis of Glutathione

• It is synthesized in 2 steps catalyzed by 2 enzymes:
  • γ-Glutamyl-Cysteine Synthase (ligase)
  • Glutathione Synthase

Forms of Glutathione

• Reduced form: one glutathione containing Thiol (SH) group, (Glutathione-SH) = (G-SH)
• Oxidized form: two glutathione connected by disulfide linkage, (Glutathione – S — S – Glutathione) = (G-S-S-G)
• Ratio of GSH : GSSG = 100 : 1

Functions of Glutathione

• Detoxification: It has a detoxication function against certain toxic compounds, combining with them to form non-toxic substances
• Hemoglobin Protection: It protects hemoglobin against oxidation by H2O2 (Hydrogen Peroxide), acting as an antioxidant
• Cell Membrane Protection: It protects the cell membranes of red blood cells (RBCs) against oxidizing agents (H2O2), protecting them against hemolysis
• Amino Acid Transport: Glutathione acts as a component of an amino acid transport system, playing a role in the absorption of neutral amino acids
• Insulin Inactivation: It inactivates insulin in the liver via insulin glutathione trans-hydrogenase, causing reductive cleavage of the 2 disulfide bridges of insulin and converting it into 2 separate polypeptide chains
• Regeneration of Vitamins: It regenerates vitamins C and E to their reduced active forms

Formation of Creatine and Creatinine

Creatine: Methyl Guanido Acetic Acid

• Creatine is a tripeptide formed from:
  • Glycine
  • Arginine
  • Methionine
• It is mainly present in skeletal muscle as Creatine phosphate, storing energy
• It is formed in 3 organs in 3 steps, with 3 reactions:
  • Transamidination occurs in the Kidney, forming Guanido-acetic acid
  • Transmethylation occurs in the Liver, forming Creatine
  • Phosphorylation occurs in Skeletal muscle, forming Creatine~P
• Creatine-phosphate is a high-energy phosphate compound stored in skeletal muscle, acting as a source of energy during the first period of muscular contraction

During the first few minutes of muscular contraction:

• The phosphate group transfers from creatine~P to ADP, forming new ATP under the effect of Creatine Kinase:
  • Creatine phosphate + ADP ↔ Creatine + ATP
• Also, creatine loses one water molecule and converts to Creatinine, which is totally excreted in urine
• Creatine is a threshold substance, meaning that when filtered by the kidney, it must be totally reabsorbed back into the blood
• Therefore, creatine is not normally present in urine
• The amount of creatine phosphate is directly related to muscle mass, as males typically have more creatine phosphate than females
  • This is because the male sex hormone (Testosterone) increases the uptake and retention of creatine in muscle

B) Creatinine

• Creatinine is the main end product of Creatine phosphate catabolism in skeletal muscle after muscular exercise
• It is a NON-threshold substance, meaning it's filtered but not reabsorbed, leading to total excretion in a constant amount
  • Serum creatinine level: 0.6-1.2 mg/dl
  • Urinary creatinine level: 1-1.2 g/24 hr urine
• Creatinine is a sensitive indicator of kidney function, because an increase indicates severe renal damage with less than 25% kidney function, as kidney unable to excrete creatinine with urine

Metabolic Disorders of Glycine Metabolism

Glycinuria

• Glycinuria results from a defect in renal tubular reabsorption of glycine
• It leads to increased urinary excretion of glycine, even though plasma glycine levels are normal

Primary Hyperoxaluria

• It is a metabolic disorder caused by failure to convert glyoxalic acid to formic acid
• Leads to oxidation of glyoxalic acid to oxalic acid, which combines with calcium to form calcium oxalate
• This leads to excretion of a high amount of Ca oxalate with urine, causing renal stones

Phenylalanine and Tyrosine

Structure

• Phenylalanine is an essential amino acid that cannot be synthesized in the body, and is a-amino, β-phenyl propionic acid.
• Tyrosine is a non-essential amino acid that can be synthesized in the body from phenylalanine, and is a a-amino, β-phenyl para hydroxy propionic acid.
• Both, however, are aromatic amino acids
• Phenylalanine is glucogenic Fumaric acid Give glucose
• Tyrosine is ketogenic Acetoacetic acid Give ketone bodies

Function of Phenylalanine

• Shares in protein biosynthesis
• Phenylalanine is the main source of tyrosine in the body:
  • Tyrosine is synthesized from phenylalanine in the liver by phenylalanine hydroxylase
  • A deficiency of phenylalanine hydroxylase will cause phenylketonuria

Catabolic Pathways of Phenylalanine

• There are 2 pathways for catabolism of phenylalanine:

Direct Pathway (Minor Pathway)

• In the liver, phenylalanine is catabolized to phenyl-acetic and phenyl-lactic, which are both excreted with urine

Indirect Pathway (Major Pathway)

• Phenylalanine is converted to tyrosine in the liver
• Tyrosine is catabolized in the liver to:
  • Fumaric acid, giving glucose
  • Acetoacetic acid, giving ketone bodies

Special Products from Tyrosine

• Tyrosine enters in synthesis of important compounds like:
  • Melanin pigment
  • Dopamine neurotransmitter
  • Catecholamines (Adrenaline and noradrenaline)
  • Thyroid hormones (T3 and T4)
  • Phenol

Formation of Melanin Pigment

• This reaction takes place in the melanocytes (pigment cells) in skin, hair, and iris
• Under the effects of the enzyme, Tyrosinase
• Melanin is synthesized by melanocyte cells
• Melanin is Present mainly in skin, hair, and iris

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Any change in the amount of Melanin pigment leads to the following disorders:

• Vitiligo: the skin loses its color, resulting in white patches appearing on the skin
  • Occurs when melanocytes are destroyed or stop function by the immune system
  • Generates starts on hands, forearms, feet, and face
• Melasma: Includes brown irregular, well-demarcated, hyper-pigmented patches appearing on face or arms
  • Common mostly in women, especially if pregnant
  • Caused by hormone changes, birth-control pills and increased sun exposure

Biosynthesis of Catecholamines

Catecholamines include:

• Dopamine
• Epinephrine and Norepinephrine
• They are synthesized in:
  • Adrenergic neurons of the nervous system
  • Chromaffin cells of adrenal medulla
• The first step in the conversion to dopamine

Steps of Catecholamines Biosynthesis

• Hydroxylation of tyrosine to L-Dihydroxy-phenylalanine (L-Dopa)
• Decarboxylation of L-Dopa to dopamine
• Hydroxylation of dopamine to Norepinephrine (Noradrenaline)
• Methylation of Norepinephrine to Epinephrine (Adrenaline)

Dopamine

• It is a chemical messenger (neurotransmitter) that is released and acts in the central and peripheral nervous system
• Constitutes about 80% of catecholamines in the brain

Function of Dopamine

• Dopamine gives feelings of pleasure, satisfaction and motivation
• It also plays a role in controlling memory, mood, sleep, learning, concentration, and other body functions
• It is also vital for control of skeletal muscles and body movement by the brain

Catabolism of Catecholamines:

• Catecholamines catabolized by 2 enzymes:
  • Monoamine oxidase (MAO): An Intra-cellular enzyme that is Present in mitochondria of adrenergic nerve endings
  • Catechol-Ortho-Methyl-Transferase (COMT): An Extra-cellular enzyme
    • It is present in all tissues, with high concentration in liver and kidney
    • Catechol-Ortho-Methyl-Transferase (COMT) is not found in nerve endings
  • The end product of dopamine catabolism is Homovanillic acid (HVA), which is excreted with urine
  • The end product of catecholamine hormones catabolism is Vanilly Mandelic Acid (VMA), which is excreted with urine

Biosynthesis of Thyroid Hormones

• This occurs in thyroid gland follicles
• The thyroid gland contains a glycoprotein called Thyroglobulin
• Thyroglobulin has about 150 tyrosine residues
• Iodide ions (I⁻) are taken up from the blood by thyroid cells and oxidized to positive ions (I⁺)
• The positive ions become incorporated with tyrosine through iodination forming Mono-iodo-tyrosine and Di-iodo-tyrosine
• Then a coupling process occurs of the new Di-iodo-tyrosine molecules with the Mono-iodo-tyrosine and Di-iodo-tyrosine
• Forms of Biosynthesis of Thyroid Hormones:
  • Tri-iodo-tyrosine (T3)
  • Tetra-iodo-tyrosine (T4) (Thyroxine)
• The Thyroglobulin then releases the T3 and T4 into plasma

Formation of Phenol

• Phenol results from Petrifaction of tyrosine by bacteria in the large intestine
• This process occurs through 4 steps: Decarboxylation; Deamination; Oxidation; Decarboxylation
• Phenol is a toxic substance, so it must be detoxified in the liver via conjugation with glucuronic acid

Disorders of Phenylalanine and Tyrosine Catabolism

Phenylketonuria (PKU)

• It is inherited disorder of phenylalanine caused by a deficiency of liver phenylalanine hydroxylase
• Characterized by a failure of conversion of phenylalanine into tyrosine
• Phenylalanine is increased in blood and converted to phenyl-pyruvic and phenyl-lactic acid in blood and excreted urine, so it is called phenylketonuria
• It mostly affects infants and children
• Common symptoms include: Failure to walk and talk; Failure to grow; Mental retardation; Eczema of the skin; Mousy odor of skin and urine

Diagnosis

- Through a blood test at the 4th day of birth (Guthrie test)
- In the urine, by ferric chloride test, which gives green color with phenylalanine

• The usual treatment for PKU is infants should be fed on low phenylalanine milk/diet till 6 years old

Alkaptonuria

• It is inherited metabolic disorder occurs due to defect in homogentisic acid oxidase
• Causes accumulation of homogentisic acid, giving urine an unusual dark brown to black color Most notably:
  • The homogentisic acid in urine is oxidized into a black alkapton pigment that causes urine to darken in air
  • There also is accumulation of homogentisic acid in cartilages and joints that can lead to Arthritis and pigmentation of connective tissues

Albinism

• It is due to a deficiency of the tyrosinase enzyme in melanocyte cells of the skin, hair, and eye The results are such that:
  • Melanin pigments will not be formed, leading to white color of skin and hair
  • The iris of the eye is colorless but appears red due to its blood vessels
    • The patient will be referred to as Albino and may be sensitive to light
  • Common albinism is separated by the site affected:
  • Eye: Ocular albinism
  • Skin: Cutaneous albinism
  • Eye and skin: Occulo-cutaneous albinism

Parkinson's Disease

Most Common:

• Parkinson's disease is a movement and mood disorder that mainly effects persons 60 years old an over
• The common cause is the absence of Dopamine. Dopamine is a neurotransmitter in the brain that comes as a result of a degeneration of brain cells responsible for dopamine synthesis and secretion
• Symptoms include: Tremors; Rigidity in all muscles; Loss of motor control Most diagnosis patients appear like a statue, with mask face and a Shuffling gait
• Most treatments is achieved based on administration of L-dopa, which is the precursor of Dopamine, relieving the condition

Tryptophan

• It is an essential, heterocyclic amino acid.
• It is α-aminoβ-indole propionic acid.
• It is glucogenic and ketogenic amino acid.

Tryptophan Catabolism

• Tryptophan catabolism takes place mainly in the liver
  • Tryptophan is oxidized to:
    • Alanine, which forms glucose
    • Acetoacetyl CoA, which forms ketone bodies

Functions of Tryptophan

• Biosynthesis of Niacin (vitamin B3)
  • Niacin (Nicotinic acid) is a member of the B-complex vitamins
  • Tryptophan will synthesize niacin in the liver
    • Every 60 mg of Tryptophan will convert to 1 mg of niacin
  • Niacin enters in the formation of NAD and NADP
  • Lack of niacin will cause Pellagra
    • Symptoms are the 3 D diseases: Diarrhea, Dermatitis, and Dementia
  • Biosynthesis of Serotonin hormone (5-HT)
    • Also called 5-Hydroxy-tryptamine (5-HT)
    • 1% of Tryptophan will convert to serotonin
    • Requires vitamin B6 (Pyridoxal phosphate (PLP)

Main Steps of Synthesis include:

• Hydroxylation of Tryptophan to 5-Hydroxy-tryptophan
• Decarboxylation of 5-Hydroxy-tryptophan to 5-Hydroxy-tryptamine (Serotonin):.

Sites of Serotonin Synthesis and Secretion:

• Argentaffin cells of intestinal mucosa
• Mast cells and platelets
• Hypothalamus and brain stem
• Smooth muscle cells

Biological functions of Serotonin:

• When there is an injury in the body, serotonin will be released by mast cells and platelets: This will cause Vasoconstriction and enhance/aggregate platelets to stop any bleeding that may has occurred
• Stimulates peristaltic movement of the intestine
• Plays a role as a stimulatory neurotransmitter in the brain:
  • Serotonin increases brain activity, regulates appetite
  • Responsible for mood elevation, and decreases brain activity

Catabolism of Serotonin

• Serotonin is catabolized by Monoamine oxidase (MAO), to form 5-hydroxyindole acetic acid (5HIAA)

The Action of All Anti-Depressant Drugs

• Depends on the inhibition of MAO, and serotonin catabolism
• Leads to increased serotonin level, causing euphoria and psychic stimulation
• Decreased Serotonin level will cause and may lead to Schizophrenia

Biosynthesis of Melatonin (Hormone)

• A hormone synthesized in the pineal body, and in the peripheral nerves of human during night only
• Derived from tryptophan and serotonin: Refer to the synthesis of serotonin
• Melatonin is derived from serotonin by Acetylation and Methylation
• Secretion of melatonin is due to darkness increasing the sensation to fall asleep
• Decreased secretion is due to light; Release of melatonin is inhibited by light entering the eye and transmitting the sensation to the gland

Functions of Melatonin in Human

• It is anti-aging, anti-oxidant, anti-cancer hormone; It acts as free radicals scavenger and induces synthesis of antioxidants
• It has a sleep-inducing effect, and regulates the circadian rhythm
• Melatonin is used in pharmacology for treatment for insomnia and sleep disorders