Specialized Amino Acid Derivatives and Inborn Errors of Metabolism)

Tryptophan

Key Biological Derivatives: Niacin (Vitamin B3), Serotonin, Melatonin

Board Exam Significance: A deficiency in tryptophan absorption causes a Niacin deficiency, leading to Pellagra

Tyrosine

Key Biological Derivatives: Catecholamines (Dopamine → Norepinephrine → Epinephrine), Melanin, Thyroxine (T4/T3)

Board Exam Significance: Derived from Phenylalanine. Deficiencies in the enzymes handling Tyrosine lead to severe metabolic diseases (Albinism, Alkaptonuria).

Glutamate

Key Biological Derivatives: GABA (Gamma-aminobutyric acid), Glutathione

Board Exam Significance: ‘blank’ is the primary excitatory neurotransmitter, but it decarboxylates into GABA, the primary inhibitory neurotransmitter.

Histidine

Key Biological Derivatives: Histamine

Board Exam Significance: Requires Vitamin B6 as a cofactor for decarboxylation. Major mediator of allergic responses and gastric acid secretion

Arginine

Key Biological Derivatives: Nitric Oxide (NO), Urea, Creatine

Board Exam Significance: Nitric Oxide is a potent vasodilator synthesized by NO Synthase (NOS).

Glycine

Key Biological Derivatives: Porphyrin (Heme), Purines, Glutathione

Board Exam Significance: Glycine is the most basic amino acid and acts as an inhibitory neurotransmitter in the spinal cord.

Glutathione (GSH)

A tripeptide formed from Glutamate, Cysteine, and Glycine. It is the body's master intracellular antioxidant. It protects red blood cells from oxidative damage (hydrogen peroxide) by oxidizing its sulfhydryl (—SH) group to form a disulfide bond (GSSG).

Neuropeptides (Substance P & Endorphins)

• Substance P: An 11-amino-acid peptide that transmits pain signals from the periphery into the central nervous system.

• Endorphins & Enkephalins: Endogenous opioid peptides that bind to the same receptors as morphine. They act as natural painkillers by inhibiting the release of Substance P.

Insulin

Contains 51 amino acids organized into an A-chain and a B-chain connected by disulfide bridges.

Oxytocin & Vasopressin (ADH)

Nonapeptides (9 amino acids) secreted by the posterior pituitary. Structurally, they differ by only two amino acids but have vastly different functions (uterine contraction vs. water retention).

Hartnup Disease

• Defect: Autosomal recessive mutation in the transport proteins for neutral amino acids (especially Tryptophan) in the renal tubules and intestinal epithelium.

• Mechanism: The body cannot absorb Tryptophan. Since Tryptophan is the precursor for Niacin (Vitamin B3), the patient develops a severe Niacin deficiency.

Phenylketonuria (PKU)

• Defect: Deficiency of the enzyme Phenylalanine Hydroxylase (or its cofactor BH4).

• Mechanism: The body cannot convert Phenylalanine into Tyrosine. Phenylalanine accumulates and converts into toxic phenylketones, which damage the developing brain. Tyrosine becomes an essential amino acid because the body can no longer synthesize it.

Albinism

• Defect: Deficiency in the enzyme Tyrosinase.

• Mechanism: The body cannot convert Tyrosine into Melanin (the pigment responsible for skin, hair, and eye color).

Alkaptonuria (Black Urine Disease)

• Defect: Deficiency of Homogentisate oxidase (an enzyme in the Tyrosine degradation pathway).

• Mechanism: Homogentisic acid accumulates in the blood and tissues.

Hartnup Disease: Clinical Presentation

Pellagra-like symptoms (The 3 Ds: Dermatitis, Diarrhea, Dementia).

Phenylketonuria (PKU): Clinical Presentation

Severe intellectual disability, microcephaly, hypopigmentation (light skin/hair, due to lack of Tyrosine for melanin), and a characteristic "musty" or "mousy" body odor.

_{^{Management: Strict lifelong restriction of dietary Phenylalanine (avoiding the artificial sweetener Aspartame).}}

Albinism: Clinical Presentation

Lack of pigmentation, severe photosensitivity, and increased risk of skin cancers (squamous and basal cell carcinomas).

Alkaptonuria (Black Urine Disease): Clinical Presentation

The hallmark sign is urine that turns black upon standing (oxidation in the air). It also causes bluish-black pigmentation of cartilage (ochronosis) and early-onset debilitating arthritis.

Maple Syrup Urine Disease (MSUD)

_{^{A dangerous metabolic disorder affecting the branched-chain amino acids (BCAAs)}}

• Deficient Enzyme: Branched-chain alpha-ketoacid dehydrogenase (requires Vitamin B1 / Thiamine as a cofactor).

• Accumulated Amino Acids: Leucine, Isoleucine, and Valine (and their corresponding alpha-ketoacids).

PhLE Mnemonic: "I Love Vermont maple syrup" (Isoleucine, Leucine, Valine).

Homocystinuria

_{^{A disorder of methionine metabolism that presents somewhat similarly to Marfan syndrome.}}

• Deficient Enzyme: Most commonly Cystathionine beta-synthase (requires Vitamin B6 / Pyridoxine as a cofactor).

• Accumulated Substance: Homocysteine and Methionine.

Maple Syrup Urine Disease: clinical hallmarks

• Sweet, maple syrup-smelling urine (caused by the accumulation of isoleucine).

• Severe central nervous system defects, intellectual disability, and death if untreated.

Management: Dietary restriction of BCAAs.

Homocystinuria: Clinical Hallmarks

• Marfanoid habitus: Tall, thin stature with long extremities and arachnodactyly (spider-like fingers).

• Downward lens subluxation in the eyes (unlike Marfan syndrome, which is usually upward).

• High risk of early, severe cardiovascular thrombosis (blood clots), which is the leading cause of early death.