🧪 General Principles🟢 BasicAmino Acid Metabolism

Aminoacidopathies

Inherited enzyme defects in amino-acid metabolism, each blocking one step and causing a characteristic build-up.

Key points

Phenylketonuria is a deficiency of phenylalanine hydroxylase, so phenylalanine accumulates; untreated it causes intellectual disability and a musty body odour, and fair skin and hair are classically associated.
It is one of the classic newborn-screened disorders, because the harm is preventable if the block is caught at birth.
Alkaptonuria is a deficiency of homogentisate oxidase; homogentisic acid builds up, turning the urine black on standing and darkening cartilage (ochronosis).
Maple syrup urine disease blocks the breakdown of the branched-chain amino acids leucine, isoleucine and valine, giving urine that smells of maple syrup.
Homocystinuria most often reflects a deficiency of cystathionine beta-synthase and can resemble Marfan syndrome, but the lens dislocates downward (in Marfan it is upward) and there is a tendency to clot.
Albinism is not an aminoacidopathy but sits in the same tyrosine pathway: it is a defect of melanin synthesis, classically from tyrosinase deficiency.

Phenylketonuria smells musty, alkaptonuria turns black, maple syrup disease smells sweet — the odour or colour often names the block.

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🧪 General Principles🟢 BasicAmino Acid Metabolism

Inherited enzyme defects in amino-acid metabolism, each blocking one step and causing a characteristic build-up.

Phenylketonuria is a deficiency of phenylalanine hydroxylase, so phenylalanine accumulates; untreated it causes intellectual disability and a musty body odour, and fair skin and hair are classically associated.
It is one of the classic newborn-screened disorders, because the harm is preventable if the block is caught at birth.
Alkaptonuria is a deficiency of homogentisate oxidase; homogentisic acid builds up, turning the urine black on standing and darkening cartilage (ochronosis).
Maple syrup urine disease blocks the breakdown of the branched-chain amino acids leucine, isoleucine and valine, giving urine that smells of maple syrup.
Homocystinuria most often reflects a deficiency of cystathionine beta-synthase and can resemble Marfan syndrome, but the lens dislocates downward (in Marfan it is upward) and there is a tendency to clot.
Albinism is not an aminoacidopathy but sits in the same tyrosine pathway: it is a defect of melanin synthesis, classically from tyrosinase deficiency.

Phenylketonuria smells musty, alkaptonuria turns black, maple syrup disease smells sweet — the odour or colour often names the block.