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Errors in metabolism - Biochemistry
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BIOCHEMISTRY – LECTURE 9 PART 1inborn errors in metabolism
Inborn errors of metabolism are a group of disorders that result from mutations
in genes which code for enzymes involved in catalysing reactions in metabolic
pathways. On first inspection, the cause and effect seem simple – a broken link
in a chain results in a broken chain. There will be the same effect regardless of
which enzyme in a pathway is missing/non-functional. Things are a bit more
complicated. The effects can be very different depending on which enzyme is
defective, as it may cause different product formation.
There can be said to be 3 main effects of a defective enzyme in a pathway:
Decrease in final product formation
Build-up in substrate
Increased formation of other metabolites
The biochemical and physiological effect of an error is due to one or more of
these 3 factors.
A new born baby often has no symptoms due to the mother’s blood removing
excess substrate and providing any missing end products. Only after a day or
two after birth do symptoms present, when the baby’s biochemistry has to fend
for itself. The protection the foetus has in utero from the mother’s biochemistry
has implications for testing. It takes some time for any inability to make
product/use substrate to become apparent. Testing too early can result in a
misleading result (falsely negative for the disease).
There is a glycogen storage disease known as glucose 6-phosphatase deficiency.
It effects two main pathways – glucose production from glycogen and it blocks
gluconeogenesis. Glycogen accumulates in the liver, causing the organ to
enlarge (hepatomegaly). Glycogen accumulation in platelets affects their
function, predisposing affected people to bleed inappropriately. The inability to
conduct gluconeogenesis, changes the fate of pyruvate to lactate, which may
cause acidosis. Glucose 6-phosphatase deficiency has effects on health due to:
o Inability to produce a product (glucose)
o Build up of substrate (glycogen and pyruvate)
o Formation of secondary metabolite (lactate).
This has incidence of 1 in 100,000 births. This deficiency has the effect of
causing severe, fasting hypoglycaemia (low blood glucose). Affected people are
unable to produce glucose, and have to rely on dietary glucose as their only
source of glucose. Initially, treatment involves frequent feeding, including
overnight. In older children, starch-rich meals can help maintain healthy glucose
concentration in the blood. Balance between preventing hypoglycaemia and
preventing glycogen build up.
Phenylketonuria (PKU) aka phenylalanine hydroxylase deficiency is an example
of a condition where substrate accumulation is responsible for many of the
potential clinical problems. It also exemplifies how a ‘broken link’ in a metabolic
chain can have diverse effects. It is caused by a mutation in the gene coding for
the enzyme, phenylalanine hydroxylase, which catalyses the formation of
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