Metabolic uses of amino acids ● building blocks for protein synthesis ● precursors of...

Metabolic uses of amino acids

● building blocks for protein synthesis

● precursors of nucleotides and heme

● source of energy

● neurotransmitters

● precursors of neurotransmitters and hormones

Outline of amino acid degradation

● The liver is the major site of degradation for most amino acids, but muscle and kidney dominate the degradation of specific ones

● Nitrogen is removed from the carbon skeleton and transferred to α-ketoglutarate, which yields glutamate

● The carbon skeletons are converted to intermediates of the mainstream carbon oxidation pathways via specific adapter pathways

● Surplus nitrogen is removed from glutamate, incorporated into urea, and excreted

Amino acid breakdown pathways join mainstream carbon utilization at different points of entry

Transamination of amino acids

The reaction mechanism of transamination

The ping pong bi bi mechanism of transamination

Nitrogen disposal and excretion

● Nitrogen accruing outside the liver is transported to the liver as glutamine or alanine

● In the liver, nitrogen is released as free ammonia

● Ammonia is incorporated into urea

● Urea is released from the liver into the bloodstream and excreted through the kidneys

The urea cycle, part 1: carbamoylphosphate synthetase

The urea cycle, part 2: subsequent reactions

The urea cycle in context

The urea cycle spans mitochondria and cytosol

The glucose-alanine cycle

Nitrogen transport by glutamine

The central role of glutamate in nitrogen disposal

Control of ammonia levels in the liver lobule

Regulation of the urea cycle

Hereditary enzyme defects in the urea cycle

● may affect any of the enzymes in the cycle

● urea cannot be synthesized, nitrogen disposal is disrupted

● ammonia accumulates, as do other metabolites depending on the deficient enzyme

● treatment

○ protein-limited diet

○ arginine substitution

○ alternate pathway therapy

Asparagine degradation

Serine dehydratase

Serine-pyruvate transaminase

Degradation of leucine

Degradation of phenylalanine and tyrosine

Phenylketonuria (PKU)

● homozygous defect of phenylalanine hydroxylase

● affects one in 10,000 newborns among Caucasians; frequency differs with race

● excess of phenylalanine causes symptoms only after birth; intrauterine development normal

● cognitive and neurological deficits, probably due to cerebral serotonin deficit

● treatment with phenylalanine-restricted diet

● some cases are due to reduced affinity of enzyme for cofactor THB, can be treated with high dosages of THB

The Guthrie test for diagnosing phenylketonuria

Ochratoxin A inhibits phenylalanyl-tRNA synthetase

Tyrosinemia

● homozygous defect of fumarylacetoacetate hydrolase

● fumarylacetoacetate and preceding metabolites back up

● fumaryl- and maleylacetoacetate react with glutathione and other nucleophiles, causing liver toxicity

● the drug NTCB inhibits p-hydroxyphenylpyruvate dioxygenase, intercepting the degradative pathway upstream of the toxic metabolites

● dietary restriction of tyrosine required to prevent neurological deficit

Metabolic uses of amino acids ● building blocks for protein synthesis ● precursors of...

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