14. Post-Absorption Processing of Proteins Flashcards
What biologically active compounds are amino acids precursors for?
Haem Nucleic acids Hormones Neurotransmitters Biologically-active peptides
What determines the half life of a protein?
N-terminal residue
Which amino-acid sequences are rapidly degraded?
Proline
Glutamic acid
Serine
Threonine
What two ways are proteins degraded?
Ubiquitin-proteasome system
Lysosomal enzymes
What does the concentration of amino acids in circulation between meals depend on?
Utilisation by tissues
Release following protein degradation
How is alanine used in hepatic gluconeogenesis?
Alanine is made in muscle by the transamination of pyruvate
Liver coverts the carbon skeleton of alanine to glucose
What does it mean for an amino acid to be glucogenic?
Its breakdown yields TCA cycle intermediates
Used to make glucose: gluconeogenesis
What does it mean for an amino acid to be ketogenic?
Breakdown yields acetoacetate or acetyl-CoA
Ketogenesis
Which amino acids are glucogenic?
Alanine Arginine Asparagine Aspartate Cysteine Glutamate Glutamine Glycine Proline Serine Histidine Methionine Threonine Valine
Which amino acids are both glucogenic and ketogenic?
Tyrosine
Isoleucine
Phenylalanine
Tryptophan
Which amino acids are ketogenic?
Leucine
Lysine
Which amino acids are formed from essential amino acids?
Cysteine
Tyrosine
Hydroxylysine
Which enzymes play a role in forming amino acids from amphibolic intermediates?
Transaminases
Glutamine synthetase
Glutamate dehydrogenase
What reaction does phenylalanine undergo to make metabolic intermediates?
Enzyme: phenylalanine hydroxylase
Produces tyrosine
Tyrosine converted to fumarate or acetoacetate
Which disorder is caused by a deficiency of phenylalanine hydroxylase?
Phenylketonuria
How is glutamine synthesised?
Glutamate converted to glutamine by glutamine synthetase
Reaction is useful for storing free ammonia in a non-toxic form
What coenzyme is used by transaminases?
Vitamin B6
What is the mostly widely used ‘donor-acceptor’ transaminase couple?
Glutamate
a-ketoglutarate
Which amino acids use the glutamate - a-keoglutarate pair as part of their degradation?
Aspartic acid Asparagine Arginine Isoleucine Leucine Lysine Tryptophan Tyrosine Valine
Which enzyme converts alanine and a-ketoglutarate to pyruvate and glutamate?
ALT
Which enzyme converts oxaloacetate and glutamate to aspartate and a-ketoglutarate?
AST
What reaction does ALT catalyse?
Alanine and a-ketoglutarate
to
Pyruvate and glutamate
What reaction does AST catalyse?
Oxaloacetate and glutamate
to
Aspartate and a-ketoglutarate
What are the two steps in disposal of amino acids?
Transamination
Oxidative deamination
What are the two steps in synthesis of amino acids?
Reductive amination
Transamination
Where does the urea cycle take place?
Liver
How is amino nitrogen transported in circulation?
Alanine (made from pyruvate in muscle)
Glutamine (made from glutamate in most tissues)
What is free ammonia produced in the body by?
Breakdown of purines and pyrimidines
Breakdown of amines and monoamines
Breakdown of glutamine
Bacterial digestion of urea
Where do the two amino groups on urea come from?
Carbamoyl phosphate
Aspartate
How much energy is required to produce urea?
3 ATP molecules per molecule of urea
What are normal levels of free ammonia in blood serum?
5-50mmol/L
What is acquired hyperammonaemia caused by?
Liver disease or failure
What is hereditary hyperammonaemia caused by?
Inherited urea cycle defect
What are the effects of hyperammonaemia?
CNS toxicity
Tremors, slurred speech, vomiting, cerebral oedema
Coma, death
What is the most common urea cycle disorder in humans?
Ornithine transcarbamylase deficiency
What is the function of ornithine transcarbamylase?
Converts carbamoyl phosphate and ornithine into citrulline
What are the symptoms of an OTC deficiency?
Hyperammonaemia Vomiting Refusal to eat Progressive lethargy Coma
