Amino Acid Metabolism Flashcards
how many grams of protein do our bodies turn over a day?
300g
intake vs excretion
protein 100g/day
carbon dioxide, urea and ammonia 100g/day
what is the source of the amino acid pool?
diet and some body proteins if required
classifications of amino acid dependent on availability
essential, non-essential and semi-essential
essential amino acids
histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine
amino acids that cannot be synthesised de novo
conditionally essential
arginine, cysteine, glutamine, glycine, proline, serine, tyrosine
non essential
alanine, asparagine, aspartate and glutamate
enzymes involved in dietary absorption of amino acids
pepsin- non specific, maximally active at low stomach pH
proteolytic enzymes of the pancreas in the intestinal lumen
trypsin and chymotrypsin act in the duodenum
amino peptidases, digest proteins from the amino terminal end
what do trypsin and chymotrypsin prefer cleaving?
chymotrypsin- peptides where the amino acid N terminal bond is bound to trypsin, tyrosine, phenylalanine or leucine
trypsin- cleaves peptide chains normally at the carboxyl end bound to arginine or lysine
how are di and tripeptides taken up into the intestinal cells?
transported in a hydrogen ion symporter called PepT1
another important transporter
active sodium linked transport, along with the sodium-potassium ATPase maintaining the sodium concentration gradient
why does protein turnover occur?
damaged/incorrectly produced/folded proteins need to be removed
signalling proteins need to be removed and produced as needed
enzymes are often up/down regulated as a part of regulatory mechanisms
how do we know which proteins to break down?
proteins are tagged with ubiquitin using ubiquitin ligase
the tased protein is then directed into the proteasome for degradation
ubiquitin ligase structure + function of different subunits
ligases are made from three components E1, E2, E3
E1 & E2 are responsible for activating the ubiquitin
E3 recognises the protein to be ubiquinated
How does E3 work?
able to recognise damaged/misfolded proteins
it can also recognise certain n-terminal residues which signal the half life of a protein
syndrome related to ubiquitination
Angelman’s syndrome
- a genetic disease with features of severe motor and intellectual disability
- caused by mutations in ubiquitin ligase
overall regulation of protein synthesis/breakdown
insulin
- net anabolic effect by stimulating chain initiation; effects on transcription are protein specific
- inhibition of protein breakdown
thyroid hormones, cortisol
- net catabolic effect
anabolic steroids- net anabolic effect, such as testosterone
how else can amino acids be classified?
unionised or zwitterions
side chain
first two divisions of side chain
hydrophobic, non polar and hydrophilic, polar
further divisions of hydrophobic + amino acids
aromatic
- glycine
- alanine
- valine
- leucine
- isoleucine
- methionine
- proline
alkyl
- tryptophan
- phenylalanine
further divisions of hydrophilic + amino acids
neutral
- tyrosine
- serine
- threonine
- cysteine
- glutamine
- asparagine
acidic
- glutamic acid
- aspartic acid
basic
- lysine
- histidine
- argenine
explain the concept of positive and negative nitrogen balance
positive nitrogen balance is associated with periods or growth and repair, when the mass intake of nitrogen is greater than the loss from the body, greater protein pool
negative nitrogen balance is associated with burns, fevers and periods of fasting, where the mass intake is lower than the loss from the body, decrease in protein pool
what stops catabolism for going forward with some amino acids?
presence of an alpha amino group prevents oxidative breakdown
how is this overcome?
alpha amino groups must be removed before catabolism can proceed, the nitrogen can be incorporated into other compounds or excreted
the process if called deamination and produced an alpha-veto acid and ammonia
3 different types of deamination
oxidative
non-oxidative
hydrolytic
explain oxidative deamination + which amino acids
amino acid becomes oxidised by NADP+ to form an alpha keto acid alpha ketoglutarate and NADPH + H+ and an ammonium ion
catalysed by glutamate dehydrogenase
glutamate
profile on glutamate dehydrogenase
mostly used in the liver and kidney
NAD+ used as coenzyme in oxidative deamination
NADP+ in reductive amination
direction of reaction depends on substrate availability
regulation of glutamate dehydrogenase
deamination allosterically modulated by high ADP and GDP
amination allosterically modulated by high ATP and GDP
what is a process that happens to amino acids other than glutamate?
transamination
explain transamination
amino acids are funnelled into glutamate to enable breakdown
amino transferase catalyses the conversion of one amino acid into an alpha keto acid and then another keto acid into an amino acid
specific amino transferase examples
alanine transaminase
ala + alpha ketoglutarate to pyruvate and glutamate
aspartate transaminase
asp + alpha ketoglutarate to oxaloacetate and glutamate
what do all transamination reactions require?
the cofactor pyridoxal phosphate, derived from pyridoxine vit B6
how does pyridoxal phosphate work?
forms a Schiff base with a lys residue in the active site of the transaminase
which amino acids under transamination + what do they produce and their relevance ?
alanine- pyruvate keto acid, can be added into glycolysis
aspartate- oxaloacetate- added to TCA cycle
leucine- 2-oxo-3-methyl valerate- oxidised in muscle
isoleucine- 2-oxo-4-methyl valerate- oxidised in muscle
valine- 2-oxo-3-methyl butyrate oxidised in muscle
what ketoacid does the deamination of glutamate produce + function?
alpha ketoglutarate, which is a substrate in the TCA cycle
2 examples of non oxidative deamination
serine to pyruvate and ammonium, catalysed by serine dehydratase
threonine to alpha ketobutyrate and ammonium, catalysed by threonine dehydratase
why are the enzymes called dehydratases?
because the dehydration precedes deamination
why are serine and threonine directly deaminated?
they have an OH group
explain hydrolytic deamination + where this occurs
glutamine to glutamate, with water to ammonium catalysed by glutaminase
intestinal cells and renal cortex
why is amino acid release from muscle important?
muscle has the largest store of protein in the body, typically 5-7kg
during starvation, glucose continues to be oxidised, so it must come from non-glucose sources
what percentage of total free amino acids does skeletal muscle store?
50%
stages of the Cahill cycle in muscle
- glucose to pyruvate in glycolysis
2. pyruvate to alanine and an alpha ketoacid in transamination
stages of Cahill cycle in liver
- alanine transamination to pyruvate and release of ammonia
2. pyruvate to glucose via gluconeogenesis
what is glutaminase + functions ?
mitochondrial tissue specific isozyme that converts glutamine to glutamate
helps generate urea
expressed in kidneys to generate ammonium ions to help in acid-base balance
expressed I neurones to assist neurotransmission
reaction that form glutamine + where it occurs
glutamate to glutamine catalysed by glutamate synthetase
ammonium ion used
ATP to ADP and Pi
muscle
what is waste nitrogen excreted as in humans?
90% urea, the rest 10% ammonia
why is ammonia released in small amounts?
toxic so limited, however some still needed to maintain acid-base balance
why is urea excreted instead?
safe, non toxic means to excrete nitrogen
what is urea produced from?
ammonium ions
urea cycle stages
- HCO3- + NH4+ to form carboxyl phosphate, catalysed by carboxyl phosphate synthesise-I along with 2ATP to form 2ADP and Pi. occurs in mitochondria
- carbomoyl phosphate to citrulline
- citrulline to argininosuccinate
- argininosuccinate to arginine
- arginine to urea and ornithine
- ornithine combined with carbamoyl phosphate to produce citrulline
what is the urea cycle linked to + how?
TCA cycle through argininosuccinate
regulation of urea cycle
enzymes are coordinately up regulated by glucagon and glucocorticoids
carboxyl phosphate synthesise is the controlling step
how else can amino acids be classified
based on possible metabolic fates, such as glycogenic if enter TCA cycle or ketogenic via acetyl CoA or mixed
glucogenic amino acids
ala gly arg his met asn asp cys glu gln pro ser thr val
ketogenic
leu
lys
both
ile
phe
trp
tyr
how is ammonia transported to the liver?
in the form of glutamine
glutamate + ammonia forms glutamine
three branched chain amino acids
leucine, isoleucine and valine
explain stages of branched chain metabolism
- branched chain amino acids undergo transamination to form keto acids
- branched chain amino acid dehydrogenase complex converts the keto acids into acetyl CoA derivatives
- can then be converted into acetyl CoA or succinyl CoA to join the TCA cycle
