Protein Metabolism Flashcards
Key entry points into gluconeogenesis, conversion to what keto-acids?
Alanine converted to pyruvate. Glycine to Serine to pyruvate or 3-phosphoglycerate.
Glutamate to alpha-ketoglutarate
Aspartate to OAA.
Which are the branched chain amino acids and why are they important?
Valine, Leucine and Isoleucine.
Very hydrophobic, important for protein structure.
Can be used for oxidation in muscle in temporary starvation, in place of fatty acids.
(Also essential!)
[BCKDH mutation in MSUD limits their metabolism –> acidosis!]
What are transaminase reactions for? (give an example using alanine and alpha-ketoglutarate) [which aa don’t get involved?]
Transaminase reactions (by aminotransferases) transfer amino groups from one amino acid to a keto-acid to produce a different (non-essential) amino acid.
alanine and alpha-ketoglutarate –> pyruvate and glutamate
Only threonine and lysine don’t participate
What is Maple Syrup Urine disease? MSUD
AKA Branched-chain ketoaciduria
Inherited defect in BCAA metabolism leads to acidosis and neurological symptoms, seizures, retardation. Unless BCAAs restricted in diet.
Autosomal recessive mutation in BCKDH (branched-chain ketoacid dehydrogenase) leading to build up of these ketoacids.
What and where is carbamoyl phosphate synthetase?
Makes carbamoyl phosphate from CO2, 2ATP and NH4+ (from glutamate dehydrogenase)
Found in liver, in mitochondria. (lots of CO2 in mitochondria!)
How do amino groups from glutamate enter Urea cycle? (2 ways, key enzymes?)
1) Oxidative deamination: Glutamate dehydrogenase in liver (promoted by low energy indicators ADP and GDP) using NAD+, producing a-ketoglutarate for TCA cycle, NH4+ and NADH +H+. Made into carbamoyl phosphate in mitochondria and attached to ornithine, to make citrulline
2) Aspartate aminotransferase: transfers amino group from glutamate to OAA to produce aspartate, which attaches to citrulline, producing argininosuccinate, [then releasing fumarate, leaving arginine]
Most common urea cycle metabolism disorder?
Triad of symptoms?
Diagnostic test and explanation:
Management?
Ornithine Transcarbamoylase deficiency.
X-linked recessive mutation. (female carriers make experience vomiting, migraine and lethargy after eating a lot of protein)
Triad of symptoms: Hyperammonaemia, encephalopathy and alkalosis.
Urinary orotic acid, caused by excess carbamoyl phosphate entering Pyrimidine synthesis pathway in cytosol.
Give essential amino acids, and arginine, which cannot be made without urea cycle. Benzoic acid, [conjugates glycine, which is in dynamic equilibrium with ammonia because of direct synthesis by glycine synthase.]
How does liver deaminate all common amino acids?
Transamination to form glutamate (using alpha-ketoglutarate) (named after AA having amino group removed, e.g. tyrosine transaminase)
Or rarely direct deamination, then incorporated to form a glutamate (e.g. histidase for histidine)
Describe steps and purpose of Glucose-Alanine cycle?
Muscle amino acid such as leucine (abundant) deaminated by transaminase, transferring amino group to alpha-ketoglutarate forming glutamate.
ALT (alanine transaminase) transfers amino group from glutamate to pyruvate, forming alanine [inhibits PyK], which enters bloodstream.
In liver, alanine converted back to pyruvate by ALT, producing glutamate, GDH deaminates to alpha-ketoglutarate and enters Urea cycle.
Used during fasting state, allows oxidation of BCAAs for energy in muscle, whilst eliminating NH3, returns carbon-skeletons to liver for gluconeogenesis to supply brain, and also back to muscle.
Mechanism of the glutamine mediated transport of NH4 from Extrahepatic tissues and why?
Each glutamine used to transfer 2 amino groups to liver (where Urea cycle takes place)
Glutamate formed by adding amino group to alpha-ketoglutarate, (e.g. reverse reaction of GDH). Glutamine synthase adds second amino group to glutamate forming glutamine (using ATP).
Glutamine transported in blood to liver and deaminated back to glutamate by glutaminase. Free NH4+ enters Urea cycle (as can glutamate via GDH and aspartate transaminase.)
[2 amino groups transported per glutamine (Q).]
Which amino acids are ketogenic? (which exclusively)
why?
Leucine and lysine converted to acetoacetate or AcetylCoA only. Which can only generate fatty acids or ketones, and when occurs in fasted state –> ketogenic only.
Amino acids converted into TCA cycle intermediates, (alpha-ketoglutarate,succinate,fumarate,OAA) or pyruvate, are gluconeogenic.
Isoleucine I, tyrosine Y, phenylalanine F, tryptophan W, and threonine T are both. WIFTY.
Amino acids Arginine R, Aspartate D and Glycine G as biosynthetic precursors?
Arginine makes positive polyamines that coat DNA, critical for cell growth. Spermine, spermidine, putrescine.
Aspartate makes pyrimidines.
Glycine makes purines, glutathione and creatine.
Amino acids Histidine, Serine, Phenylalanine/Tyrosine, Tryptophan, and Valine as biosynthetic precursors?
(essential) Histidine H –> histamine
Serine S: (cell membrane) Phosphatidylserine and Sphingolipids
(essential) Phenyalanine F/ Tyrosine Y: Dopamine, adrenaline, noradrenaline
(Essential) Tryptophan W: Serotonin and nicotinic acid for NAD+.
(essential) Valine: Pantothenic acid (VitB5) for CoA synthesis.
Result of tyrosinase deficiency/absence?
Normally produces melanin in melanocytes from tyrosine/L-DOPA.
So absence leads to Albinism!
DDC inhibitors and MAO-I s mechanism of action?
Treatment of Parkinsonism.
Dopamine decarboxylase DDC inhibitors like carbidopa prevents breakdown of LDOPA in blood to dopamine that can’t cross BBB.
MAOIs prevent breakdown of dopamine, noradrenaline and serotonin in neurons (mitochondria)
What is Hartnup disease? aetiology, pathophysiology, symptoms, treatment?
Autosomal recessive inherited deficiency in Neutral Amino acid transport/uptake.
Particularly tryptophan, W, deficiency causes lack of serotonin and niacin (–>NAD+, absence causes brain and skin problems)
When treated with sufficiently high protein diet, most patients are asymptomatic but can cause Cerebellar ataxia, psychiatric symptoms, pellagra (niacin deficiency)-like skin rashes. on exposure to sunlight, or other stresses
Also supplement with Niacin (vitamin B3)
What is Gaucher’s disease?
Deficiency of glucocerebrosidase enzyme (found in lysosomes). (lysosome storage disease)
Can’t break down glucocerebrosides –> Build up of fatty material (glycosyl-sphingosine) causes Spleen enlargement, anaemia and bone lesions, neurological disorders
(effects WBCs most because they contain many lysosomes)
Variable but often progressive and life threatening, can present at any age.
Briefly what is Hurler syndrome?
A rare autosomal recessive LysosomeStorageDisease LSD.
(can’t break down particular long sugar chains)
causes deformities, mental retardation and deafness.
Describe lysosomes and their contents?
Organelles that bud off of golgi bodies, about size of mitochondria.
Acidic contents (around pH5.0) maintained by proton pump.
Enzymes including cathepsins and other proteases, phosphatases, nucleases, glucosidases, phospholipases etc.
[Primary lysosomes fuse with vacuoles containing target material to form secondary lysosomes to digest material.]
What is Tay-Sachs disease?
Autosomal recessive rare Lysosome storage disease. (ashkenazi jews)
Deficiency of Beta-hexosaminidase A, can’t breakdown gangliosides (accumulate in lysosomes of neurons)
Normally infantile onset (fatal neural degeneration, +cherry red spot of fovea)
Rarely juvenile (death) or Adult onset (like schizophrenia and other neural problems).
What are the 3 types of autophagy?
Macroautophagy is characterised by formation of large double-membraned autophagosome around damaged organelles (specific: pexophagy,ribophagy,mitphagy), that then fuses with lysosomes for digestion. [Requires Atg proteins for formation and fusion.] This is constitutive, but a non-selective process is stimulated by glucagon in starvation, suppressed by insulin.
Microautophagy is where material is directly engulfed by lysosomal membrane in a non-selective, consitutive manner.
Chaperone mediated autophagy CMA is a highly regulated process involving Hsc70 chaperone selecting water-soluble cytoplasmic proteins for unfolding and transport into lysosomes individually.
Describe the structure of the proteasome and how it is selective for damaged proteins?
20S core of proteasome contains alpha and beta subunits (of which beta have catalytic sites)
Narrow channel to access catalytic site, not accessible by normal, correctly folded proteins. The 19S protein complex has the ATP-dependent unfoldase activity required to unfold proteins targeted by ubiquitination.
20S and 19S together form 26S proteasome.
Describe the process of ubiquitination:
E1: Ubiquitin activating enzyme: attaches ubiquitin to its own cysteine and activates it using ATP
Transferred to an E2 (conjugating enzyme) cysteine, which binds one of many E3 ubiquitin ligases (specific to substrate proteins).
E3 mediates transfer from E2 to epsilon amino group on lysine on target protein
lysine 48 polyubiquitination.
Describe 2 pathways of Apoptosis following Fas activation:
Extrinsic: FasL activates Fas. FADD activated, activate Caspase 8 (cysteine protease, cleaves around aspartate residues)
Caspase 8 activates Caspase 3 activates CAD, chops DNA.
Intrinsic pathway: Caspase 8 can also cause release of cytochrome C from mitochondria, activates Caspase 9 (apoptosome) activates Caspase 3.
PKB also phosphorylates BAD, releasing antiapoptotic Bcl2, preventing pore formation by Bax and NOXA.
