Lecture 4: Alcohol+ Protein Metabolism Flashcards
Case study: Pesticide Poisoning. Guy has continued exposure to aromatic weak acids (e.g. DNC).
Symptoms = Low body temp, no subcutaneous fat. Explain symptoms.
- aromatic weak acids readily penetrate the mitochondrial membrane & act as uncoupling agents
- collapses p.m.f and uncontrolled metabolism occurs
- large amount of metabolic fuel (fatty acids) + O2 used
- less ATP, more heat = high body temp = sweating to cool down
Ketogenic a/a
Lysine, Leucime
Both glucogenic and ketogenic
Tyrosine, Threonine, Phenylalanine
Which a/a do pregnant women and children need more of? (Conditionally essential)
Arginine, thyrosine, cysteine
Where do carbon atoms for non-essential a/a come from?
- intermediates of glycolysis (C3)
- PPP (C4&C5)
- Krebs Cycle (C4&C5)
Why is removal of nitrogen from a/a important?
To allow carbon skeleton of a/a to be metabolised
How is NH3 toxic (Hyperammonaemia) and state symptoms.
- associated with blurred vision, tremors, coma, death
- Disrupts energy supply to brain
- affects pH and interferes with neurotransmitter release n synthesis
Which aminotransferase enzyme is routinely measured to test for liver damage?
- ALT (Arginine Aminotransferase)
- AST (Aspartate Aminotransferase)
What is the difference between homocysteine and homocystine
- Homocystine is two homocysteine molecules joint by a disulphide bond.
- Homocystine is the oxidised form of homocysteine
What is the mode of inheritance of homocystinuria?
Autosomal recessive
How would PKU disrupt blood brain barrier?
- Phenylalanine is a large neutral amino acid (LNAA) and competes for transport across the blood brain barrier via LNAAT.
- Excess phenylalanine saturate this transporter
- Inhibiting protein/neurotransmitter synthesis = brain development affected.
Briefly explain how the metabolism of alcohol can cause damage to the liver.
- Intermediate metabolite, acetaldehyde is a toxic metabolite.
- Increased acetyl coA, more fat synthesized, converted to triacylglycerol. Decreased NAD/NADH ratio. No oxidation of FA.
- Less lipoproteins formed. Unable to transport fat out of liver. Results in ‘fatty’ liver.
Desc. the metabolism of alcohol
Ethanol —> Acetaldehyde —> Acetate
[alcohol dehydrogenase] [aldehyde dehydrogenase]
- Acetate –> acetyl CoA (enter TCA or FA synthesis)
What is the recommended limit of alcohol for both men and women in a week? What is the kinetics displayed by alcohol removal?
- 14 units spread over 3 days in a week
- Exhibits zero-order kinetics (eliminated in linear fashion) = undergo constant removal regardless of plasma conc.
Which compound formed in alcohol metabolism is toxic and what keeps its effects to a minimum?
- Acetaldehyde
- Aldehyde dehydrogenase has a very low Km
How does alcohol damage affect liver?
[3 disease]
- Fatty liver
- Alcoholic hepatitis
- Alcoholic cirrhosis (damage)
What is the response to chronic alcohol consumption?
- Decrease NAD+ –> lactate X converted to pyruvate –> lactate ⬆️ –> lactic acidosis + kidney ability to excrete uric acid ⬇️–> monosodium urate crystals form –> phagocytes remove, die –> release lysosomal enzyme –> cell lysis & inflammation –> gout
- Decrease NAD+ –> X glycerol metabolism –> ⬇️gluconeogenesis –> hypoglycaemia
- Increased acetyl~CoA –> ⬆️FA + ketone bodies + TAG –> lower lipoprotein synthesis –> fatty liver
Explain the mechanism of action of Disulfiram
- Inhibit aldehyde dehydrogenase –> acetaldehyde accumulate –> ‘hangover’
- Used with other methods to stop drinking
What diseases are oxidative stress linked to?
- Cardiovascular disease
- Alzheimer’s disease
- Rheumatoid arthritis
- Multiple sclerosis
- Cancer
- COPD
What are free radicals?
An atom/molecule that has unpaired electrons and is very reactive
What are RNS and give some examples
- Superoxide react w NO to form peroxynitrite
- Not a free radical –> powerful oxidant
What are ROS and give some examples
- Superoxide and Hydroxyl radical
- H2O2 X a radical but react with Fe2+ to produce radicals
How does ROS affect DNA?
- Reacts with base: lead to mispairing –> mutation
- Reacts w sugar: Strand break/mutation
- Mutation can lead to cancer
How does ROS cause damage to proteins?
- Reacts w proteins –> disulphide bond form –> fragment/ change in protein structure –> loss of function/change of func –> protein degradation
- Lead to misfolding/crosslinking –> Heinz bodies in RBC
What is the function of disulphide bonds?
- Important role in folding and stability of proteins
- Formed between thiol groups of cysteine
How does ROS damage lipids? x2
- Lipid peroxidation (atherosclerosis)
- ROS react w polyunsaturated FA –> lipid radical/ lipid peroxyl radical (after O2) –> chain reaction, react with more FA by extracting H atom –> Lipid bilayer integrity fails
What are some sources of biological oxidants?
Endo:
- ETC
- Peroxidases
- NADPH oxidases
- NO oxidases
Exo:
- Radiation
- Drugs (Primaquine, anti-malarial)
- Pollutants
- Toxins (Paraquat, herbicide)
How is the ETC a source of ROS?
- Occasionally electrons escape ETC –> react w O2 –> superoxide
What is a respiratory burst?
- Rapid production of superoxide and H2O2 by phagocytes to kill bacteria
- Neutrophils and monocytes
What is chronic granulomatous disease?
- Genetic defect in NADPH oxidase complex –> ⬆️risk to bacterial infections
- Pneumonia, Impetigo (skin infection), cellulitis (infection in deeper layer of skin)
What are some cellular defences against ROS and RNS?
- Superoxide dismutase (SOD):
i) Converts superoxide to H2O2 and O2 - Catalase:
i) Converts H2O2 to H2O & O2
II) Declining levels associated w grey hair - Glutathione:
i) Thiol group of Cys- donates e- to ROS
ii) Reacts with another GSH to form disulphide
[catalysed by glutathione peroxidase]
*requires selenium
iii) GSH reduced back by glutathione reductase –> transfer e- to NADPH
What vitamins are cellular defences and how?
- Vit E: Prevent peroxidation –> reduce free radical by donating H atom in nonezymatic reaction
- Vit C: Regenerate reduced form of Vit E
BONUS: Carotenoids, Uric acid, Melatonin
What are Heinz bodies?
- Hb in RBC form disulphide bonds –> aggregates
- Increased mechanical stress when squeezing through capillaries
- Sign of G6PDH deficiency
- Removed by spleen
How does paracetamol cause damage? How is overdose treated?
- High levels –> NAPQI ⬆️–> Lipid peroxidation, damage to proteins + DNA
- Acetylcysteine replenish glutathione levels
Why is creatinine measured?
- Breakdown product of creatine/creatine.P
- Produced at a constant rate, proportional to muscle mass
- Indicator of renal function: raised plasma level + low urine level = damage to nephrons
Bonus: Plot graph of 24hr urine creatinine muscle mass X muscle mass
Describe the different types of nitrogen balance
- N equilibrium: Intake = Loss
[Normal state, No change in body protein]
2.Positive N balance: Intake > Loss
[Growth/Pregnancy/Recovering from malnutrition, ⬆️in total body protein]
- Negative N balance: Intake < Loss
[Never normal/Trauma/Infection/Malnutrition, ⬇️body protein]
Name glucogenic, ketogenic and both a.a & desc. their function
- Glucogenic: Alanine
(a. a can be used to make glucose) - Ketogenic: Lysine, Leucine
(a. a can be used to make ketone bodies, acetyl CoA) - Both: Tyrosine
When does mobilisation of protein reserves occur? What inhibits/stimulates it?
- Occurs in liver
- Extreme stress (starvation)
- Inhibit: Insulin + GH, Stimulate: Cortisol (glucocorticoid)
What are the conditionally essential a.a?
- Children and pregnant women
- ACT = Arginine, Cysteine, Tyrosine
Where does the carbon atoms for a.a synthesis come from?
- TCA cycle
- PPP
- Glycolysis
Where does the amino group for the synthesis of a.a come from?
- Transamination from other a.a
- Ammonia
What is tyrosine, cysteine and tryptophan used to synthesise?
- Tyrosine: T3,T4 & melanin
- Cysteine: Glutathione
- Tryptophan: Melanin
Why is removal of nitrogen from a.a essential? Which pathways facilitate the removal?
- Allow carbon skeleton of a.a –> oxidative metabolism
- Nitrogen –> urea
- Pathways: Transamination & Deamination
What is transamination? What enzyme catalyses the reaction, what is their mode of action?
- Transfer amine group from one a.a to another
- Aminotransferase enzyme use α-ketoglutarate –> trf to glutamate
- Require coenzyme pyridoxal phosphate (Vit B6)
What are some key aminotransferase enzymes? What do they test for?
- Alanine (ALT), Aspartate (AST)
- Liver function test
- High in conditions: Viral hepatitis, toxic injury, autoimmune liver disease
What is deamination? Where does it occur? Give some examples of enzymes that can perform deamination.
- Changes amino group –> ammonia (very toxic) –> urea
- Done in liver and kidney
- Keto acids –> metabolism
- Enzymes: Glutamate dehydrogenase, glutaminase
Features of urea
- High N content
- Inert, water sol, excreted in urea, osmotic role
Where does the urea cycle occur? What regulates it?
- Occur in liver & 5 enzymes
- High protein diet activate enzyme, Low represses it
What are some consequences and causes of defects in the urea cycle?
- Autosomal recessive genetic disorders deficiency in one of enzymes
- Leads to: hyperammonaemia, accumulation of urea cycle intermediates
What are some symptoms of defects in urea cycle and its treatment?
- Symptoms: Vomiting, Lethargy, Mental retardation, Seizures, Coma
- Low protein diet, replace a.a with keto acids
N.B Severe case = die, Mild = symptoms show late
Why is ammonia toxic?
- Readily diffusible, toxic to brain
- Affects pH (alkaline)
- Interfere w TCA cycle/ protein synthesis
What are mechanisms used to transport a.a?
- Glutamine
- A.a + glutamate –> glutamine
- Transported in blood, cleaved by glutaminase (form glutamate & a.a –> urine) - Alanine
- Amine group transfered to glutamate (transamination) –> T. to pyruvate forms alanine
- Alanine (blood) –> liver convert back to pyruvate –> amino group (urea) + pyruvate (TCA)
How are defects in a.a metabolism detected and treated? What are some examples?
- Rare
- Heel prick test
- Phenylketonuria (PKU),
- Homocystinuria
- Treatment: X a.a in diet, modify diet at early age (effect less)
What is phenylketonuria (PKU)? Treatment?
- Deficiency in phenylalanine hydroxylase
- Accumulation of phenylalanine in tissue –> phenylketones in urine
- T: Low PhA diet enriched w tyrosine, avoid artificial sweeteners + high protein foods (milk, meat, eggs)
What are the affected pathways in PKU and what are its symptoms?
- X phenylalanine hydroxylase –> PhA X converted to tyrosine –> X T3,T4, melanin synthesis
- S: microcephaly (small head), hypopigmentation, intellectual disability, delayed development
**CAN BE AVOIDED W EARLY INTERVENTION
What is homocystinuria? Treatment?
- Problem breaking down methionine –> ⬆️homocystine
- Autosomal recessive
- Defect in cystathionine β-synthase
- T: low methionine diet, avoid high protein foods, X nuts, cysteine/vit B6 supplement
Effect of homocystinuria?
- Accumulation of homocysteine due to cystathionine β-synthase deficiency (require vit B6 co-factor)
- Elevated H.C –> cardiovascular disease
How does protein turnover occur?
- Free amino acids from proteolysis of muscle proteins and digestion (also used for synthesis of muscle proteins)
- Free a.a go to the liver –> amino group removed form urea –> urine
- Free a.a –> carbon skeleton used (glucogenic + ketogenic) –> energy
