Protein And Amino Acid Metabolism Flashcards
Nitrogen metabolism
Major nitrogen containing compounds -
AA,
proteins (both these are the main nitrogen containing compounds),
purines and pyrimidines, creatine phosphate, NT and hormones
Creatinine - a clinical marker
Breakdown product of creatine & creatine phosphate in muscle
Usually produced at constant rate depending on muscle mass (unless muscle is wasting)
Excreted in urine per day - Men 14-26 mg/kg, Women 11-20 mg/kg
Filtered via kidneys into urine
Creatinine urine excretion over 24h proportional to muscle mass
Provides estimate of muscle mass
Also commonly used as indicator of renal function (raised level on damage to nephrons)
Nitrogen balance
N equilibrium - intake = output - no change in total body protein, normal state in adult
Positive N balance - intake > output - increase in total body protein, normal state in growth and pregnancy or in adult recovering form malnutrition
Negative N balance - intake < output - net loss of body protein. Never normal, causes trauma, infection or malnutrition
Protein turnover
Free amino acids are AA that have not been encorporrated into proteins yet
2 outcomes for these free AA - they are either encoporated into cellular proteins or they are sent to the liver to be broken down
The carbon skeleton is either broken down by glucogenesis or ketogenesis to generate energy.
The amino group is converted into urea and then excreted in the urine (to prevent the build up of toxic ammonia
Glucogenic and ketogenic AA
E.g. of AA - glucogenic = Cysteine/valine,
Ketogenic = Lysine + Leucine,
Both glucogenic and ketogenic = Isoleucine/Tyrosine
Mobilisation of proteins reserves
Occurs during extreme stress (Starvation) or during hormonal control (e.g. when insulin and growth hormone are present protein synthesis increase and proteins degredation decreases (vice versa for when glucocorticoids are present))
Excessive breakdown of protein can occur in Cushing syndrome (excess cortisol) - weakness skin sturcutre leading to striae formation - in this disease fat mobilisation occurs and is mainly depositied in the abdomen causing the skin to stretch - this plus the fact that the disease causes muscle weakness so the abdomen muscles are not strong enough to resist the stretch so you get excessive stretch marks
9 essential AA - Isoleucine, lysine, threonine, histidine, leucine, methionine, phenylalanine, tryptophan, valine
AA synthesis:
In addition to dietary intake, body can synthesise some amino acids it requires (the non-essential amino acids)
Carbon atoms for non-essential amino acid synthesis come from:
Intermediates of glycolysis (C3)
Pentose phosphate pathway (C4 & C5)
Krebs cycle (C4 & C5)
Amino group provided by other amino acids by the process of transamination or from ammonia
Synthesis of important nitrogen containing compounds
In addition to protein synthesis (requires all 20 amino acids) amino acids also required for synthesis of other important compounds (requires specific amino acids).
Removal of nitrogen from AA
Removal of amino group is essential to allow carbon skeleton of amino acids to be utilised in oxidative metabolism
Once removed nitrogen can be incorporated into other compounds or excreted from body as urea Two main pathways facilitate removal of nitrogen from amino acids
Transamination and Deamination
Transamination
This involves swapping the amino group of 2 AA, pushing these amino groups to the AA glutamate and aspartate - as these can be fed into the urea cycle
Most aminotransferase enzymes use α-ketoglutarate to funnel the amino group to glutamate.
Exception to rule is aspartate aminotransferase which uses oxaloacetate to funnel amino group to aspartate
All aminotransferases require the coenzyme pyridoxal phosphate which is a derivative of vitamin B6
ALT converts alanine to glutamate
Aspartate aminotransferase (AST) Converts glutamate to aspartate
Plasma ALT and AST levels measured routinely as part of liver function test
Levels particularly high in conditions that cause extensive cellular necrosis such as:
• Viral hepatitis
• Autoimmune Liver Diseases
• Toxic injury
Death Cap mushroom can cause acute liver failure if ingested (increases Plasma ALT levels up to 20x normal)
Deamination
Liberates amino group as free ammonia
Mainly occurs in liver & kidney
Keto acids can be utilised for energy
Also important in deamination of dietary D-amino acids (found in plants and microorganisms)
Ammonia (and ammonium ions) very toxic and must be removed. Ultimately converted to urea or excreted directly in urine
Several enzymes can deaminate amino acids
Amino acid oxidases
Glutaminase
Glutamate dehydrogenase
At physiological pH, ammonia (NH3) is converted to ammonium ion (NH4+)
Urea
High nitrogen content
Non-toxic
Extremely water soluble
Chemically inert in humans (bacteria can break it down to release NH3)
Most urea is excreted in urine via the kidneys
Also performs useful osmotic role in kidney tubules
Urea cycle
Aspartate and glutamate easily feed into urea cycle
All enzymes in the cycle are either up regulated when the demand is needed or down regulated if the demands are lower
The cycle occurs half in the M and half in the cytoplasm
Ammonia can also feed into the cycle by being converted into Carbamoyl phosphate (using 2 ATP)
Arginine to Ornithine release water and Urea
Therefore getting rid of the amine groups of AA safely
Amount of urea cycle enzymes normally related to need to dispose of ammonia
High protein diet induces enzyme levels
Low protein diet or starvation represses levels
Cycle is inducible but not regulated
Refeeding syndrome - Can occur when nutritional support given to severely malnourished patients
Ammonia toxicity significant factor (urea cycle down regulated) - the person hasn’t got the capacity in urea cycle to process a high protein diet, so a build up of toxic ammonia occurs
Re-feed @ 5 to 10 kcal/kg/day. Raise gradually to full needs within a week
Defects in the urea cycle
Autosomal recessive genetic disorders caused by deficiency of one of enzymes in the urea cycle
Occur ~1 in 30,000 live births
Mutations cause a partial loss of enzyme function
Leads to - hyperammonaemia and accumulation/excretion of urea cycle intermediates
E.g. Arginase deficiency
Clinical picture - (NH3 toxicity)
Severity depends on nature of defect and amount of protein eaten
Severe urea cycle disorders show symptoms within 1 day after birth. If untreated, child will die.
Mild urea cycle enzyme deficiencies may not show symptoms until early childhood
Management - Low protein diet and Replace amino acids in diet with keto acids
Symptoms - Vomiting, Lethargy, Irritability, Mental retardation, Seizures, Coma
Biochemical basis of ammonia toxicity
Readily diffusible and extremely toxic to brain
Blood level needs to be kept low (25-40 µmol/L)
Several potential toxic effects:
- Interference with amino acid transport and protein synthesis
- Disruption of cerebral blood flow
pH effects (its alkaline, so could cause alkalosis in blood)
- Interference with metabolism of excitatory amino acid neurotransmitters (e.g. glutamate and aspartate)
- Alteration of the blood–brain barrier
- Interference with Krebs cycle (reacts with α-ketoglutarate to form glutamate)
