Metabolism 5.1 Protein Metabolism
1. Metabolic Overview
Learning Outcomes
Describe amino acid catabolism and ammonia metabolism.
Explain clinical relevance of creatinine, defects in amino acid metabolism, protein/energy deficiency consequences, and specific defects (phenylketonuria, homocystinuria).
Lecture Outline
Covers amino acids, synthesis, nitrogen balance, protein turnover, catabolism, transamination, deamination, ammonia transport, urea cycle, and specific disorders (PKU, homocystinuria).
Metabolic Overview: Protein and Energy Metabolism (Stage Model)
Diet Components: Proteins, Carbohydrates, Lipids (Triacylglycerols), Alcohol.
Stage 1: Digestion
Breaks down into amino acids, monosaccharides, glycerol, fatty acids.
Stage 2: Nitrogen Pool and Energy Intermediates
Gluconeogenesis from glucogenic amino acids.
Glycolysis, glycogenolysis, lipogenesis.
\beta-oxidation of fatty acids.
Lactic acid \rightleftarrows Pyruvate for energy and gluconeogenesis.
Stage 3: Urea Cycle
Removal of nitrogen as urea (ammonia detoxification).
Stage 4: Electron Transport Chain and ATP Production
Overall Purpose: Ammonia detoxification via urea, preservation of carbon skeletons for energy storage or glucose production.
Key Metabolites/Energy Carriers: NADH, NAD^{+}, FADH{2}, FAD, O{2}, CO_{2}, ATP, ADP, Pi .
2. Amino Acids and Nitrogen Balance
Major Nitrogen-Containing Compounds
Amino acids, proteins.
Purines & pyrimidines (DNA/RNA).
Porphyrins (heme), creatine, neurotransmitters (e.g., dopamine), some hormones (e.g., adrenaline).
Important concepts: nitrogen balance, protein turnover.
Amino Acids Structure
20 standard amino acids, each with a unique side chain (R groups).
Components: Amino group (-NH_{2}), Carboxyl group (-COOH), Alpha carbon, and Side chain (R).
General formula scaffold: NH_2-CH(R)-COOH.
In nitrogen balance, the body manages nitrogen from amino acids; the rest forms various nitrogen-containing compounds.
Essential vs. Non-Essential Amino Acids
Essential Amino Acids (9 from diet): Isoleucine, Lysine, Threonine, Histidine, Leucine, Methionine, Phenylalanine, Tryptophan, Valine.
Conditionally Essential Amino Acids: Arginine, Tyrosine, Cysteine (in specific situations like children/pregnant individuals).
Non-Essential Amino Acids: Can be synthesized endogenously from metabolic intermediates.
Sources of Carbon Skeletons for Amino Acid Synthesis
Intermediates of glycolysis (C3).
Pentose phosphate pathway (C4 and C5).
TCA cycle intermediates (C4 and C5).
Amino group provided by other amino acids via transamination or from ammonia.
Amino Acids for Other Nitrogen-Containing Compounds
Arginine \to nitric oxide (NO).
Cysteine \to hydrogen sulfide, glutathione.
Tyrosine \to catecholamines, melanin, thyroid hormones.
Glycine \to purines, glutathione, haem, creatine.
Glutamate \to GABA.
Tryptophan \to nicotinamide, serotonin (5-HT), melatonin.
Histidine \to histamine.
Serine \to sphingosine.
Nitrogen Balance States
Zero Nitrogen Balance (N equilibrium): Intake = output; no net change in total body protein; normal in healthy adults.
Positive Nitrogen Balance: Intake > output; net gain in body protein; normal during growth, pregnancy, or recovery from malnutrition.
Negative Nitrogen Balance: Intake < output; net loss of body protein; never normal; caused by trauma, illness, burns, or malnutrition.
Illustrative Magnitudes (70 kg male):
Body proteins \approx 2 kg N-containing compounds \approx 60 g N.
Amino acid pool \approx 16 g N.
Dietary N intake \approx 16 g N/day, waste N excretion \approx 14 g N/day (in balanced state).
Creatinine as a Clinical Marker
Breakdown product of creatine and creatine phosphate in muscle.
Produced at a constant rate proportional to muscle mass; excreted in urine.
Urinary excretion over 24 h estimates muscle mass.
Elevated blood creatinine suggests renal (kidney) dysfunction/nephron damage.
Reference/excretion ranges: Men: 14-26\ \text{mg/kg/day}; Women: 11-20\ \text{mg/kg/day}.
3. Protein Catabolism and Nitrogen Removal
Protein Fuel Stores and Mobilization
Major Fuel Stores (illustrative):
Triacylglycerol: weight \sim 15\ \text{kg} ; energy content \sim 6 \times 10^{5}\ \text{kJ}.
Glycogen: weight \sim 0.4\ \text{kg} ; energy \sim 4.0 \times 10^{3}\ \text{kJ}.
Muscle protein: weight \sim 0.6\ \text{kg} ; energy \sim 1.0 \times 10^{5}\ \text{kJ}.
Mobilization: Occurs under extreme stress (starvation/trauma) and is hormonally controlled.
Hormonal Effects:
Insulin and growth hormone: \uparrow protein synthesis; \downarrow protein degradation.
Glucocorticoids (e.g., cortisol): \downarrow protein synthesis; \uparrow protein degradation.
Clinical Note: Excessive protein breakdown in Cushing’s syndrome (excess cortisol) weakens skin structure and can cause striae.
Dietary Protein and Nitrogen Balance in Different States
Positive Nitrogen Balance in Pregnancy (anabolic state):
Decreased nitrogen excretion, increased fetal and maternal protein synthesis.
Hormonal milieu can influence urea cycle enzymes.
Higher blood glucose can reduce amino acid catabolism.
Negative Nitrogen Balance in Hypothyroidism:
Reduced metabolic stimulation.
May lead to weight gain and altered protein metabolism (reduced synthesis and degradation rates).
Dietary Protein Digestion and Protein Turnover
Dietary protein digestion yields free amino acids; cellular proteins undergo proteolysis and turnover.
Synthesis of new amino acids and proteins from carbon skeletons and amino groups.
Fate of Amino Acids after Uptake:
Some become glucogenic amino acids and feed into gluconeogenesis.
Some become ketogenic amino acids that form ketone bodies or acetyl-CoA.
Ammonia is removed as part of nitrogen metabolism, most commonly via the urea cycle.
Removal of Nitrogen from Amino Acids
Essential step to enable carbon skeletons to enter oxidative metabolism.
Fate of Removed Nitrogen: Converted to urea and excreted, or incorporated into other compounds.
Two Main Pathways:
Transamination: Transfers \text{NH}_2 from an amino acid to a ketoacid, creating a new amino acid.
Deamination: Removes ammonia (\text{NH}_3) from an amino acid.
Transamination is used to funnel amino groups into glutamate via \alpha-ketoglutarate.
Glucogenic and Ketogenic Amino Acids
Glucogenic Amino Acids: Precursors of gluconeogenesis; yield pyruvate or TCA cycle intermediates.
Ketogenic Amino Acids: Form acetyl-CoA or acetoacetyl-CoA.
Mixed Amino Acids: Can give rise to both glucogenic and ketogenic products.
Transamination and Aminotransferases
\text{NH}_2 group transfer from an amino acid to \alpha-ketoglutarate \to glutamate.
Most aminotransferases use \alpha-ketoglutarate to funnel amino groups to glutamate.
Resulting keto acids can be used for energy.
Cofactor: Pyridoxal phosphate (PLP), a derivative of vitamin B6.
Example: Alanine \rightleftarrows pyruvate; Aspartate \rightleftarrows oxaloacetate.
Diagnostic Markers: Aminotransferases
Key Enzymes Measured in Liver Function Tests:
Alanine aminotransferase (ALT): converts alanine to glutamate.
Aspartate aminotransferase (AST): converts aspartate to glutamate.
High levels of ALT and AST indicate liver cell necrosis.
AST/ALT ratio > 2 suggests alcohol-related liver disease.
Deamination and Ammonia Handling
Mainly occurs in liver and kidney; releases \text{NH}2 as free ammonia (\text{NH}3).
Ammonia is highly toxic and must be removed.
Ammonia is converted to urea or excreted directly in urine.
Deaminating Enzymes: Amino acid oxidases, glutaminase, and glutamate dehydrogenase.
Dietary D-amino acids also undergo deamination.
Ammonia Toxicity and the Brain
Ammonia readily crosses membranes and the blood–brain barrier (BBB).
Brain ammonia levels must be kept low (e.g., 25-40\ \mu\text{mol/L}).
Toxic Effects include:
Interference with amino acid transport and protein synthesis.
pH disturbances (alkalosis).
Disruption of TCA cycle through reaction with \alpha-ketoglutarate to form glutamate.
Alteration of BBB and cerebral blood flow.
Interference with metabolism of excitatory amino acids (glutamate, aspartate).
Transport of Ammonia
Main pathways: amino acids \to glutamate \to glutamine.
Glutamate can be converted to glutamine via glutamine synthetase for safe transport.
Brain and peripheral tissues exchange nitrogen via alanine and glutamine cycles.
Glucose–Alanine Cycle: Alanine shuttles \text{NH}_3 equivalents from muscle to liver for urea synthesis.
In liver, \text{NH}3 is incorporated into glutamine or alanine precursors; in liver, \text{NH}3 enters the urea cycle.
4. Urea Cycle and Disorders
Urea: Properties and Purpose
Contains a high nitrogen content; non-toxic and highly water soluble.
Chemically inert in humans; bacteria in the gut can hydrolyze urea to ammonia.
Synthesised in the liver by the urea cycle; most urea excreted in urine via kidneys.
Also helps osmotic balance in kidney tubules.
Structural formula: \text{NH}{2}\ \text{-C(=O)-}\ \text{NH}{2}.
Primary vehicle for nitrogen excretion in mammals.
The Urea Cycle: Steps and Enzymes
Overall concept: convert toxic ammonia into non-toxic urea for renal excretion.
Key Steps and Enzymes (in order):
Carbamoyl phosphate synthetase I (CPS1): Mitochondrial enzyme; \text{NH}3 + \text{CO}2 \to \text{carbamoyl phosphate}.
Ornithine transcarbamylase (OTC): Citrulline synthesis from carbamoyl phosphate and ornithine.
Argininosuccinate synthetase (ASS1): Citrulline + aspartate \to argininosuccinate; uses ATP.
Argininosuccinate lyase (ASL): Argininosuccinate \to arginine + fumarate.
Arginase: Arginine \to urea + ornithine (reused in the cycle).
Energy Cost: The cycle consumes high-energy phosphate bonds (e.g., \sim 4 ATP equivalents per urea formed).
Cellular Localisation: CPS1 is mitochondrial; other steps occur in the cytosol.
Regulation of the Urea Cycle and Refeeding Considerations
Urea cycle enzyme levels are normally matched to ammonia disposal needs.
Inducible by high protein intake; repressed by low protein or starvation.
Refeeding Syndrome Risk:
Occurs when severely malnourished patients receive nutrition too quickly.
Start at about 5-10\ \text{kcal/kg/day} and increase gradually.
Goal: raise to full needs within a week to avoid ammonia toxicity due to low urea cycle activity.
Urea Cycle Disorders (Autosomal Recessive)
Deficiency of one urea cycle enzyme leads to hyperammonemia and accumulation/excretion of cycle intermediates.
Common Defects and Implications:
CPS1 deficiency.
Ornithine transcarbamylase (OTC) deficiency (most common).
Argininosuccinate synthetase deficiency (citrullinemia).
Argininosuccinate lyase deficiency (ASA/argininosuccinic aciduria).
Arginase deficiency (argininemia).
Clinical: Spectrum from severe neonatal to mild childhood presentations; management includes dietary protein restriction and providing keto acids as amino acid surrogates.
Ornithine Transcarbamylase (OTC) Deficiency: Clinical Details
Severity depends on the defect and protein intake.
Severe urea cycle disorders often present within 1 day of birth; untreated can be fatal.
Mild deficiencies may present later in childhood.
Management: Low-protein diet and replacement amino acids with keto acids.
Common Symptoms: Vomiting, lethargy, irritability, developmental delay, seizures, coma.
5. Amino Acid Metabolism Disorders
Amino Acid Metabolism Disorders: Overview and Management
More than 50 inherited diseases affecting amino acid metabolism.
Many disorders are extremely rare (<1:250,000) but collectively significant in paediatric genetics.
Often involve partial loss of enzyme activity; untreated cases can lead to intellectual impairment.
Treatment: Focuses on restricting specific amino acids in the diet.
Newborn screening (heel-prick test) enables early detection.
Examples: PKU, maple syrup urine disease (MSUD), homocystinuria. Heel-prick test is part of newborn screening programs.
Phenylketonuria (PKU)
Most common inborn error of amino acid metabolism (\sim 1\ \text{in}\ 1.5\times 10^{4} births).
Defect: Phenylalanine hydroxylase (PAH) deficiency; autosomal recessive (chromosome 12).
Phenylalanine accumulates in tissue, plasma, and urine; phenylketones appear in urine (oxidize to phenylacetate, giving a musty odor).
Treatment: Strict low phenylalanine diet plus tyrosine supplementation; avoid artificial sweeteners containing phenylalanine; avoid high-protein foods.
Pathway Consequences: Phenylalanine \to tyrosine pathway is impaired; downstream products (noradrenaline, adrenaline, dopamine, melanin, thyroid hormones) are reduced; protein synthesis can be affected.
Early intervention can prevent severe cognitive impairment and other complications.
PKU Biochemical Pathways and Symptoms (Untreated)
Accumulation of phenylalanine leads to metabolic and neuromotor issues.
Transporter/transamination disruptions affect multiple pathways; early dietary control mitigates adverse outcomes.
Observed Symptoms without Early Treatment: Severe intellectual disability, developmental delay, microcephaly, seizures, hypopigmentation.
Homocystinurias
Rare disorders (\sim 1\ \text{in}\ 344,000), most commonly due to defect in cystathionine \beta-synthase (CBS); methionine synthase defect possible.
Autosomal recessive; excess homocystine (oxidised homocysteine) excreted in urine.
Accumulation of homocysteine and methionine causes disease manifestations.
Dietary Management: Low methionine diet; avoid methionine-rich foods (milk, meat, fish, cheese, eggs; nuts and peanut butter also contain methionine).
Supplements: Cysteine, vitamin B6 (pyridoxine), betaine, B12, folate.
Homocystinuria: Pathway and Clinical Features
Pathway: Methionine \to homocysteine \to cystathionine (requires active CBS and vitamin B6 co-factor).
Deficiency leads to elevated plasma homocysteine, associated with cardiovascular risk.
Symptoms: Lens dislocation, long limbs and fingers, intellectual disability.
Alternative enzyme defect: methionine synthase can also be involved.
6. Practical Implications and Takeaways
Summary of Key Concepts
Non-essential amino acids can be synthesized from glycolysis, PPP, and the Krebs cycle intermediates.
Amino acid catabolism involves removal of nitrogen via transamination and deamination; carbon skeletons are diverted to glucose (glucogenic) or ketone bodies/acetyl-CoA (ketogenic).
Ammonia is highly toxic and rapidly converted to urea; urea cycle defects cause hyperammonemia.
Amino acid metabolism disorders can be diagnosed in newborns via heel-prick screening and managed through restricted amino acid diets.
PKU is the most common congenital amino acid metabolism disorder; defective phenylalanine hydroxylase leads to phenylalanine accumulation and widespread metabolic effects.
Homocystinuria results from CBS or methionine synthase defects, with elevated homocysteine and methionine; dietary and co-factor therapy can mitigate symptoms.
Practical and Clinical Implications
Creatinine measurement is routinely used to assess renal function and estimate muscle mass.
Understanding nitrogen balance helps in managing nutrition in pregnancy, illness, burns, and malnutrition.
In burn care, early aggressive nutrition is critical to minimize catabolism; monitor hormonal and metabolic shifts during ebb and flow phases.
Refeeding syndrome risk requires careful nutritional ramp-up to prevent ammonia-related complications.
Practical Takeaways
The liver handles nitrogen disposal via the urea cycle; defects can lead to dangerous hyperammonemia.
Transamination and deamination are central to amino acid catabolism; PLP (vitamin B6) is a key cofactor.
PKU and homocystinuria are classic inherited metabolic disorders with specific dietary management strategies.
Monitoring nitrogen balance and creatinine provides clinical insights into nutritional status and renal function.
Burns and severe illness dramatically shift protein metabolism, necessitating tailored, cautious nutrition plans to avoid catabolic loss and complications.