Protein metabolism and nitrogen balance Flashcards
Learning outcomes
- Explain the role of amino acids in protein metabolism
- Outline the hormonal control of protein metabolism
- Name the major waste products of protein and amino acid catabolism and describe their metabolism/excretion
- Indicate how nitrogen balance is measured and conditions which may lead to positive or negative nitrogen balance
- Explain the relationship between energy metabolism and protein metabolism
Amino acids
Building blocks of proteins:
There are 20 amino acids in the human body.
• Common features
• Acidic group - COOH
• Amino group -NH2
• 10 amino acids cannot be synthesised so must be derived from diet-the other 10 are required for protein formation but are “non-essential”
Amino acids link through peptide bonds between groups (hydrolysis reaction, +H2O)
Circulation of absorbed amino acids
Proteins are largely absorbed in the ileum as amino acids via secondary active transport.
•20-30 g/day required to offset obligatory losses.
•Digestion occurs over hours so only small quantities are absorbed at any one time.
•Normal blood levels: 35-65 mg/dl, excess circulating AAs are taken up by cells within 5-10 min.
•High protein turnover between cells/tissues allows rapid redistribution.
•Movement across membranes by facilitated diffusion or secondary active transport.
•Amino acids are actively reabsorbed in proximal tubules but if threshold is exceeded are excreted in urine.
Metabolism of amino acids
- Diet (digestion) > amino acids
- Amino acids > a) wide range of proteins and some metabolism <> ~250g protein
b) deamination (~20g protein pd) a) fatty acids/ ketone bodies
Ammonia > urea (through urea cycle)
Storage of amino acids
- Unlike glucose and fatty acids free amino acids cannot be stored.
- Free amino acids quickly combine to form peptides and intracellular proteins.
- When proteins are required, intracellular proteins are degraded into AAs by lysosomal enzymes and are then transported into the blood.
- Reverse equilibrium of proteins
- Maintains low levels of circulating AAs
- AAs are constantly available
- The liver has a large capacity for storing proteins, but kidneys and the intestinal mucosa are also used.
- Each cell has a limited capacity protein storage and excess amino acids are used immediately for energy or are converted to fat/glycogen.
- Influenced by hormones – growth hormone and insulin increase protein synthesis while glucocorticoids mobilise amino acids.
Plasma proteins
Albumin -Generates colloid osmotic pressure which prevents capillary plasma loss.
Globulins -Innate and acquired immunity e.g., gG, IgA, IgM.
Transferrin -Carriage of ferrous ions in circulation.
Fibrinogen -Mediates clot formation and repairs leaks in the circulatory system.
Plasma proteins are a readily available AA source• Majority plasma proteins are formed by the liver (up to 30 g/day).
• Production increases in certain stress conditions e.g., severe burns, renal disease.
• Plasma protein synthesis decreasesin liver disease causing oedema e.g., cirrhosis.
• Up to 400 g/day protein turnover.
• Total protein to plasma protein ratio is steady ~33:1.
• Whole body protein depletion can be readily treated with i.v.plasma proteins
Stages of protein metabolism in liver
- Transamination -for synthesis of non-essential AAs
- Deamination- for degradation and energy generation
- Oxidation- to generate energy by gluconeogenesis or ketogenesis
- Excretion- via urea cycle
Synthesis of amino acids
•Transamination– an amino group is transferred from an amino acid to anα-keto acid by an aminotransferase/transaminase.
• Requires formation of appropriate α-keto acids which act as precursors for the reaction.
-Glutamine is the main AA substrate, asparagine, glutamic acid, and aspartic acid are also used.
• Aminotransferases use vitamin B6as a co-factor – B6 deficiency severely decreases protein synthesis.
Protein as a source of energy
Excess amino acids are degraded by the liver starting with deamination by aminotransferases.
•Deamination is the removal of amino groups from amino acids and involves transamination in a reverse process to the synthesis of amino acids.
• Oxidative deamination:
AA (aminotransferase)> A-ketoacid >
A-ketoglutarate> glutamate >
NAD+ + H2O (glutarate dehydrogenase) > NADH + NH4+ >
urea
α-ketoglutarate can enter the TCA cycle (gluconeogenesis), or its amino group can be transferred or released as ammonium in the urea cycle.
Energetic use of deaminated amino acids
Glucogenesis
• e.g, α-ketoglutarate can enter the TCA cycle to generate glucose.
•Ketogenesis
• Acetyl CoA or acetoacetylCoA can enter the TCA cycle or can generate ketones.
Amino acid classifications
Glucogenic amino acids- Alanine, Arginine, Asparagine, Aspartic acid, Cysteine, Glutamic acid, Glutamine, Glycine, Histidine, Methionine, Proline, Serine,Valine
PITTT- both glucogenic and ketogenic
- Phenylalanine
- Isoleucine
- Threonine
- Tryptophan
- Tyrosine
Ketogenic- leucine and lysine
Urea formation and the urea cycle
• Ammonia (NH3) released during deamination is highly toxic and is rapidly ionised to NH4+.
• NH4+ is converted to urea which may then be excreted by the kidneys.
Urea contains two NH2 groups- one from NH4+ (carbamoyl phosphate), one from aspartate
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ornithine, carbamoyl phosphate, citrulline, aspartate, argininosuccinate, fumarate, arginine
Temporal fat stores
Carbohydrate– only few hundred grams stored as glycogen
•Protein– rapid depletion of readily accessible stores by gluconeogenesis
•Fat– primary source of stored energy so used until depleted (ketosis)
•Protein- largely utilised only when all carbohydrate and fat stores depleted
• Chronic starvation leads to protein depletion
Hormonal regulation of protein metabolism
Growth hormone
• Promotes synthesis of cellular proteins
• Increases AA membrane transport
• Increases RNA transcription/translation
Insulin
• Promotes cellular uptake of amino acids
• Inhibits protein catabolism
• Increases RNA transcription/translation
• Decreases gluconeogenesis
Growth hormone and insulin• Both are required for effective protein synthesis• Synergistic interaction - act on different AAs?• NB Lack of insulin ⇒increased plasma AAs ⇒energy/gluconeogenesis ⇒muscle wasting
Testosterone
• Transient muscle growth – induces contractile proteins.
Oestrogen
• Muscle growth but minor relative to testosterone.
Thyroxine• Increases cell metabolism ⇒activates anabolic/catabolic protein pathways.
• If carbs and fat are low increases protein degradation, if adequate carbs and fat increases protein synthesis.
Glucocorticoids
• Increase protein breakdown, increasing levels of circulating AAs, hepatic and plasma proteins ⇒gluconeogenesis.
Inborn errors of protein metabolism
Relatively rare and many are inherited autosomal recessive disorders
Enzyme defects in the urea cycle e.g.
• Citrullinemia – deficiency in arginosuccinate synthase results in the accumulation of ammonia and other toxic substances in the blood. Deficiencies in enzymes involved in metabolism of AAs e.g.
• Tyrosinemia – defective liver tyrosine aminotransferase leads to progressive liver disease, mental retardation, osteoarthritis.
• Phenylketonuria – mutations in PAH gene lead to a lack of phenylalanine hydroxylase causing damage to CNS, learning/behavioural disabilities, and epilepsy.
