Lecture 6' Protein metabolism

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Lecture 6. Protein metabolism
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Protein Turnover

• continually synthesised catabolised
• no protein synthesised for life
• turnover can occur within cells
• eg. enzymes
• turnover may involve cell death and replacement

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Protein Turnover

• rates of breakdown replacement different for
  different proteins
• For example.
• some enzymes have a ½ life of hours
• red blood cells have a ½ life of 120 days

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Protein Turnover

• daily turnover
• 3 - 4g/kg BW/day
• eg. if body mass 70kg
• daily turnover 210-280g/day
• dietary intake ? ¼ - ½ this amount ? 43 - 140g/day

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Protein Turnover

• ? most Protein made from recycling amino
  acids
• ie. from breakdown of body protein
  re-assembly into new proteins

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Protein types

• proteins vary in size shape
• depends upon side group size shape
• shapes can determine function
• shapes can determine rate of digestion
• electrical charge (ve or -ve) on side groups
• bind various substances
• fibre, ions, water

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Protein types

• Hydrophilic proteins
• ve or -ve charged side groups
• attracted to H20 molecules
• Hydrophobic proteins
• neutral polypeptides (no charge)
• repel H20 molecules
• influences reactions in bloodstream, in cells

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Protein Quality

• Biological Value of Protein
• based on NITROGEN RETENTION
• higher the protein quality
• more Nitrogen retained
• more protein synthesis than
• catabolism

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Nitrogen Balance

• if dietary protein ADEQUATE
• synthesis catabolism of nitrogenous compounds
  (protein) in balance
• ie. Nitrogen intake excretion are equal
• ? total body protein content remains STABLE

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Nitrogen balance

• if dietary protein intake
• INADEQUATE
• muscle protein catabolism can exceed synthesis
• ie. Nitrogen excretion exceeds dietary intake
• ? increased LOSS of body protein
• NEGATIVE NITROGEN BALANCE

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Biological value (BV) of Protein

• missing essential amino acids?
• protein synthesis impaired
• if a non-essential amino acid missing
• another AA dismantled
• required side chain, C- amino groups assembled
  into new AA

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Biological value (BV) of Protein

• impaired protein synthesis
• remaining amino acids de-aminated
• Nitrogen excreted
• C-chain available to make other amino acids

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Biological value (BV) of Protein

• Biological value of absorbed nitrogen
  RETAINED
• BV of EGG protein 100
• all absorbed protein retained
• BV does NOT have to 100 to support human growth

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Biological value of some protein foods

• EGG 100
• MILK 93
• BEEF 75

• FISH 75
• CORN 72

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• PROTEIN DIGESTION

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Enzymes specific to protein digestion

• Pepsinogen ? Pepsin
• pepsinogen secreted by stomach lining (gastric
  mucosa)
• needs acid environment
• conversion to pepsin
• stomach HCl pH 1.2
• begins digestion of protein

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Enzymes specific to protein digestion

• PROTEIN denatured
• ?
• stomach HCl
• cleaved
• denatured di tri-peptides
• PROTEIN ?
• PEPSIN

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Enzymes specific to protein digestion

• PROTEASE
• in pancreatic secretions (bicarbonate solution)
• needs alkaline environment
• pH of small intestine 8.0
• main site of polypeptide digestion

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Enzymes specific to protein digestion

• POLYPEPTIDES
• hydrolysis ? Protease
• DI-PEPTIDES, TRI-PEPTIDES
• and AMINO ACIDS

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Protein Food IN

Partial protein digestion by HCl pepsin
Further protein digestion in SI by pancreatic
enzyme (protease) di- tri-peptides further
digested by peptidase enzyme in cells of SI wall
Total 92 absorption
Amino acids absorbed into PORTAL VEIN
transported to liver
Minimal protein excretion in faeces
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Absorption of Protein

• actively absorbed across SI wall
• requires energy (ATP)
• requires specific carriers
• is NOT improved by eating amino acid supplements

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Excess protein intake

• Protein IN greater than needed?
• NOT stored as protein
• AAs de-aminated first
• Nitrogen excreted (as urea in urine)
• C-fragments available for
• new AAs
• gluconeogenesis
• oxidation
• fatty acid synthesis (rarely)

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Excess protein intake

• Gluconeogensis?
• C-fragments incorporated into glucose
• GLUCONEOGENIC AAs
• Fatty acid synthesis?
• C-fragments used in fatty acid synthesis
• KETOGENIC AAs
• Leucine ONLY solely ketogenic AA
• ? fat stores? (unlikely)
• ? AA oxidation? (exponential)

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Excess protein intake Potential Health problems

• risk of dehydration
• ? urea excretion in urine
• risk of ? calcium excretion (in urine)
• current dietary scare re milk
• milk leads to Osteoporosis
• protein leaches calcium from bones?
• Unlikely
• but does bind Ca in SI

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Single amino acid supplementation

• single AAs do not occur in foods
• competition with other amino acids for
  absorption
• transport carrier occupied with ? single AA
• risk of other AA deficiencies
• if ? absorption of essential AAs
• protein catabolism body breakdown

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Single amino acid supplementation

• EXAMPLES
• Lysine
• prevention or treatment of viral infections
  (herpes)
• only useful if Lysine-deficient
• Tryptophan
• to relieve depression, insomnia
• no supportive scientific evidence
• some deaths (EMS)

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Recommended Reading

• Whitney Rolfes (2002)
• Understanding Nutrition 9th Ed.
• Chapter 6 Protein Amino Acids
• (excellent chapter)
• Wahlqvist (2002)
• Food Nutrition
• Chapter 14 Protein