Bean Facts

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Bean Facts
Human Nutrition HLTH 2001 Random Facts
Scientists discover how to make beans less
explosive TWO strains of bacteria have been found
to be the key to making beans flatulence-free.
http//www3.interscience.wiley.com/cgi-bin/abstrac
t/104540109/ABSTRACT
Pictures from http//www.americanbean.org/
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Lecture 7. Protein metabolism
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Answers to Homework
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Answers to Homework

• Sedentary individual (70kg body mass)
• 56 - 70g protein/day protein FOOD

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Answers to Homework

• Endurance athlete (75kg body mass)
• 90 - 105g protein/day protein FOOD

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Answers to Homework

• Strength Power athlete (100 kg body mass)
• 120 - 170g protein/day protein FOOD

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Protein Turnover

• continually synthesised catabolised
• Anabolic vs catabolic reaction see Fig 7.1 p217
• 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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Deamination

• When amino acids are metabolised for energy or
  used to make fat, they must be deaminated first
• Resulting products are
• Carbon structure without its amino group (keto
  acid)
• Ammonia (NH3)

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Deamination

• If have source of NH3 then body can make
  nonessential amino acids from keto acids

R side group
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Transamination

• Generation of amino acids requires nitrogen
• transferring an amino group from one amino acid
  to its corresponding keto acid generates a new
  amino acid and new keto acid
• By many of these reactions, the liver can
  synthesize the nonessential amino acids
• Transamination requires vitamin B6 coenzyme see
  figure 7.15

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Transamination

• amino acid1 keto acid2 ÜÞ keto acid1 amino
  acid2

Amino acid1
Keto acid2

Keto acid1
Amino acid2
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Uptake of amino acids for transport

• Monomers (free amino acids) pass through the wall
  and into the bloodstream and are taken up by
  cells.
• Excess dietary amino acids are converted to
  common metabolites that are precursors of
  glucose, fatty acids, and ketone bodies
  (metabolic fuels).

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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
• Zero nitrogen balance

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Positive Nitrogen Balance

• if dietary protein intake is in excess then
• POSITIVE NITROGEN BALANCE
• Growing infants and children
• Pregnant women
• Recovery from illness
• Retaining protein to add
• Blood
• Bone and skin
• Muscle cells

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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
• Most likely amino acids to be limiting are
• Lysine, methionine, threonine, tryptophan
• 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 polypeptides
• 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
• DI-PEPTIDES, TRI-PEPTIDES

? Protease
? Peptidase
hydrolysis
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)

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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
• Up to 3g per day when taken in divided doses with
  meals
• Tryptophan
• to relieve depression, insomnia
• no supportive scientific evidence
• some deaths due to eosinophilia-myalgia syndrome
  (EMS)
• What is a good dietary source?

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Single amino acid supplementation

• EXAMPLES
• Arginine
• Converted into
• Nitric oxide via nitric oxide synthase
• Urea via arginase

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Recommended Reading

• Whitney Rolfes (2002)
• Understanding Nutrition 9th Ed.
• Chapter 6 Protein Amino Acids
• (excellent chapter). Chpt 14 looks at needs for
  athletes and Chpt 7 looks at protein metabolism
• Wahlqvist (2002)
• Food Nutrition
• Chapter 14 Protein