The ppt contains the short and precise information about the protein.

1. Proteins
– Polymers of 20 amino acids
– Found throughout the body
• >40% - skeletal muscle
• >25% body organs
• Make up >50% of cell content
– Functional categories
– Enzymes
– Hormones
– Structural proteins
– Immunoglobulins
– Transport proteins
» Albumin, transthyretin, heme proteins, transferrin,
ceruloplasmin

2. Amino Acids
• Structure
– Central C atom
– At least one amino group [-NH2]
– At least one carboxyl group [-COOH]
– Side chain [R]
• The side chains of AA bestow protein structure and functional role

3. Table 7-1a1, p. 178

4. Table 7-1a2, p. 178

5. Table 7-1b, p. 179

6. • Classification
• Essential
• Non-essential
• Conditionally essential
– Nonessential aa may become essential
» Organ failure, immature organ function
» Inborn errors of metabolism

7. Sources of Protein
• Dietary
– Animal and plant products
• Endogenous protein
– Desquamated mucosal cells
• 50 g/day
– Digestive enzymes and glycoprotein
• 17 g/day

8. Digestion
• Mouth
– No appreciable digestion
• Stomach
– Digestion begins here
– Gastric juice (HCl)
• Released from gastric parietal cells
• Its release is stimulated by gastrin, gastrin-releasing
peptide, acetylcholine, histamine
• Denatures quaternary, tertiary and secondary structures
• Break apart hydrogen and electrostatic bonds but not
peptide bond
• Activates pepsinogen to pepsin

9. – Pepsin
• Digests proteins by attacking peptide bonds
adjacent to the carboxyl end
– Digestion products are large polypeptides, some
oligopeptides, free amino acids
• Small intestine
– Partially digested proteins (acidic chyme) enter SI
and stimulate the release of the hormones, These
hormones stimulate acinar cells of pancreas to
release proenzymes and alkaline pancreatic juice
(bicarbonate-neutralizes the chyme) into the
intestine

10. – Pancreatic juice zymogens (proenzymes)
• Trypsinogen, chymotrypsinogen,
procarboxypeptidases A and B, proelastase,
collagenase
• Converted to active enzymes in the SI
• These active enzymes digest polypeptides into
tripeptides, dipeptides and free amino acids
– Intestinal enzymes in the lumen of the SI and
within mucosal cells complete protein digestion

11. Table 7-2, p. 182

12. Fig. 6-6, p. 190

13. Protein Absorption
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AA and small peptides are transported via
group specific amino acid or peptide
transport systems (carrier mediated) and co-
absorbed w/ sodium requiring energy (from
the Na+ gradient, Na-K ATPase)

14. • AA absorption/transport into enterocyte
– Different mechanisms
– Most AA are absorbed in the proximal SI

15. Table 6-4, p. 192

16. Fig. 6-7, p. 193

17. Peptide Transport
• Primarily di- and tripeptides
• Represent the primary system for AA absorption (67% vs
33% free AA absorbed)
• Transport system different from those that transport AA
(primarily PEPT1 transporter)
• Appear to be absorbed more rapidly than free AA
• Associated with movement of protons
• Peptides are hydrolyzed by cytoplasmic peptidases to
generate free intracellular AA

18. Fig. 6-8, p. 193

19. Intestinal Basolateral Membrane
Transport of AA
• Diffusion and Na-independent transport
– Primary
• Na-dependent pathways are important when
the AA concentration in the lumen are low
– Provide the enterocyte with AA to meet its needs

22. AA Metabolism
• Intestinal cell AA use
– Not all AA are transported out of the intestinal cell and
into circulation
– Many of the AA absorbed are used along the villus for
protein synthesis
– Within intestinal cell, AA may be used for energy or
synthesis of new compounds
• Structural proteins for new intestinal cells
• Nucleotides
• Apoproteins for lipoprotein formation
• New digestive enzymes
• Hormones
• N-containing compounds
– Partially metabolized either to other AA or to other
compounds
• Use approximately 30-40% of essential AA from diet
• Use approximately up to 90% glutamate in the diet

23. Glutamine in Intestinal Cells
– Used extensively by intestinal cells as a source of
energy
– Stimulate cell proliferation in the GI mucosa
– Enhance synthesis of heat shock proteins (stress
proteins)
– Catabolized to form glutamate, -keto-glutarate,
alanine, proline, ornithine, citrulline, glutathione

24. Intestinal Aspartate Metabolism
– Metabolism of aspartate from the diet generally
occurs within the intestinal cells
– Very little aspartate is found in portal blood

25. Intestinal Arginine Metabolism
– Up to 40% of dietary arginine is oxidized by intestinal
cells yielding
• Citrulline and urea
– Enzymes carbomyl phosphate sythetase I and ornithine
transcarbomylase and enzymes of the urea cycle are
present in intestinal cells
– Citrulline is released in the blood and taken up by the
kidney (main organ responsible for provision of
arginine to body tissues)
arginine
citrulline

26. Intestinal Methionine and Cysteine
Metabolism
• Up to 52% of methionine is metabolized in the gut
• Cysteine generated from methionine or from diet is used
to make glutathione
– Metabolized to taurine (70-90%)
– Metabolized to pyruvate and sulfite (10-30%)
• Glutathione
– Tripeptide made in the enterocyte (and other cells) from
glutamine, glycine and cysteine
– Functions with glutathione peroxidase to destroy lipid peroxides
and hydrogen peroxides
– Reduce ROS -damage cellular DNA, proteins, PUFA in intestinal
cell membranes
– Found in most cells of the body

27. • AA Absorption into Extraintestinal Tissues
– Carrier system similar to the intestinal
basolateral membrane
– Hepatocytes
• System N
– Na dependent
– Transports glu and his
• System A
– Induced by glucagon
– Provides AA substrates for gluconeogenesis
• System Gly
– Na dependent
– Transports gly
• Hormones and cytokines influence AA transport
– Kidneys
– γ-glutamyl cycle-important in transporting AA through membranes
of renal tubular cells, erythrocytes, neurons

28. Protein synthesis
– Liver primary site of uptake for most AA
• 20% of AAs used for synthesis of proteins and N-containing
compounds
• Many proteins synthesized in the liver will remain in the
liver
– Others are released into the plasma
– Insulin promotes cellular uptake and use of AA for
protein synthesis
•  movement of AA transporters to the membrane and
activity of several transporters
• Antagonizes enzyme responsible for AA oxidation (e.g.
phenylalanine hydroxylase inhibited by insulin)
– Glucagon stimulates the use of some AA for
gluconeogenesis

29. Fig. 7-19, p. 194
Use of AA for Anabolism

30. Plasma Proteins
• Synthesized in liver and released in the blood
• Total protein in human plasma
– 7.5 g/dL
• Used to assess individual’s protein status
• Albumin
– Most abundant of the plasma protein
–Maintain
• Oncotic pressure
• Transport nutrients (vitamin B6, Zn, Ca, Cu, fatty acids, AA
tryptophan)
• Transport some drugs and hormones
–Long half life
• 14-18 days
• Not a good indicator of visceral protein status

31. Plasma Proteins
– Blood clotting proteins
• Prothrombin (blood coagulation
– Immunoprotection
• Immunoglobulins
– Nutrient transport
• Haptoglobin
– Free hemoglobin transport)
• Ceruloplasmin
– Cu transport
• Transferrin
– Fe transport
• RBP
– Vitamin A transport
• Transthyretin (prealbumin)
– Thyroid hormone and retinol transport
– More sensitive indicator of visceral protein status
» Half life-2 days and 12 hours
• Others

32. Plasma Proteins
• Acute-phase proteins
– C-reactive protein
– Serum amyloid A
– Stimulate immune system, promote wound healing,
chelation and removal of free Fe from circulation to
prevent its use by bacteria
• Stress/heat shock proteins
– Synthesized in response to stress including heat and
oxidative stress
– Functions are still unclear
• Facilitate 2 and 3 protein structure formation
• Repair of denatured or injured proteins
• Transport old proteins for disposal

33. AA Catabolism
– Occurs to varying degrees in different tissues both in
the fed and fasting state
– Liver takes up about 50 - 65% of AA after a meal
• Main site of catabolism for essential AA, except for
branched chain AA
• Rate of catabolism of AA differs
– BCAA catabolyzed much slower in the liver than in muscle
• AA are catabolyzed in different regions of the liver
– Periportal hepatocytes catabolyze most AA except glutamate and
aspartate (metabolized by perivenous hepatocytes)
– Steps
• Removal and disposal of amino group
– AA undergo deamination and transamination to remove amino
group
• Catabolism of C skeleton

34. Deamination of AA
• Only remove amino group; no direct transfer to
another compound
– Enzymes
• Dehydratases, lyases, dehydrogenase
• Ammonia readily used by periportal hepatocyte for
urea synthesis

35. Transamination of AA
– Transfer of amino group from one AA to an AA C-
skeleton or α-keto acid
– Important for synthesis of nonessential AA
– Enzyme: Aminotransferase (require vit B6)
• Aspartate aminotransferase (AST) and alanine
aminotransferase (ALT)
– Among the most active
– Involve three key AA: ala, glu, asp
• AST
– Higher conc. in the heart than liver, muscle and other tissues
• ALT
– Higher in liver than heart but also moderate amounts in kidney
• Conc. increases with trauma or disease to an organ

36. Metabolism of C-skeleton
– AA → NH3 + C-skeleton/-keto acid
– Usage depends on original AA
– Fate of AA C skeleton depends on the nutritional
state and the AA
• Energy
• Glucose
• Ketone bodies
• Cholesterol
• Fatty acids
– Energy
• Complete oxidation of AA yields
– Energy, CO2/HCO3
-, and Ammonium
• Used when diets are inadequate in energy

37. Metabolism of C-skeleton
• Glucogenic
– Form glucose thru gluconeogenesis
– Liver primary; also kidney
– Can be degraded to pyruvate or an intermediate
in the Krebs Cycle
– Conversion of AA to glucose accelerated by high
glucagon:insulin
• Not receiving enough dietary CHO, infection, trauma,
diabetes, liver disease

38. Metabolism of C-skeleton
• Ketogenic
– Catabolism of AA form non-Kreb’s cycle
intermediates
• Acetyl CoA or acetoacetate
– AA catabolized to ketone bodies during
inadequate CHO intake

39. Metabolism of C-skeleton
• Cholesterol
– Leu the only AA that generates HMG CoA
– Other AA generate acetyl CoA which can be
metabolized for cholesterol production
• Fatty acid production
– Excess energy and protein intakes coupled with
adequate CHO intake
– C-skeleton of AA can be used for FA synthesis

40. Food Proteins and Protein Quality
• Requirements and recommended dietary
intakes are usually expressed in terms of
dietary protein rather than amounts of
individual amino acids
– Because requirements are met by a mixture of
food proteins
– Dietary protein is usually measured not as protein
but as nitrogen and then N is converted to protein
by use of a factor (Protein = 6.25 x N)
• Actual factor for milk is 6.38 and for wheat is 5.7

41. Protein Turnover
• Process by which body protein is continually
degraded and resynthesized
• Affected by food intake and nutritional status
• Mediated via hormones
– Insulin, glucagon, GH, glucocorticoids
• Insulin
– Increase protein synthesis
– Decrease protein degradation
– + N balance (N intake > N output)
• Protein contains 16% N (6.25 factor in converting N to protein)
• Counter regulatory hormones (glucagon,
epinephrine, glucocorticoids)
– Promote overall protein degradation
– - N balance (N intake < N output)

42. Protein Quality
• Both digestibility and AA content affect protein quality
• Digestibility [d = (I-F)/I]
– Amounts of AA absorbed following ingestion of a given protein
– Only that part of the protein that is digested can contribute AAs
to meet requirements
– Animal protein
» 90-99% digestible
– Plant proteins
» 70-90% digestible
» Many plant proteins, especially when eaten raw, are less
digestible partly because they are contained within cell
walls which are resistant to mammalian digestive enzymes
and are broken down only by the GI microflora

43. Protein digestibility values
Protein source Digestibility
Egg 97
Milk, cheese 95
Meat, fish 94
Peanut butter 95
Soy protein isolate 95
Soy flour 86
Wheat, refined 96
Wheat, whole 86
Rice, Polished 88
Corn, whole 87
Millet 79
Maize 85
Oatmeal 86
Peas 88
Beans 78

44. Protein Quality
– High quality or complete proteins
– Contains all indispensable AA in approximate amounts
needed by humans
– Mostly foods of animal origin (exception gelatin, soy protein)
– Low quality or incomplete proteins
• Limiting AA
» Indispensable AA present in lowest quantity in food
» Lysine, methionine + cysteine, tryptophan or threonine
are normally limiting in the mixed proteins of human
diets
» Complementation of proteins (e.g. corn and beans) or
supplementation of the diet with a small amount of high
quality protein can be of significant benefit

45. Table 7-7, p. 222

46. Evaluation of Protein Quality
• Chemical or aa score
– AA with the lowest score in relation to the reference protein
(egg) becomes the limiting AA and determines the AA or
chemical score for the protein
• Protein digestibility corrected AA score (PDCAAS)
• Protein efficiency ratio
– Body weight gain on a test protein per gram of protein
consumed
• Biological value
– Amount of N retained for maintenance and/or growth vs
the amount of N which is absorbed
• Net protein utilization
– Measures retention of food N consumed rather than food N
absorbed