Digestion and absorption of proteins Protein enters the stomach and the low pH:

23. *** Amino acid: building block for protein, protein may contain a chain of several hundred amino acids • Molecular weight of 2-3,000 • Amino acids contain an acid group and an amine group • Amino acids are bonded together by peptide bond • About 21 regular occurring amino acids (10 are essential in that they must be in the diet)

24. Dietary Essential AA -- an amino acid required by an animal and can not be synthesized by the animal in the amounts needed; and therefore must be present and available in the diet • Arginine • Histidine • Isoleucine • Leucine • Lysine • Methionine – S-containing, Cystine may provide ½ the reqmt • Phenylalanine -- tyrosine may provide ½ of the requirement • Threonine • Tryptophan • Valine • Nonessential AA -- required by the animal but can be produced in adequate amounts

25. Limiting amino acid: the essential amino acid in a feed or ration that becomes depleted first during protein synthesis • This restricts further protein synthesis until a new supply of amino acids comes along • Therefore an animal's absolute requirement is not for protein per se, but for required amounts of each essential amino acid • Not the same amounts and proportions of amino acids in ingested food and what is to be synthesized in the body

26. Digestion and absorption of proteins • Protein enters the stomach and the low pH: • denatures the protein • activates pepsinogen to form pepsin • Pepsin is a endopeptidase • random cleavage of peptide bonds within the protein molecule • form shorter chains of amino acids

27. Digestion and absorption of proteins • Proteins next enter the small intestine • trypsinogen is activated by enterokinase to form trypsin; activated trypsin further activates more trypsinogen • chymotrypsinogen is activated by trypsin to form chymotrypsin Note: pepsin, trypsin and chymotrypsin are all endopeptidases • Procarboxypeptidase is activated by trypsin to form carboxypeptidase • this is an exopeptidase which cleaves at the carboxyl end

28. Digestion and absorption of proteins • With the proteolytic action of all of these enzymes the protein is reduced to free (non-associated) amino acids • AA's are absorbed into the mucosal cell to the blood by active transport; some AA's share the same transport system or use the same carrier • Circulating AA's are taken into peripheral cells facilitated by insulin *** Important, the animal's requirement is not for protein but for specific proportions of AA's: the proportion of AA's in the proteins which are eaten is not necessarily the same proportion of AA's needed for the proteins to be synthesized.

29. Factors that affect protein digest and metabolism • Enzyme inhibitors and other anti-nutritional factors in the feed • trypsin and chymotrypsin inhibitors in some feeds • decrease the activity of these enzymes • moderate heat treatment is effective in reducing the effects of these inhibitors • Heat treatment • Maillard reaction: lysine + sugar + heat yields a Maillard product which is indigestible • in some dried feeds • in some forages that have heated due to molding

30. Factors that affect protein digest and metabolism • Amino acid deficiency and or imbalance • brain senses that a deficiency exists thereby reducing intake • Amino acid antagonism: one AA affects the utilization of another • the most common form is competition for active transport sites (arginine and lysine)

31. I. IntroductionII. Chemistry of Proteins A. Amino Acids (AA) 1. Proteins are made of many AA connected together (polymer of AA’s)

32. Amino Acids (AA) (cont) 2. AA’s have an identical structure with the exception of the side groups (R) a. AA’s contain the element nitrogen (N) b. Differences in the side group are what make each AA unique c. Around 22 different AA’s

33. Amino Acid Structure

34. Amino Acids (AA) (cont) 3. Essentiality of amino acids (EAA) a. Essential AA’s 1. AA’s that cannot be made by the body and therefore must be supplied by the diet b. Nonessential AA’s 1. AA’s that can be made by the body c. Conditionally essential AA’s 1. AA’s that are normally made providing the precursor is available

35. Proteins 1. Consist of the AA’s linked together in different combinations a. Sequence of AA’s determines the conformation (3-D structure) which dictates the type and function of the protein • AA’s are connected to each other through a peptide bond a. condensation reaction

36. Peptide Formation H2O

37. Insulin (peptide hormone)

38. Digestion and Absorption of proteins A. Process of Digestion taking the large protein molecule and breaking it down to smaller subunits (AA’s) 1. Mouth a. Mechanical

39. Process of Digestion (cont) 2. Stomach a. Chemical breakdown (denaturation) 1. Protein loses its 3-D structure in the low pH (acid) of the stomach caused by HCl b. Enzymatic breakdown (proteases , peptidases ) 1. pepsinogen is converted to the enzyme pepsin at low pH

40. Process of Digestion (cont) 3. Small intestine a. Pancreatic and intestinal proteases 1. Enzymes are specific for certain peptide bonds connecting AA’s b. Proteins are broken down into oligo-, di- and tri- peptides

41. Insulin (peptide hormone)

42. Protein Digestion

43. B. Process of Absorption 1. Intestinal cells absorb single AA’s, di- and tri-peptides a. AA’s must pass through the cell membrane, through the cell and then through the cell membrane again b. Absorption occurs through transporters specific for certain AA’s 1. Transporters located in cell membrane 2. Absorption requires energy

44. Absorption of NutrientsActive Transport

45. Protein Absorption as Amino Acids

46. Protein Digestion to Amino Acids PEPSIN cleaves on the N-terminal side of Y/W/F • Digestion of food protein --in stomach --in small intestine protein Y Y W Y peptides Y W trypsin (N-term to K/R) chymotrypsin (C-term to bulky hydrophobes) carboxypeptidase (from C-terminus) aminopeptidase (from N-terminus) Free amino acids

47. Ubiquitination and hydrolysis of a protein- 2004 Nobel Prize in Chemistry - different rates in protein degradation has a physiologic basis protein Ubiquitination enzymes catalyze the attachment of numerous molecules of ubiquitin to the protein targeted for degradation. The proteasome catalyzes ATP-dependent hydrolysis of the substituted protein, releasing peptides and ubiquitin.

48. Overview of Amino Acid Oxidation 4 1 citric acid cycle a-keto acid a-amino acid a-keto Glu Glu 2 3 NH4+ excretion

49. Overview of Amino Acid Oxidation 4 1 citric acid cycle a-keto acid a-amino acid a-keto Glu Glu 2 3 NH4+ excretion

50. O O- O O O- O O O oxaloacetate phenyl lactate O- -O O pyruvate a-keto levulate O O- O a-keto glutarate O -O O- O O Alpha Keto Acids • All amino acids can be converted into (or made from) an alpha keto acid • Some alpha keto acids are significant in their own rights. To which amino acids are these most closely related?