9/27/20131PROTEINRole/Function of Proteins• Structure of Body• Regulate Metabolism• Immune Protection• Transport and Storage• Acid/Base & Fluid BalanceProteins are the least efficient and most expensive source of energy!9/27/20132Protein in Animal Diets• Monogastric• Quality of dietary protein dictates quality of protein absorbed• Amount required varies with stage of life• $$• Digestibility in the foregut is critical• Dietary non-protein nitrogen (NPN) not efficiently utilized• Ruminant• Quality of dietary protein is less important• More economical• Contribution of microbes to animal needs• Microbial crude protein formation• Requires energy• Requires source of nitrogen• Degraded intake protein• NPN• N recyclingProtein• Composed of C, H, O, N (S, P)• Component of every body tissue• Vary widely • Chemical composition• Physical properties• Size, shape• Solubility• Biological function9/27/20133Protein• Protein = amino acids and peptide bonds• Amino acids are the building blocks of proteinamino acidamino acidCarboxyl Terminal Amino Terminal Amino Acid Structure• 20 commonly found• 10 required in animal diets = essential• Basic amino acid structure:Amine groupCarboxyl group9/27/20134Amino Acid Structural Isomers• Amino acids in L and D forms• L found in nature and used more efficiently by animals• Synthetic amino acids often racemic mixtures• D methionine used effectivelyClassification of Amino Acids• Classification• Structure• Net charge• Polarity• Essentiality• Classification of Side Chains (R)• Neutral• Aliphatic• Aromatic• Sulfur• Acidic• Basic• Heterocyclic9/27/20135Side Chain Classification larger version available on eLearningCommon Abbreviations:Essentiality• Essential• Amino acids not synthesized in sufficient amounts by the animal• Required in the diet• Nonessential• Adequately synthesized by the animal• No specific requirement• Differs with species, stage of life, stage of production9/27/20136Essential Amino Acids (PVT TIM HALL)• Arginine• Histidine• Isoleucine• Leucine• Lysine • Methionine• Phenylalanine• Threonine• Tryptophan• Valine• Proline• Taurine• Serine• GlycineProtein Structure• Functional role of proteins are determined by their structure and organization• Key levels of organization:• Primary• Secondary• Tertiary• Quaternary9/27/20137Protein Structure• Primary• Sequence of amino acids• Peptide bonds• Side chains of each amino acid makes the protein unique• Secondary• Twisting of chains into coiled structures• α helix• β sheet• Achieved through weak bonding• H bondsProtein Structure• Tertiary• Grouping of secondary structures• 3-D folding• H and disulfide bonds• Quaternary• Several tertiary structures linked together• Large and complex9/27/20138Denaturation of Protein• “Native” form of protein is most biologically active• Disruption of 3-D structure destroys the function• Denaturation = break H bonds• Heat • ChemicalsUse of bST in Milk Production• What is the concern?• Why does the industry use it?• Why has the FDA declared the practice safe?9/27/20139Crude Protein• Value listed on feed tags and animal requirement • Chemically determined by analysis of a sample for N• The average composition of protein = 16% N• Factor of 6.25 used to convert %N in a sample to CP• CP = N x 6.25• This value tells you nothing about protein qualityPROTEIN DIGESTION9/27/201310Protein Digestion• Both monogastric and ruminant animals begin protein digestion in the stomach (abomasum) and continue the majority of digestion in the small intestine• Absorption of di and tri peptides and amino acids• The difference between the digestive systems is the modification that rumen microbes make to the dietary protein• Protein that enters the small intestine is different from that which is fedProtein Digestion• The Challenge: whole proteins (dietary or microbial) are not absorbed• The Solution: digestive enzymes • Secreted as inactive pre-enzymes (zymogens)• Stomach/Abomasum• Small Intestine• Break peptide bonds• Create free amino acids and small peptides for absorption• Digestion begins in the stomach/abomasum9/27/201311Protein Digestion – Stomach (Abomasum)• HCl secretion• Denatures 4o, 3o, and 2ostructures• Peptide bonds unaffected• Activates pepsinogen• Pepsin• Endopeptidase• Cleaves peptide bonds on amino side of aromatic amino acids• Peptide fragments• Protein leaves stomach as mixture of insoluble protein, soluble protein, peptides, and amino acidsPepsinogen PepsinHClProtein Digestion - PepsinLeu – Val – His – Arg – Met – Phe – Tyr – Lys – Asp Which peptide bonds does pepsin break?• Amino side of aromatic amino acidsAmino endCarboxyl end9/27/201312Protein Digestion – Small Intestine• Mixture from stomach further digested by pancreatic enzymes• Pancreatic peptidases• Trypsinogen• Chymotrypsinogen• Procarboxypeptidases A and B• Proelastase• CollagenaseProenzymes(zymogens)Protein Digestion – Small Intestine• Zymogens must be converted to active forms• Trypsinogen Trypsin• Enteropeptidase is secreted from brush border• Once trypsin is formed:• Trypsinogen Trypsin• Chymptrypsinogen Chymotrypsin• Procarboxypeptidase Carboxypeptidase• Proelastase Elastase•EnteropeptidaseTrypsinTrypsinTrypsinTrypsin9/27/201313Pancreatic Peptidases• Trypsin• Autocatalytic• Endopeptidase• Peptide bond on carboxyl side of basic amino acids• Chymotrypsin• Endopeptidase• Peptide bond on carboxyl side of aromatic amino acids• Carboxypeptidase• Exopeptidase• Carboxy terminal end• Zn dependentBrush Border Peptidases• Produced in the small intestine enabling absorption to occur in the distal intestine (ileum)• Aminopeptidase• Exopeptidase• Cleaves from amino terminal end• Dipeptidases• Cleaves bond between dipeptides• Enzyme also found in the enterocyte9/27/201314Protein DigestionLys –Met –Val –Glu–Phe –Cys–Leu–His –Ile -AsnAmino terminalCarboxyl terminalTrypsin Inhibitors• Small proteins or peptides• Present in plants, organs, and fluids• Soybeans, peas, beans, wheat• Pancreas• Colostrum• Importance?• Inactivates by heat• Raw soybeans vs. soybean meal9/27/201315Products of Digestion• Enzymatic digestion in the stomach (abomasum) and small