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Transcript
Lecture 6a- 8 February 2016 Most of this lecture taken from Chapters 6,7 of Rolfes et al(Understanding Normal and Clinical Nutrition (Nutrition 2104/2106 text) Lecture 6a-8 February 2016 Protein metabolism and regulation Outline of lectures 6a,b Amino acids • • • • • • Definition of amino acid Structure of amino acid Non-essential amino acids Essential amino acids Conditionally essential amino acids Class exercise Outline of lectures 6a,b Proteins • Definition of protein • Amino acid chains • Amino acid sequence • Protein shapes • Protein functions • Protein denaturation • Digestion • Absorption • Transport Outline of lectures 6a,b Metabolism • Amino acid anabolism • Protein anabolism • Protein catabolism • Amino acid catabolism • Nitrogen balance • Regulation of amino acid and protein metabolism More detailed comments Amino acids Definition of amino acid -building blocks of proteins Structure of amino acid -each amino acid has an amino group(NH2), an acid group(COOH), a hydrogen atom, and a distinctive side group all of which are attached to central carbon atom -some amino acids also contain sulphur Amino acids Non-essential amino acids -body can synthesise these for itself -foods usually deliver these to the body but it is not essential for food to deliver these amino acids-usually Amino acids Essential amino acids -cannot make on own or cannot make sufficient amounts of these amino acids -therefore they are required in the diet Amino acids Conditionally essential amino acids -sometimes a non-essential amino acid becomes essential -two reasons for conditional essentiality Class exercise -find all essential amino acids -find all non-essential amino acids -find an example of a conditionally essential amino acid -what foods contain all essential amino acids? -what foods are lacking in one or more essential amino acids? -how does one overcome issue of diet lacking in one or more essential amino acids? Class exercise Essential amino acids Histidine Isoleucine Leucine Lysine Methionine Phenylalanine Threonine Tryptophan Valine Class exercise Non- Essential amino acids Alanine Arginine Asparagine Aspartic acid Cysteine Glutamic acid Glutamine Glycine Proline Serine Tyrosine Class exercise Conditionally Essential amino acids Phenylalanine makes tyrosine If not enough phenylalanine in diet or conversion is reduced then tyrosine becomes essential Proteins Definition of protein -compounds composed of carbon, hydrogen, oxygen, nitrogen and sometimes sulphur atoms, arranged into amino acids linked in a chain Proteins Amino acid chains -dipeptides, tripeptides, polypeptides Proteins Amino acid sequence -20 amino acids give tremendous variability unlike starch Proteins -Protein shapes -side groups- attract or repel water -attract or repel other side groups on amino acids in amino acid sequence Proteins Protein denaturation -if change protein structure lose function Proteins Digestion mouth-no digestion stomach-acid digestion leads to denaturation -acid converts pepsinogen to pepsin -pepsin makes large polypeptides into smaller polypeptides and some amino acids Proteins Digestion small intestine-pancreatic and intestinal proteases digest polypeptides to: • • • • oligopeptides ( oligo means few; one definition suggests 2-40 amino acids though this definition varies), tripeptides, dipeptides and amino acids -peptidases split most of dipeptides and tripeptides to single amino acids Proteins Digestibility Proteins vary in their digestibility due to variable dietary protein structures as well as other food ingredients and hence accessibility to protein digestive enzyme active sites Animal proteins are generally very digestible and plant proteins are generally less digestible than animal proteins Complete plant proteins -soy - very digestible - close to many animal proteins’ digestibility -quinoa - about 10 % less digestible than soy Proteins Digestibility Digestibility affects availability (WWFQ) of essential and non-essential and conditionally essential amino acids Proteins Digestibility Current gold standard for protein digestibility is: Protein Digestibility Corrected Amino Acid Score (PDCAAS) PDCAAS = mg of limiting amino acid in 1 g of test protein / mg of same amino acid in 1 g of reference protein) x true digestibility percentage. True digestibility = human digestibility - faecal digestibility e.g. of reference protein = milk protein = 100 Proteins Digestibility FAO has recently proposed a new gold standard for protein digestibility -Digestible Indispensable Amino Acid Score (DIAAS) DIAAS % = 100 x [(mg of digestible indispensable amino acid in 1 g of dietary protein)/(mg of the same indispensable amino acid in 1 g of reference protein)]
