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Proteins

Proteins. Chapter 5 Nutrition for Sport and Exercise Dunford & Doyle. In many sports, well-developed muscle is necessary for successful performance. p153. Learning Objectives. Describe amino acids and how the structure of a protein affects its function

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Proteins

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  1. Proteins Chapter 5 Nutrition for Sport and Exercise Dunford & Doyle

  2. In many sports, well-developed muscle is necessary for successful performance. p153

  3. Learning Objectives • Describe amino acids and how the structure of a protein affects its function • Describe the digestion, absorption, transportation, and metabolism of amino acids • Explain protein metabolism and the processes associated with skeletal muscle protein synthesis and breakdown • Explain daily protein recommendations for athletes and the amount and timing of protein intake before, during, and after exercise

  4. Learning Objectives • Describe the effects of low protein and energy intakes on training, recovery, performance, and health • Translate protein recommendations into daily food intake and assess an athlete’s dietary protein intake • Evaluate dietary supplements containing amino acids and proteins for safety and effectiveness

  5. Protein Basics • Functions optimally only when energy intake is sufficient • Plays many functional roles including: tissue growth, enzymes, hormones, immune system response, and can provide energy • Basic component of protein is the amino acid • The amount recommended for athletes is typically higher than for nonathletes • Supplements are no more or less effective than food proteins • Consumption after exercise is important

  6. Amino Acids • Contain C, H, O, and N • Chemical compounds that contain an amino group (NH2) and a carboxyl group (COOH) • Amino acids contain approximately 16% Nitrogen • 1 g Nitrogen in every 6.25 g protein • Body uses 20 different amino acids to make various proteins • Amino acids have side chains, acid groups, basic groups, and rings

  7. Basic Structure of an Amino Acid

  8. Dispensable vs. indispensable • Indispensable (or essential) • 9 amino acids • The body cannot manufacture them • Dispensable (or non-essential) • 11 amino acids • Can be manufactured in the liver • 6 of these 11 are conditionally indispensable b/c during times of stress the body cannot manufacture a sufficient amount • Ex/ illness, injury, prolonged endurance exercise

  9. Structure and Function of Protein

  10. Proteins are found in both plant and animal foods.

  11. Protein Quality • Determined based on amounts and types of amino acids and the extent to which they are absorbed • Protein digestibility amino acid score (PDCAAS) – the internationally accepted method for determining protein quality • Scored on a scale of 0 to 1 (1 being the highest score obtainable and 0 being the lowest) • Several proteins have a score of 1

  12. Protein Quality Scores

  13. Protein Quality • Animal proteins are termed “complete” • Contain proper amounts and types of all the indispensable amino acids • Higher bioavailability • Plant proteins are termed “incomplete” • Missing one or more of the indispensable amino acids or the proper concentrations • Complementary proteins • Combining two incomplete plant proteins to provide all the indispensable amino acids • Ex/ beans and rice, beans and corn tortillas

  14. Beans and corn provide complementary proteins in the vegetarian meal.

  15. Basic Structure of Proteins • Definitions: • Peptide – 2 or more amino acids combined • Dipeptide – 2 amino acids • Tripeptide – 3 amino acids • Polypeptide (protein) – 4 or more amino acids • Most proteins are polypeptides

  16. Structure of Polypeptides • The structure of a protein determines its function • Primary structure • Determines how a protein functions • Based on information contained in DNA and RNA • Secondary structure • Result of bonding of amino acids in close proximity • These bonds give more rigidity or stability to the protein, such as in collagen

  17. Structure of Polypeptides 3. Tertiary structure • Result of interactions of AAs not in close proximity • Polypeptide then forms a loop, which results in clustering of AAs • Cluster of AAs may have positive or negative charge, and may accept or repel other compounds (such as H20) 4. Quaternary structure • Protein is made up of more than 1 polypeptide • Proteins can interact with other molecules • Insulin and hemoglobin are examples

  18. The Functions of Proteins in the Body

  19. Functions of Proteins - 5 Major Categories 1. Enzymes • Needed to catalyze (speed up) chemical reactions • Quaternary structure allows enzymes to interact with other compounds • Each enzyme interacts with its specific substrate (like a key fits into a specific lock) • Purpose of enzymes is to regulate speed of chemical reactions • “Rate limiting enzymes”

  20. Functions of Proteins - 5 Major Categories 2. Hormones • Act as chemical messengers to regulate metabolic reactions • Examples of protein-based hormones: • Insulin • Glucagon • Human growth hormone • Secondary and quaternary structure are important for proper function

  21. Functions of Proteins - 5 Major Categories 3. Structural proteins • Provide rigidity and durability • Include proteins of muscle and connective tissue, and protein found in skin, hair, and nails • Ex/ actin, myosin, collagen • Secondary (3-dimensional shape) and tertiary structure (interconnected bonds) are important for proper function

  22. Functions of Proteins - 5 Major Categories 4. Transport proteins • Transportation of compounds in the blood, plasma, or lymph • Ex/ lipoproteins (transport lipids) and hemoglobin (transports oxygen and CO2) • Quaternary structure allows flexibility and capability of changing shape when necessary

  23. Functions of Proteins - 5 Major Categories 5. Immune system proteins • The immune system protects the body from invasion of foreign particles, such as viruses and bacteria • Activation of lymphocytes to produce antibodies • Antibodies are compounds made of polypeptide chains • Antibodies fit specific virus or bacteria (like a key in a lock) • Glutamine and arginine help regulate immune system

  24. Digestion of Proteins • Denatured in the stomach by HCl • Hydrochloric acid (HCl) activates pepsin • Pepsin: enzyme released in stomach that breaks down large polypeptides into smaller units (peptides) • Broken down further in the small intestine by other digestive enzymes (pancreatic juices, brush border enzymes) into small polypeptides, tripeptides, dipeptides, and free AAs • Chymotrypsin & trypsin – proteolytic enzymes in pancreatic juice acting in the duodenum • The GI tract cannot absorb anything larger than a tripeptide

  25. Absorption of Proteins • Primarily in the jejunum and ileum • 2/3 in the form of di- or tri-peptides; 1/3 as individual AAs • Indispensable amino acids absorbed more quickly than dispensable • “Predigested” protein supplements • Protein exposed to enzymes during processing

  26. Absorption of Proteins • Exogenous vs. endogenous sources • Exogenous AAs originate from outside body • Food (2/3) • Endogenous AAs originate from inside the body • Mucosal cells shed into GI tract, GI secretions (1/3) • Digested/absorbed similar to exogenous but are often absorbed in lower GIT including colon • Once inside mucosal cells, dipeptides and tripeptides are broken down into free AAs • Free AAs will be incorporated into cellular proteins or released into blood via portal vein (liver)

  27. Digestion & Absorption of Dietary Protein

  28. Transportation of Proteins • 50-65% of AAs absorbed from a meal will go to liver; the remainder will be released as free AAs into the blood and become part of the AA pool • Blood amino acid concentration is increased for several hours after a protein-containing meal • Liver monitors supply of AAs and determines which AAs will be transported to which tissues • Liver considered a “clearinghouse” for AAs

  29. Transportation of Proteins • However, the Branched Chain Amino Acids (BCAAs) circulate immediately in the plasma and are taken up by skeletal muscle cells • BCAAs are leucine, isoleucine, valine • Liver lacks sufficient BCAA transferase enzyme, which is needed to transfer the BCAAs to tissues • Skeletal muscle cells, heart, kidneys, and adipose tissue contain high levels of BCAA transferase enzyme

  30. Nitrogen Intake, Turnover, and Excretion

  31. Amino Acid Pool • Free amino acids circulating in blood or fluid found within or between cells • Average of 150 g of amino acids in pool • ~80 g is glutamine • More dispensable than indispensable amino acids in the pool • Always in flux because of protein turnover • Food intake • Exercise • Breaking down or building of tissues

  32. Anabolism and Catabolism • Anabolism • Building complex molecules from simple molecules • Catabolism • Breaking down complex molecules into simple molecules • The use of protein for energy is a catabolic process – protein must be broken down into AAs to be metabolized for energy

  33. Protein Anabolism and Catabolism • Deamination – the removal of an amino group from the AA • Alpha-keto acid is formed, which is referred to as the carbon skeleton • Transamination – the transfer of an amino group to another carbon skeleton • Process used to form dispensable AAs in liver from indispensable AAs • Amino acids not used by liver at time of absorption become part of amino acid pool

  34. Deamination of an Amino Acid

  35. Protein Anabolism • Major function of the liver • AAs will be incorporated into liver enzymes or plasma proteins • Albumin – transport protein • Other proteins synthesized and released in response to injury and infection • Creatine synthesis is completed in the liver • Anabolic state occurs when synthesis of proteins is greater than their breakdown • AAs from AA pool are being incorporated into protein synthesis

  36. Protein Catabolism • Amino acids are not “stored” for future use like CHO (glycogen) and fat (adipocytes) • If they are not used for building proteins, then they are catabolized • Protein can be removed from skeletal muscles under extreme conditions, such as starvation • The body tries to protect the skeletal muscle from being used in this manner

  37. Protein Catabolism • Amino acids can provide energy • When amino group is removed or transferred, the carbon skeleton can be oxidized to produce energy • Protein-sparing effect is the consumption of sufficient kcals in the form of CHO and fat, which protects protein from being used as energy • However, if CHO and fat intake insufficient, protein must be used to meet energy needs • Not the preferred source of energy for exercise • Yield ~ 4 kcals/g

  38. Protein Catabolism • Carbon skeleton of AA can be used in Krebs cycle: • Different entrance points based on structure • Six amino acids are commonly broken down in muscle cells for energy: • Leucine, isoleucine, and valine (BCAA’s) • Endurance exercise results in an increased oxidation of leucine (in particular) • Aspartate, asparagine, and glutamate • Proteolysis is breakdown of muscle, which is stimulated by stress hormone cortisol • May occur during endurance exercise (stress)

  39. Amino Acids used for ATP Production

  40. Prolonged endurance exercise results in increased protein metabolism, particularly in the later stages of a triathlon. Figure 5-10 p165

  41. Protein Catabolism • Amino acids can be used for gluconeogenesis = production of energy from a non-CHO source • 18 of the 20 amino acids can be used to produce glucose • Only leucine and lysine cannot • Alanine produced in the muscle is a prime example • Glucose-alanine cycle • Muscle produces pyruvate – pyruvate converted to alanine – alanine take up by liver – liver can convert alanine to pyruvate for use in gluconeogenesis to produce glucose

  42. Glucose-Alanine Cycle

  43. Protein Catabolism • Total energy from AAs generally 3-5% • Amino acids rarely provide more than 10% of total energy • Amino acid catabolism produces ammonia • Ammonia is toxic to body • Ammonia must be converted to urea in the liver, transported in blood to kidneys, and excreted in urine • Small amount of nitrogen is lost every day in urine

  44. Nitrogen Balance • Protein metabolism is always changing – body is in constant state of protein turnover • ~ 30-40 g protein excreted in urine each day, which is equivalent to ~ 5-7 g of nitrogen • 6.25 g protein = 1 g Nitrogen • Nitrogen Balance: the difference between total nitrogen (protein) intake and total nitrogen loss (via urine and feces)

  45. Nitrogen Balance • When intake is equal to loss, a state of nitrogen balance is achieved • MPS = MPB • When intake is greater than loss, a state of positive nitrogen balance occurs • MPS > MPB • When intake is less than loss, a state of negative nitrogen balance occurs • MPS < MPB

  46. Positive Nitrogen Balance

  47. Negative Nitrogen Balance

  48. Nitrogen Balance • Most adults are in nitrogen balance • i.e. - amount of protein being synthesized is equal to amount being degraded • Adults who want to be in a “growth” state must achieve positive nitrogen balance • Pregnancy • Desire to increase skeletal muscle mass • Hypertrophy: refers to an increase in muscle size due to increase in size of muscle cells due to enlargement (not increase in number of muscle cells) • Greatest increase is in myofibrillar proteins

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