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Understanding Proteins and Amino Acids: Structure, Functions, and Metabolism

Explore the world of proteins and amino acids, crucial molecules in the body, their structures, functions, metabolism, and vital role in various bodily processes. Learn about essential and nonessential amino acids, protein synthesis, digestion, and metabolic fate.

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Understanding Proteins and Amino Acids: Structure, Functions, and Metabolism

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  1. Proteins and Amino Acids

  2. What Are Proteins? Large molecules Made up of chains of amino acids Are found in every cell in the body Are involved in most of the body’s functions and life processes The sequence of amino acids is determined by DNA

  3. Structure of Proteins Made up of chains of amino acids; classified by number of amino acids in a chain Peptides: fewer than 50 amino acids Dipeptides: 2 amino acids Tripeptides: 3 amino acids Polypeptides: more than 10 amino acids Proteins: more than 50 amino acids Typically 100 to 10,000 amino acids linked together Chains are synthesizes based on specific bodily DNA Amino acids are composed of carbon, hydrogen, oxygen, and nitrogen

  4. Structural Differences Between Carbohydrates, Lipids, and Proteins Figure 6.1

  5. The Anatomy of an Amino Acid Figure 6.2b

  6. Peptide Bonds Link Amino Acids Form when the acid group (COOH) of one amino acid joins with the amine group (NH2) of a second amino acid Formed through condensation Broken through hydrolysis

  7. Condensation and Hydrolytic Reactions Figure 6.3

  8. Essential, Nonessential, and Conditional Essential – must be consumed in the diet Nonessential – can be synthesized in the body Conditionally essential – cannot be synthesized due to illness or lack of necessary precursors Premature infants lack sufficient enzymes needed to create arginine

  9. Structures of 20 common amino acids: Amino acids are grouped based upon the properties and structures of side chains.  1) aliphatic (R groups consist of carbons and hydrogens) glycine - R=H smallest a.a. with no chiral center alanine - R=CH3 methyl group valine R = branched; hydrophobic; important in protein folding leucine R= 4 carbon branched side chain isoleucine R = 2 chiral centers proline R = ring; puts bends or kinks in proteins; contains a secondary amino group  2) aromatic (R groups have phenyl ring) phenylalanine - very hydrophobic tyrosine - hydrophobic, but not as much because of polar groups tryptophan – “Absorb UV light at 280 nm --> used to estimate [protein] 3) sulfur-containing R groups methionine - sulfur is internal (hydrophobic) cysteine - sulfur is terminal --> highly reactive; can form disulfide bonds

  10. Contd…  4) side chains with alcohols serine - b-hydroxyl groups --> hydrophilic threonine - “  5) basic R groups histidine - hydrophilic side chains - + charged at neutral pH lysine – “ arginine - strong base 6) acidic R groups and amide derivatives aspartate - b carboxyl group - confer - charges on proteins glutamate - g carboxyl group   asparagine - amide of aspartate - side groups uncharged --> polar glutamine - amide of glutamate – “Amide groups can form H bonds with atoms of other polar amino acids”.

  11. Functions of Proteins • Catalysts - enzymes for metabolic pathways • Storage & Transport - e.g. myoglobin and hemoglobin • Structural- e.g. actin, myosin • Mechanical work - movement of flagella and cilia, microtubule movement during mitosis, muscle contraction • Decoding information - translation and gene expression • Hormones & Hormone receptors • Specialized functions - e.g. antibodies

  12. Structure of the Protein Four levels of structure Primary structure Secondary structure Tertiary structure Quaternary structure Any alteration in the structure or sequencing changes the shape and function of the protein

  13. Denaturing Alteration of the protein’s shape and thus functions through the use of Heat Acids Bases Salts Mechanical agitation Primary structure is unchanged by denaturing

  14. Denaturing a Protein Figure 6.5

  15. Protein Digestion: Part 1 Figure 6.6

  16. Protein Digestion: Part 2 Figure 6.6

  17. Protein Digestion: Part 3 Figure 6.6

  18. Protein Digestion: Part 4 Figure 6.6

  19. Amino Acid Absorption Amino acids are absorbed in the small intestine Amino acids are transported to the liver from the intestines via the portal vein In the liver, amino acids are Used to synthesize new proteins Converted to energy, glucose, or fat Released to the bloodstream and transported to cells throughout the body Occasionally proteins are absorbed intact

  20. Amino Acid Metabolism Liver metabolizes amino acids, depending on bodily needs Most amino acids are sent into the blood to be picked up and used by the cells Amino acid pool is limited but has many uses Protein turnover – the continual degradation and synthesizing of protein

  21. Protein Synthesis Figure 6.8

  22. Deamination When the amino acid pool reaches capacity the amino acids are broken down to their component parts for other uses First deamination must occur Carbon-containing remnants are Converted to glucose, if they are glucogenic amino acids, through gluconeogensis Converted to fatty acids and stored as triglycerides in adipose tissue

  23. Metabolic Fate of Amino Acids Figure 6.7

  24. How Does the Body Use Protein? Functions of protein Provide structural and mechanical support Maintain body tissues Functions as enzymes and hormones Help maintain acid base balance Transport nutrients Assist the immune system Serve as a source of energy when necessary

  25. How Much Protein Do You Need? Healthy, nonpregnant adults Should consume enough to replace what is used every day The goal is nitrogen balance Pregnant woman, people recovering from surgery or injury, and growing children Should consume enough to build new tissue

  26. Nitrogen Balance and Imbalance Figure 6.12

  27. Not All Protein Is Created Equal High quality protein Is digestible Contains all essential amino acids Provides sufficient protein to synthesize nonessential amino acids It helps to be aware of: Amino acid score Limiting protein Protein digestibility corrected amino acid score (PDCAAS) Biological value of protein rates absorption and retention of protein for use

  28. Protein Quality Complete proteins Contain all nine essential amino acids Usually animal source are complete proteins Are considered higher quality Incomplete proteins Low in one or more essential amino acid Usually plant sources are incomplete

  29. Protein Needs Protein intake recommendations 10–35% of total daily kilocalories Adults over 18 0.8 g/kg daily American College of Sports Medicine, the American Dietetic Association, and other experts advocate 50–100% more protein for competitive athletes participating in endurance exercise or resistance exercise Typically this population eats more and therefore gets additional protein

  30. Best Sources of Protein Proteins are abundant in Dairy foods Meats Poultry Meat alternatives such as dried beans, peanut butter, nuts, and soy 3 oz serving of cooked meat, poultry, or fish Provides 21–25 grams of protein About 7 g/oz About the size of a deck of cards Adequate amount for one meal

  31. Best Sources of Protein Figure 6.14

  32. Protein Bars Are marketed as convenient and portable Can be High in saturated fat and/or sugar Low in fiber Expensive A peanut butter sandwich is portable and lower in saturated fat and sugar and higher in fiber than some protein bars

  33. Eating Too Much Protein Risk of heart disease Risk of kidney stones Risk of calcium loss from bones Risk of colon cancer Displacement of other nutrient-rich, disease preventing foods

  34. Eating Too Little Protein Protein-energy malnutrition (PEM) Protein is used for energy rather than its other functions in the body Other important nutrients are in short supply More prevalent in infants and children 17,000 children die each day as a result

  35. Too Little Protein Without adequate protein Cells lining the GI tract are not sufficiently replaced as they slough off Digestive function is inhibited Absorption of food is reduced Intestinal bacteria gets into the blood and causes septicemia Immune system is compromised due to malnutrition and cannot fight infection

  36. Types of PEM: Kwashiorkor • Severe protein deficiency • Generally result of a diet high in grains and deficient in protein • Symptoms range from • Edema in legs, feet, and stomach • Muscle tone and strength diminish • Hair is brittle and easy to pull out • Appear pale, sad, and apathetic • Prone to infection, rapid heart rate, excess fluid in lungs, pneumonia, septicemia, and water and electrolyte imbalances (Image from http://www.thachers.org/pediatrics.htm) Figure 6.16

  37. Types of PEM: Marasmus • Results from a severe deficiency in kilocalories • Frail, emaciated appearance • Weakened and appear apathetic • Many cannot stand without support • Look old • Hair is thin, dry, andlacks sheen • Body temperature and blood pressure are low • Prone to dehydration, infections, and unnecessary blood clotting Figure 6.17

  38. Types of PEM: Marasmic Kwashiorkor Chronic deficiency in kilocalories and protein Have edema in legs and arms Have a “skin and bones” appearance With treatment the edema subsides and appearance becomes more like someone with marasmus

  39. Treatment for PEM Medical and nutritional treatment can dramatically reduce mortality rate Should be carefully and slowly implemented Step 1 – Address life-threatening factors Severe dehydration Fluid and nutrient imbalances Step 2 – Restore depleted tissue Gradually provide nutritionally dense kilocalories and high-quality protein Step 3 – Transition to foods and introduce physical activity

  40. Vegetarian Diet People choose vegetarian diets for a variety of reasons Ethical Religious Environmental Health Vegetarians must consume adequate amounts of a variety of food and should plan meals well

  41. Potential Benefits, Risks of a Vegetarian Diet Benefits of a healthy vegetarian diet Reduced risk of Heart disease High blood pressure Diabetes Potential risks of a vegetarian diet Underconsumption of certain nutrients Protein Vitamin B12 • Cancer • Stroke • Obesity

  42. Vegetarian Food Guide Pyramid Figure 6.18

  43. Soy Soy is increasing in popularity in the United States High-quality protein source Low in saturated fat Contains isoflavones Phytoestrogens May reduce risk of heart disease Some research suggests it may reduce the risk of cancer Some concern it may promote breast cancer

  44. Ionization of Amino Acids • All amino acids are have a neutral net charge at physiological pH (7.4). •  The a carboxyl and a amino groups and any other ionizable groups determine charge. •  Each amino acid has 2 or 3 pKa values (7 amino acids have side chains that are ionizable) (see Table 3.2). This complicates the basic titration curve, so that there are 3 inflection points rather than 2. •  At a given pH, amino acids have different net charges. •  Can use titration curves for amino acids to show ionizable groups. • The isoelectric point (pI) is the pH at which the amino acid has no net charge = zwitterion. • If pH > pI, amino acid would be negatively charged. • If pH < pI, amino acid would be positively charged. • If pH = pI, amino acid would have no charge.

  45. Can use Henderson-Hasselbalch equation to calculate the fraction of group ionized at a given pH. • a carboxyl group pKa 1.8-2.5 • a amino group pKa 8.7-10.7 • If pH< pKa, a greater amount of the group is protonated (NH3+ or COOH) • If pH > pKa, there is a greater ionization and a greater amount of unprotonated or anion form (NH2 or COO-). • If pH = pKa, then [conjugate base] =[weak acid]. • For the ionization of the carboxyl group of alanine, • pH = pKa + log [conjugate base] • ----------------------- [weak acid] • 7 = 2.4 + log [RCOO-] • ------------- • [RCOOH] •   4.6 = log [RCOO-] • ------------ • [RCOOH] •  39810:1 meaning the anion predominates greatly (almost all COOH groups are ionized).

  46. For the ionization of the a amino group of alanine, • 7 = 9.9 + log [NH2] -NH3+ ---> NH2 = H+ • ------------- • [NH3+] • -2.9 = log [NH2] • --------- • [NH3+] • 0.001:1 or 1 in 1000 molecules (undissociated group predominates) • At pI of 6.15, there is no net charge (all of the carboxyl groups are unprotonated, and none of the amino group is unprotonated). • pI = (pKa1 + pKa2)/2 • For R groups that are ionizable, the pI is not simply the average!!!

  47. Peptide Bonds • The primary structure of a protein is the linear sequence of amino acids that are covalently bonded to form a polypeptide chain. •  Formed by condensation reaction in which a molecule of water is removed. •  Each amino acid residue is called by replacing -ine or -ate with -yl • glycine ---> glycyl •  The peptide bond is a planar bond with no rotation around C-N axis. If is also in the trans form. Will talk about the consequences later

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