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Nov 21 - Proteins - Lecture Notes - LIFESCI 2N03

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McMaster University
Life Sciences
Danny M.Pincivero

LECTURE 7 LIFESCI 2N03 Proteins Nov 21, 2013 Proteins  Greek derivative “protos” = of prime importance o Implies how our bodies use proteins  Organic molecules  2ndmost abundant components of plants and animals (2dto water)  Food Sources o Animal-based food (meat, fish, milk)  Generally provide better protein than plant based; there are exceptions o Plant sources – dried beans, peas, nuts, seeds, some vegetables  Proteins are assembles of amino acids o Carboxyl, amino and R group around a central C o 20 amino acids (same 20 in all animals) – 9 essential, 11 non-essential o Essential Amino Acid – an amino acid that the body cannot synthesize; must be ingested o Non-Essential Amino Acid – an amino acid that the body can synthesize o Conditionally essential amino acid – body can produce it, given that an essential amino acid is provided  Eg/  Tyrosine is made from phenylalanine  Cysteine is made from methionine o Amino acids are unique by their side chains, which dictate their size, shape, composition, electric charge and pH Protein Structure – 4 levels 1. Primary o Formation of a dipeptide Tripeptide = 3 amino acids o Oligoeptide = 4-10 amino acids o Polypeptide >10 amino acids o Most natural polypeptides = 50-200 amino acids  called a protein o Eg/ Polypeptide length = 200 amino acids (most common length)  Number of different proteins = 2000 o Cross-linking by disulphide bridges  Eg/ Insulin – sequenced in 1953 by Frederick Sanger 2. Secondary o Additional formation of chemical bonds – 2 resulting shapes 1 LECTURE 7 LIFESCI 2N03 o -helix – hydrogen bonds between N-H and C=O groups  3.6 amino acids/turn  Single chain with different chemical bonds between amino acids that are not adjacent  7TM on adipocytes that insulin binds to have -helices  Eg/ Myosin molecule – coiled coil; polypeptide  Biomolecular motor  -pleated sheet – hydrogen bonds; parallel layering of polypeptide portions  Sequence of amino acids folded up onto another group of amino acids giving that portion of amino aids a sheet-like appearance  Eg/ Albumin – fatty acids transporter 3. Tertiary o Protein structure loses symmetry o Involves complex protein folding o Non-polar amino acids inside, polar amino acids outside  Polar – ability to form electrical charge with water 4. Quaternary o Proteins that contain more than 1 polypeptide chain; each chain is referred to as a sub-unit of the protein o Eg/ Hb (hemoglobin) – 4 polypeptide chains = 2 sub-units of 1 type (-unit) and 2 of another type (-unit) – called an 2 2tetramer Protein Denaturation  Loss of protein “shape” – energy input overcomes dissociation energy of chemical bonds  Factors – pH, heat, alcohol, oxidation, mechanical agitation  Eg/ egg whites (approximately 15% protein, no CHOL, 40 different proteins) o Ovalbumin = 54% (385 amino acids, glycoprotein) o Ovotransferrin = 12% o Heat – denatures proteins  63˚C – ovotransferrin denatures  80˚C – ovalbumin denatures; makes egg white firm 2 LECTURE 7 LIFESCI 2N03 o Mechanical agitation – same effect as heat, but mixes air with water portion to produce “foam-like” structure; whisking egg white in a dish  Eg/ Raw Egg Whites o Avidin (egg white protein) binds to biotin (B-vitamin)  produces vitamin B deficiency o Ability to absorb amino acids is diminished if eggs are consumed raw – by cooking, protein is denatured and broken up (which is what our body is trying to do)  Yolks – contain 4 fat soluble vitamins and large amounts of cholesterol, and fatty acids (saturated and unsaturated)  Egg whites contain protein, less calories than yolk Protein Functions 1. Mechanical o Collagen – provides structures to all tissues; exists in different quantities in all cells (muscle tendon, ligament); stiffer tissue has more collagen o Biomolecular motors –myosin, kinesin, dynein (ATPases) 2. Enzymes o Catalyzes chemical reactions 3. Hormones o Insulin, TSH, leptin o TSH – from pituitary gland; made up of amino acids o Leptin – secreted by fat cells that regulate feeding behaviour; reduced sensation of hunger 4. Immune Function o Antibodies, blood proteins, neutralizes action of antigens o Antibodies – blood proteins; function to neutralize antigens 5. Fluid Balance o Albumin and globulin in blood – cause osmotic balance; regulate fluid balance in the body; large proteins, stay confined within CV system; osmotic pressure they set up draws fluid form extracellular tissue; set up concentration that draws water o Too big to leave CV system, but attracts fluid out of interstitial space (fluid outside of CV system that surrounds cells) o Decrease in blood proteins causes fluids to accumulate in tissue; edema common sign of protein energy malnutrition 6. Acid-base Balance o Blood pH = 7.4 o Blood proteins bind acidic or alkaline atoms/molecules  Eg/ Hemoglobin – can bind protons and buffer a decreasing pH 7. Transport Functions o 70-80%of energy expenditure at rest; ATPases (Na+ K+ and Ca+)  Na+/K+ “pump” ATPase  ATPases more active in exercise o Lipoproteins 8. Energy o Oxidative deamination o Removal of nitrogen group form amino acid o C, H and O used to make glucose (glucogenic amino acid) or ketone bodies (ketogenic) – ketone bodies converted to Acetyl CoA by neurons o Excess of protein converted to fatty acids Summary of Protein Digestion  Digestion begins in the stomach o HCl – denatures proteins (pH approximately 0.8) o Pepsinogen + HCl = pepsin (active at pH = 2.5) – accounts for 10-20% of proteins digested  Small intestine – amino acids and dipeptides enter o Proteases – catalyzes large to small peptides o Trypsin and chymotrypsin – comes from pancreas; catalyzes large to small peptides o Peptidases (microvilli in intestinal wall) o Catalyzes tri- and di-peptides into amino acids Protein Absorption  Duodenum and jejunum 1. Facilitated diffusion – membrane proteins 2. Active transport o “Na exchanger” o Na re-entry into CV system brings in amino acids (leucine, isoleucine, valine) 3 LECTURE 7 LIFESCI 2N03 o Problem – too much of one amino acid may saturate the transporters, therefore absorption of another (essential) amino acid  When amino acids are absorbed – amino acid “pool” o Carbohydrates – liver and skeletal muscle o Lipids (triglycerides) – mostly in adipocytes o Proteins – everywhere Protein Synthesis  DNA, mRNA, tRNA = new protein  Encoding a new protein from blueprint – amino acids taken up by ribosomes (rRNA)  Turned on by hormones and signaling molecules  Materials necessary  Non-essential amino acids –  Essential amino acids –  Other molecules o Tryptophan – serotonin (regulates sleep), niacin o Tyrosine – norepinephrine and epinephrine o Tyrosine – thyroxin o Histidine – histamine Protein in Food  Sources – animal-based foods, eggs, milk, legumes, grains, some vegetables Recommendations (RDA’s)  Age to Quantity (g/kg body mass) o 0-6 months o 6 months – 1 year o 1-3 years o 4-6 years o 7-10 years o 11-14 years o 15-18 years (males) o 15-18 years (females) Increased Protein Needs  Infections, burns, illness, surgery  Pregnancy and lactation Athletes  “Strength/power” athletes o 1.7-1.8 grams/kg body mass o Due to accelerated protein turnover resulting from muscle “damaging” exercise  “Endurance” athletes o 1.2-1.3 grams/kg body mass o Protein used as an energy su
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