LIFESCI 2N03 Lecture Notes - Lecture 9: Tripeptide, Low-Density Lipoprotein, Alanine

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Nov 21, 25 2013
Greek derivative “protos” = of prime importance
o Implies how our bodies use proteins
Organic molecules
2nd most abundant components of plants and animals (2nd to 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
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 = 20200
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
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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
22 tetramer
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
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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
Removal of nitrogen group from amino acid
(Amino acids) C, H and O used to make glucose (glucogenic amino acid) or ketone bodies (ketogenic) ketone
bodies converted to Acetyl CoA by neurons (Kreb’s Cycle)
o Excess of protein converted to fatty acids diet that is energy sufficient
Summary of Protein Digestion
Digestion (chemical) 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 (two major proteases acting in
small intestine)
o Peptidases (microvilli in intestinal wall) enzymes that will function on small proteins
Catalyzes tri- and di-peptides into amino acids
Protein Absorption
Duodenum and jejunum
1. Facilitated diffusion membrane proteins involved in absorbing nutrients
2. Active transport membrane bound transporter that uses ATP
o “Na+ exchanger”
o Na+ re-entry into CV system brings in amino acids (leucine, isoleucine, valine)
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