LIFESCI 2N03 Lecture Notes - Lecture 9: Tripeptide, Low-Density Lipoprotein, Alanine
9 views10 pages
8 Feb 2016
School
Department
Course
Professor
LECTURE 7 LIFESCI 2N03
1
Proteins
Nov 21, 25 2013
Proteins
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
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 = 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
LECTURE 7 LIFESCI 2N03
2
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
LECTURE 7 LIFESCI 2N03
3
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)