Lecture 8 (cont) - final review.docx

Lecture 8/9 PEPTIDES Peptides are less structurally complex than larger protein molecules but they still have significant biological activities. - Ex: Glutathione is a tripeptide found in almost all organisms and is involved in protein and DNA synthesis, toxic substance metabolism, and amino acid transport - Vasopressin = antidiuretic hormone that regulates water balance, appetite, and body temperature - Oxytocin = peptide that aids in uterine contraction and lactation Functions of Proteins Of all the molecules in a living organism, proteins have the most diverse set of functions: 1) Catalysis – catalytic proteins = enzymes. Serve to accelerate biochemical reactions 2) Structure – structural proteins (cell and organismal) 3) Movement – proteins that comprise the cytoskeleton are involved in cell movement. Also active in cell division, endocytosis, exocytosis, and the ameboid movement of white blood cells. 4) Defense – blood clotting proteins, keratin (protein in skin cells), antibodies 5) Regulation – binding a hormone molecule or a growth factor to cognate receptors on its target cell changes cellular function. (ex: insulin and glucagon are peptide hormones) 6) Transport – many proteins function as carriers of molecules/ions across membrane or between cells. 7) Storage – certain proteins serve as a reservoir of essential nutrients. Ex: ovalbumin in bird eggs 8) Stress Response – capacity of organisms to survive a variety of abiotic stresses is mediated by certain proteins. Excessively high temperatures and other stresses result in the synthesis of a class of proteins called the heat shock proteins (promote the correct refolding of damaged proteins, or promote their degradation). Cells also protected from radiation by DNA repair enzymes. Recent research has also identified numerous multifunction proteins - Proteins are categorized into families based on sequence and three-dimensional shape o Superfamilies = more distantly related proteins (ex: hemoglobin and myoglobin to neuroglobin) - Also classified by shape o Globular = compact spherical molecules that are usually water-soluble (ex: nearly all enzymes are globular in shape, same with hemoglobin and albumin) o Conjugated = simple protein combined with a non-protein component.  Non-protein component = prosthetic group  Protein without its prosthetic group = apoprotein  Protein molecule combined with its prosthetic group = holoprotein o Conjugated proteins are also classified according to the nature of their prosthetic groups  Glycoproteins contain a carbohydrate component  Lipoproteins contain lipid molecules  Metalloproteins contain metal ions  Phosphoproteins contain phosphate groups  Hemoproteins possess heme groups Protein Structure Models are complicated… Space-filling and ribbon models may be useful. Levels of protein structure = primary, secondary, tertiary, and quaternary. Primary structure = specific amino acid sequence of a protein - Homologous proteins share a similar sequence and arose from the same ancestor gene - When comparing amino acid sequences of a protein between species, those that are identical are invariant and presumed to be essential for function Primary Structure, Evolution, and Molecular Diseases - Due to evolutionary processes over time, the amino acid sequence of a protein can change due to alterations in DNA sequences called mutations o Many mutations lead to no change in protein function o Some sequence positions are less stringent/variable because they perform nonspecific functions o Some changes are said to be conservative because it is a change to a chemically similar amino acid (ie: substituting the current amino acid for another one with a chemically similar side chain) Why Sequence Proteins? Knowledge is used for: - 3D structure interpretation - Protein engineering - Structure prediction - To find the gene - Identify post-translational modification - Molecular evolution studies – identifying the conservative and variable sites to trace evolutionary relationships. Supported by cytochrome c primary sequence data… Number of differences between the cytochromes c of different species - Phylogeny Primary Structure, Evolution, and Molecular Diseases… - Mutations can be deleterious, leading to molecular diseases - Sickle cell anemia is caused by a substitution of valine for a glutamic acid in beta-globin subunit of hemoglobin o Valine is hydrophobic, unlike the charged glutamic acid o The substitution for hydrophobic valine HbS: molecules aggregate to form sickle-shaped cells o These cells have low oxygen-binding capacity and are susceptible to hemolysis Secondary Structure = consists of several repeating structures - Most common structures i