Proteins Structure and Function Introducing proteins Proteins are a diverse group of large and complex polymer molecules, made up of long chains of amino acids. They have a wide range of biological roles, including: structural: proteins are the main component of body tissues, such as muscle, skin, ligaments and hair
catalytic: all enzymes are proteins, catalyzing many biochemical reactions signalling: many hormones and receptors are proteins
immunological: all antibodies are proteins. 2 of 29 Boardworks Ltd 2008 The general structure of amino acids All amino acids have the same general structure: the only difference between each one is the nature of the R group. The R group therefore defines an amino acid. amino group
carboxylic acid group R group The R group represents a side chain from the central alpha carbon atom, and can be anything from a simple hydrogen atom to a more complex ring structure. 3 of 29 Boardworks Ltd 2008 The 20 naturally-occurring amino acids
4 of 29 Boardworks Ltd 2008 Proteins Structure and Function Peptide bonds and dipeptides 6 of 29 Boardworks Ltd 2008
Polypeptides When more amino acids are added to a dipeptide, a polypeptide chain is formed. A protein consists of one or more polypeptide chains folded into a highly specific 3D shape. There are up to four levels of structure in a protein: primary, secondary, tertiary and quaternary. Each of these play an important role in the overall structure and function of the protein.
7 of 29 Boardworks Ltd 2008 Portion of polypeptide chain Amino acids Peptide bond Hydrogen bonds form
- helix Primary structure Secondary structure - the folding of the of polypeptide chain - the sequence amino acids
in a intopolypeptide an -helix or a -pleated sheet. chain Hydrogen bonds Portion of polypeptide chain Hydrogen bonds form
Hydrogen bonds Secondary structure - the folding of the polypeptide chain into an -helix or a -pleated sheet. - pleated sheet - pleated sheet - helices Amorphous
regions Tertiary structure - the secondary structures fold up to form a very precise threedimensional structure Bonds responsible for the tertiary structure Hydrogen bonds Shared electrons spend longer at these atoms, forming a slight negative charge
bonds to molecule C O- Hydrogen bonds form between these polar groups +
H N bonds to molecule hydrogen bond High temperatures and altered pH can split these bonds
Bonds responsible for the tertiary structure Ionic bonds bond to molecule O Basic group H
C Acidic group O- + Ionic bond H H
N bond to molecule Ionic bonds can be split by changing the pH Disulphide bonds CH2 CH2
Bonds responsible for the tertiary structure cysteine R group SH S HS CH2 S
CH2 disulphide bond (covalent) Disulphide bonds can be split by reducing agents Polar and Non Polar The arrangement of the atoms in some molecules is such that one end of the molecule has a positive electrical charge and the other side has a negative
charge. If this is the case, the molecule is called a polar molecule, meaning that it has electrical poles. Otherwise, it is called a non-polar molecule. Whether molecules are polar or non-polar determines if they will mix to form a solution or that they don't mix well together. Also, polar molecules are water soluble, while non-polar molecules are fat soluble. van der Waals forcesBonds responsible for the tertiary structure These are weak forces of attraction between non-polar groups Water excluded from these hydrophobic side chains helps keep the side chains together
Valine R group Phenylalanine R group CH(CH3)2 CH2 Weak van der Waals force of attraction These forces can be split by a rise in
temperature H20 NOTE: the cell is an aqueous environment 1 2 H20 3
4 H20 Hydrophobic R groups 5 6 7
8 H20 H20 H20 9 H20 2 13
3 1 12 4 6 H20 H20
13 H20 H20 H20 Globular proteins form a spherical mass with a specific 3-D H20 shape (tertiary and quaternary H20 structure)
12 H20 H20 H20 They fold up so that hydrophilic groups are on the outside and H20 H20
hydrophobic groups are inside the molecule 11 H20 H20 H20 Hydrophilic
R groups 10 5 11 7 8 H20
10 9 H20 H20 Quaternary structure This is where two or more polypeptide chains are joined to form a protein, e.g. haemoglobin collagen
Haemoglobin is an example of a globular protein with quaternary structure -chain subunit -chain subunit 4 polypeptide chains - 2 -subunits - 2 -subunits
Haem groups 4 haem prosthetic groups Fibrous proteins Fibrous protein molecules form long chains or fibres (they have primary, secondary, tertiary and quaternary structure) Their fibrous nature makes them insoluble in water... ... this makes them useful for structure and support Collagen found in skin, teeth, bones, tendons, blood vessel
walls Polypeptide chains Hydrogen bonds Fibres form a triple-helix of polypeptide chains These chains are held together by hydrogen bonds The structure of proteins 22 of 29
Boardworks Ltd 2008 Protein structure 23 of 29 Boardworks Ltd 2008 Denaturing proteins If the bonds that maintain a proteins shape are broken, the protein will stop working properly and is denatured.
denaturation: bonds broken Changes in temperature, pH or salt concentration can all denature a protein, although the specific conditions will vary from protein to protein. Fibrous proteins lose their structural strength when denatured, whereas globular proteins become insoluble and inactive. 24 of 29 Boardworks Ltd 2008 Biuret test for proteins
25 of 29 Boardworks Ltd 2008 PROTEINS dingbats say what you see NNNNNNNNNNN M S
2- -LIN -j - PA -k +y C T
+ary -d +ary
