3.1.4.1 General properties of proteins

• Proteins


  are polymers of amino acids

  
• The monomers of proteins are

  amino acids

  
• Dipeptide


  formed when two amino acids join together

  
• Polypeptide


  formed when more than two amino acids join together

  
• Amino acids have the same general structure

  ● a carboxyl group (-COOH)● an amine or amino group (-NH2)● an R group (also known as a variable side group)— attached to a carbon atom

  
• R groups generally contain carbon. The only exception is


  ● glycine — its R group consists of just one hydrogen atom
• All living things share a bank of

  only 20 amino acids— only difference between them is what makes up their R group
• Amino acids are linked together by

  ● condensation reactions— to form dipeptides and polypeptides— A molecule of water is released during the reaction

  
• Peptide bonds


  The bonds formed between amino acids

  
• The reverse reaction (hydrolysis) happens when


  dipeptides and polypeptides are broken down

  
• Hydrogen bond


  is a relatively weak bond formed between hydrogen atoms and other atoms (FON)— e.g. fluorine, oxygen, nitrogen
• Primary structure

  ● is the number and sequence of amino acids in the polypeptide chain— can determine the 3D shape or tertiary structure— therefore affect the shape of the active site in enzymes

  
• Proteins are much easier to explain if you describe their structure in four levels


  ● primary structure ● secondary structure ● tertiary structure ● quaternary structure
• The Primary Structure


  is specific for each protein — one alteration in the sequence of amino acids can affect the function of the protein

  
• Secondary structure

  ● Hydrogen bonds form between the amino acids in the chain● makes it automatically coil into— an alpha (a) helix or— fold into a beta (b) pleated sheet

  
• The many hydrogen bonds make these structures

  stable— Most channel proteins are made of alpha helices
• Disulfide bridges


  are covalent bonds between sulfur atoms— a lot stronger than the ionic and hydrogen bonds in proteins
• Tertiary structure

  ● The coiled or folded chain of amino acids is often coiled and folded further● More bonds form between different parts of the polypeptide chain— including hydrogen bonds and ionic bonds — attractions between negative and positive charges on different parts of the molecule

  
• Tertiary Structure

  ● Disulfide bridges also form whenever two molecules of the amino acid cysteine come close together— the sulfur atom in one cysteine bonds to the sulfur atom in the other

  
• The tertiary structure is

  the 3D shape of the polypeptide chain— All enzymes, antibodies and some hormones have this

  
• The tertiary structure


  creates a specific shape due to the sequence of amino acids in the chain as ● hydrogen bonds● ionic bonds● disulfide bridges (covalent bonds) — form between R groups

  
• A change to the sequence of amino acids would affect

  the secondary and tertiary structure — as these bonds would form in different places
• Quaternary structure


  proteins made of more than one polypeptide chain joined together

  
• Examples of quaternary structures

  ●Antibodies ●Haemoglobin ●Insulin ●Collagen ●Other non-protein groups may also be associated — e.g the haem group in haemoglobin
• Quaternary Structure


  is the way these polypeptide chains are assembled together— Some proteins are made of several different polypeptide chains held together by bonds
• The quaternary structure is the proteins


  final 3D structure
• Not all proteins have a quaternary structure — some are made of only one polypeptide chain.


  some are made of only one polypeptide chain
• A protiens shape determines


  its function — e.g. haemoglobin is a compact, soluble protein— makes it easy to transport— makes it great for carrying oxygen around the body
• Proteins found in living organisms have all got different structures and shapes, which makes them


  specialised to carry out particular jobs
• Enzymes

  ● roughly spherical in shape — due to the tight folding of the polypeptide chains● soluble ● often have roles in metabolism — e.g. some enzymes break down large food molecules (digestive enzymes)● other enzymes help to synthesise (make) large molecules
• Antibodies

  ● are involved in the immune response ● are found in the blood● made up of two light (short) polypeptide chains two heavy (long) polypeptide chains— bonded together● have variable regions — the amino acid sequences in these regions vary greatly.
• Transport proteins

  ● E.g. channel proteins are present in cell membranes— Channel proteins contain hydrophobic (water hating) and hydrophilic (water loving) amino acids— cause the protein to fold up and form a channel● These proteins transport molecules and ions across membranes

  
• Structural proteins


  ● are physically strong● consist of long polypeptide chains lying parallel to each other with cross‑links between them● include :— keratin (found in hair and nails) — collagen (found in connective tissue)
• Collagen has

  ● three polypeptide chains tightly coiled together — makes it strong— makes it a great supportive tissue in animals
• Proteins are also used as

  chemical messengers in the body— e.g. as hormones like insulin
• To test for a protien

  use the biuret test
• The biuret test for proteins

  ● add a few drops of sodium hydroxide solution — test solution needs to be alkaline● add some copper(II) sulfate solution

  
• If protein is present, the solution

  turns purple

  
• If there's no protein the solution

  will stay blue