1.6 - Proteins

• structure of amino acids

  the R group is a variety of different chemical groups so varies from amino acid to amino acid - can be polar

  
• formation of peptide bonds

  2 amino acids combine formig a peptide bond between the C atomof one and the N atom of the other during condensation reactions to form a dipeptide
• structure of a peptide bond

  formed between 2 amino acids

  
• structural levels of proteins

  primary

  secondary

  tertiary

  quaternary
• primary structure

  sequence of amino acids joined together during polymerisation forming a polypeptide determined by DNA

  determines the ultimate shape and function of protein
• secondary structure

  hydrogen bond form between -COOH and -NH2 so polypeptide is twisted into an alpha helix or folded into a beta pleated sheet
• tertiary structure

  chain of amino acid is coiled/folded further forming more bonds between R groups forming a 3D shape (where they occur depends on the primary structure):

  hydrogen bonds

  ionic bonds

  disulfide bridges
• ionic bonds in tertiary structure

  formed between carboxyl and amino groups not involved in forming peptide bonds

  weaker then disulfide bonds

  easily broken by changes in pH
• disulfide bridges in the tertiary structure

  fairly strong so not easily broken

  
• quaternary structure

  combination of several polypeptide chains held together by bonds

  there may be non-protein groups (prosthetic) associated with molecules
• test for proteins

  Biuret Test:

  place sample in test tube and add same volume of sodium hydroxide at room temperature to make solution alkaline

  add a few drops of dilute copper (ii) sulfate and mix gently

  no protein: solution remains blue

  protein: solution turns purple
• functions of proteins

  enzymes: metabolism (break down large food molecules), synthesise large molecules

  antibodies: immune response

  transport proteins: channel proteins have hydrophobic and hydropilic amino acids so protein forms a channel to transport molecules

  structural proteins: long polypeptide chains parallel to each other with cross-links between them
• types of proteins

  fibrous proteins

  globular proteins
• fibrous proteins

  have structural functions

  form long chains that run parallel to each other, linked by cross-bridges so are very stable

  insoluble

  e.g.: collagen
• molecular structure of collagen

  primary structure: unbranched polypeptide chain

  secondary structure: polypeptide chain is very tightly wound

  tertiary structure: chain is twisted into a second helix

  quaternary structure: 3 polypeptide chains wound together like fibres are wound together in a rope
• globular proteins

  soluble

  3D shape

  e.g.: enzymes