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OCR Biology A Notes on Proteins. Followed the H420 Specification given by the exam board
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Dipeptide = when 2 amino acids join togetherPolypeptide = when more than 2 amino acids join together
Amino acids = same general structure
Carboxyl group (COOH) and amino group (-NH2)
attached to carbon atom
Difference = R group
Proteins have Carbon, Oxygen, Hydrogen, Nitrogen,
(some have Sulfur)
Amino acids joined by peptide bonds
Water molecule released during reaction (condensation reaction)
Water molecule added during reaction to break bond (hydrolysis)
Peptide bond = C - N
Levels of protein structure:
Primary - sequence of amino acids in polypeptide chain
Secondary - Polypeptide chain doesn’t remain flat + straight. Hydrogen bonds form
between nearby amino acids in the chain - makes it coil into an alpha helix or fold into a
beta pleated sheet
Tertiary - coiled/folded chain is often coiled/folded further. More bonds form = Forms
3D structure
Quaternary - Some proteins are made of several different polypeptide chains held
together by bonds. Quaternary structure is the way these polypeptide chains are assembled
together. For proteins with more than 1 polypeptide chain, the quaternary structure =
final 3D structure
Bonds in each level:
Primary = peptide bonds between amino acids
Secondary = hydrogen bonds
Tertiary:
1. Ionic bonds - attractions between negatively-charged R and positively-charged
R groups
2. Disulfide = 2 molecules of cysteine come close together. Sulfur atom in 1 cysteine
bonds to another sulfur-contained cysteine
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