3) Proteins Summary_9744_2018

The Cell & Molecules of Life (9744) Proteins 2018 Prepared by: Mrs Selvamani Nair & Mrs Wong S H Raffles Institution (Yr 5-6) 1 Proteins Describe the structure and properties of an amino acid and describe the formation and breakage of a peptide bond  Amino Acids – Basic structural unit of proteins  Structure o consists of an α-carbon atom covalently bonded to 4 groups 1) hydrogen atom, 2) amino group (-NH2), 3) carboxyl group (-COOH), 4) variable R group  Properties of amino acids o Classified according to their R groups as uncharged (non-polar or polar) or charged o Exist as zwitterions in solution – carry both positive and negative charges o Act as buffers  Can donate or accept H+, therefore able to act as an acid or as a base – amphoteric  Essential in biological systems – sudden change in pH could adversely affect performance of proteins like enzymes  Polypeptides  Amino acids are joined by a peptide bond via a condensation reaction with the removal of one water molecule.  Further addition of amino acids results in the formation of a linear polymer called a polypeptide o Regularly repeating part, the main chain, is referred to as the backbone. o Variable part comprises the distinctive variable R groups  polypeptide folds into a specific three-dimensional shape / conformation  The nucleotide sequence in DNA determines amino acid sequence in polypeptide which determines types and locations of R groups which determines R group interactions which determines 3D structure and function of protein. (See picture with 4 R gp bonds / interactions below) Explain primary structure, secondary structure, tertiary structure and quaternary structure of p roteins, and describe the types of bonds (hydrogen, ionic, disulfide and hydrophobic interactions) that hold the molecule in shape.  4 levels of organization in the structure of proteins (a) Primary structure  Refers to the number and sequence of amino acids in a single polypeptide chain.  Linear structure maintained by peptide bonds  The sequence of amino acids (and their R groups) in a polypeptide chain determines the type and location of chemical bonds/interactions, and hence the 3D conformation and characteristics of a particular protein. (b) Secondary structure  Structure formed by regular coiling or pleating of a single polypeptide chain.  Maintained by hydrogen bonds o between C=O and N-H groups of the polypeptide backbone. o R groups are not involved  Examples of secondary structures: o -helix  Made up of a single polypeptide chain which is wound into a coiled/spiral structure.  A hydrogen bond forms between the C=O group of one amino acid residue and the N-H group of another amino acid res