Polypeptides

• Two amino acids join together (by a peptide bond, condensation reaction) to form a dipeptide; many amino acids joined like this form a polypeptide; polypeptide chains form together to form proteins

  
• All polypeptides have an “Amino” end (-NH_2, N-terminus) and a “Carboxyl” end (-COOH, C-terminus) and no branches, just a chain (may be 100s of amino acids long)
• Amino acids sequence is determined by genetic code in DNA (mRNA brings it to ribosome), number, types, and arrangements of them in a chain is infinite (potential for a huge variety of protein structures)
• Polypeptides fold up to form specific functional 3D shapes; shape is essential for protein function, it can be broken down into four levels: primary, secondary, tertiary, and quaternary
• Primary Structure is determined by DNA, all proteins have a primary structure - only peptide bonds
• Primary Structure: The specific sequence of Amino acids in the polypeptide chains determined by DNA, present in every proteins, it only has peptide bonds
• Finding the primary sequence of a protein is called protein sequencing, first done by Fredrick Singer - he sequenced insulin and won a Nobel prize for it in 1958
• Secondary Structure: Primary structure folded into regular 3D shapes, maintained by hydrogen bonds which are individually weak but since they’re formed collectively they give the secondary structure stability (peptide bond groups carry small positive and negative charges enabling hydrogen bonds)
• Secondary Structure has two types of folding:

  • α-helix: H-bonds form between the C=O of one amino acids and the N-H of another, twists it into a helix, may take up the whole chain, or just a small part
  • β-pleated sheets: H-bonds form between the C=O of one chain and the - N-H of adjacent one running anti-parallel, it gives high tensile strength - supple but no stretch
• Quaternary Structure: When a protein has more than one polypeptide chain (e.g. many functional proteins), same types of bonding as in tertiary; a polypeptide chain may have a non-protein molecule attached (called a prosthetic group, e.g. the “haem” in haemoglobin), called a conjugated protein