Biochemistry Lecture Notes PDF

Biochemistry LECTURE (12) DR. El-Sawy 1  Biochemistry Protein Structure Definition:  Proteins are formed of large number of a...

Biochemistry LECTURE (12) DR. El-Sawy 1  Biochemistry Protein Structure Definition:  Proteins are formed of large number of amino acid linked together by peptide bonds to give a polypeptide chain. Orders of protein structures Primary Protein Structure  Referred to number, type & sequence of A.A in polypeptide chain.  Any change in one of amino acids  physiological defect.  Main bond  peptide bond.  Free -NH2 group of 1st amino acid  N-terminal end.  Free -COOH end of last amino acid  C-terminal end. DR. El-Sawy 1  Biochemistry Protein Structure secondary Protein Structure α-Helix β-pleated Sheets  The polypeptide chain is twisted to right-handed coiled helix.  Formed when hydrogen bonds are formed  Each turn contains 3.6 amino acids. between 2 or more adjacent polypeptide chains  Common secondary structure in globular proteins. connected by hydrogen bonds.  The formation is spontaneous.  Stabilized by intra chain Hydrogen bonds ( )  Hydrogen bonds  inter-chain.  Carbonyl oxygen of 1st peptide bond  It is pleated due to angles of bonds.  Hydrogen of NH of the next 4th peptide bonds in chain.  Hydrogen bonds  parallel to helix axis. DR. El-Sawy 2  Biochemistry Protein Structure Super secondary structures (motifs) Def Combinations of α-helix and β-sheets to form specific shape. Function It gives protein specific structure feature to enable particular function. Helix-turn-helix (HTH) Examples Two α helices joined by a short strand of amino acids It is a major structural motif capable of binding DNA. DR. El-Sawy 3  Biochemistry Protein Structure Tertiary Protein Structure  Secondary structures are arranged to form final functional 3D structure called domain. Def  It occur due to interaction between side chains (R) of the amino acids.  Forces controlling tertiary structure : Hydrogen bond Between polar side chains of amino acids. Hydrophobic forces Between non-polar (R) groups of amino acids. Electrostatic forces Between oppositely charged (R) of amino acids. (ionic bonds, salt bridges) Disulfide bonds Between sulfur amino acids (cysteine). Bonds DR. El-Sawy 4  Biochemistry Protein Structure Quaternary Proteins Structure  Special arrangement of more than one polypeptide chains (subunits = monomers). Def  This high level of organization is essential for activity of some proteins as hemoglobin (Hb) & enzyme. Bonds  Non covalent (hydrogen or ionic bonds). Creatine kinase (CK) Dimer Haemoglobin (Hb) & lactate Tetramers dehydrogenase (LDH) Example of oligomeric proteins DR. El-Sawy 5  Biochemistry Protein Structure DR. El-Sawy 6  Biochemistry Protein Structure DR. El-Sawy 7