<prompt> The four pyrole rings in hemoglobin molecule are joined together by?Disulphide bridgesMethylene bridgesHydrogen bondsAlpha bonds  <response>: Ans. B. Methylene bridges. Ref. Harpers Biochemistry 26th pg. 270-278 The four pyrrole rings of protophyrin IX are held together by methylene bridges.Structure of Hb Hb Heme globulin 2? 2? chainsQ The iron is in Fe state in the heme of functional Hb. Iron can form six Co-Ordinated Bonds. Iron is held at the centre of the heme by 4 nitrogen of porphyrin rings pyrrole rings. Iron bind internally by 5th linkage with amino acids of Histidine.Q The co-ordinated position of Fe is a available to bind to O2. The four Pyrole rings are joined by Methylene or Methylidene bridge -C bridge. Hb-intemally contains mostly Hydrophobic amino acids forming Heme pocket and Hydrophilic amino acids on their surfaces. The Heme pocket of a-subunit are adequate for entry of O2 while b-subunits Heme pockets blocks entry of O2 by a valine residues. 