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2. Hemoglobin & Heme
  The hemoglobin molecule is made up of four polypeptide chains, (αβ)2, here colored Alpha, Beta, Alpha, Beta. The chains are bound to each other by non-covalent bonds. A 4-chain quaternary structure is called a tetramer. There are four heme-iron complexes, one in each chain.
  Each chain holds a heme group containing one Fe++ atom. The largely hydrophobic heme is bound non-covalently in a hydrophobic protein pocket (which we'll see later).
  The heme-iron complexes are colored red because they give hemoglobin its red color.
  Now the heme molecules have been colored by chemical element:
C O N Fe
  A single heme molecule with its atoms shown "spacefilling".
  Here is how iron is attached to the rest of the heme molecule.
C O N Fe
(Double bonds are shown as single sticks.)
  An elemental oxygen O2 molecule binds to the ferrous iron atom in the lungs where oxygen is abundant, and is released later in tissues that need oxygen.
  The position of bound elemental oxygen in one chain of hemoglobin.
  Pocket occupied by the heme-bound oxygen in one polypeptide chain.
  A histidine nitrogen binds to the iron, helping to anchor its position. A second histidine nitrogen anchors the bound molecular oxygen.
  A spacefilling view of one hemoglobin polypeptide chain with the heme and the nearby histidines colored by chemical elements:
C O N Fe
How does oxygen get in? Notice that there is no open path large enough for O2 to get from the outside to the heme iron, and later to escape. But protein molecules are not rigid and unmoving, unlike this molecular model. Flexibility and thermal motion are believed to open a transient pathway that enables rapid entry and exit of oxygen (Terrell 2018, Vallone 2004, Ref. 3B).

   

What Do You See?
  1. What is the main function of hemoglobin?
  2. How many protein chains are in each hemoglobin molecule?
  3. How many heme-iron complexes are in each hemoglobin molecule?
  4. When saturated with oxygen, how many oxygen atoms are bound to each hemoglobin molecule?
  5. What holds the polypeptide chains together in one hemoglobin molecule?
  6. What holds each iron atom in place?
  7. What does oxygen bind to in hemoglobin? (Remember, two sides of the oxygen molecule each bind to different things.)

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