Heme, the prosthetic portion of hemoglobin, is synthesized
in nucleated red cells in the bone marow.
Four pyrrole units are arranged to form ringed structures:
heme, porphyrinogen, and porphyrin. When broken
down (catabolism) these ringed structures form linear tetrapyrrole
molecules, the bile pigments, ie. biliverdin.
The biosynthesis of heme is outlined schematically on the
following cards: 1) synthetic sequence, 2) synthetic location,
and 3) enzymes.
Notes: a) The rate limiting synthetic step is the formation
from glycine and succinyl coA of d- aminolevulinic acid
b) Several key steps take place in mitochondria or on mitochondrial
c) Porphyrinogens are colorless, unstable, ringed tetrapyrroles
that are readily and irreversibly oxidized to red, stable
porphyrins, which are excreted in urine or bile.
d) Only one porphyrin, protoporphyrin IX, is retained and
is converted to heme.