The apoprotein is an important intermediate on the folding pathways of
many haem proteins, yet a detailed structure of such an intermediate
has remained elusive. Here we present the structure of apocytochrome b
(562) obtained by NMR spectroscopy. The apoprotein has a topology simi
lar to the holoprotein. Nevertheless, significant differences in helix
-helix packing between the two are evident. Much of the haem binding p
ocket in the apoprotein is preserved but exposed to solvent creating a
large cavern. As apocytochrome b(562) displays many of the physical c
haracteristics ascribed to the molten globule state, these results hel
p ellucidate the origin of several properties of the protein molten gl
obule.