200 



MACROMOLECULAR COMPLEXES 



The area available at the surface ( R = 160 A ) for the protein of 

 any subunit is about twice as large as the area available at the lipid- 

 protein interface ( R = 100 A ) . In the case of the hvpothetical 

 minimal unit, the area available at the lipid interface is only 420 A". 

 The protein could be folded, with the terminal groups brought to 



Fig. 11. A working hypothesis of the ultrastructure of the chromatophore. 

 The Chromatium chromatophore is described as a hollow sphere about 320 A 

 in diameter with a cortex about 90 A thick. The pigment molecules (B) aligned 

 in a monolayer are bounded internally by a phospholipid monolayer (A) and 

 externally by a 60-A-thick protein layer. The "minimal unit" of composition has 

 been used as a structural subunit. The protein has been folded and is related 

 directly to two chlorophyll molecules. On the average, the protein is related 

 indirectly to one carotenoid molecule and ten phospholipid molecules. 



the lipid surface as a-helices. Such an arrangement provides an 

 opportunity for visualizing a specific relationship between the ter- 

 minal amino acid sequence and the polar groups of the porphyrin. 

 In addition, a particular spatial orientation could be specified by the 

 pitch of the helix. Disulfide bridges between the subunits would 

 account for the observed stability of the chromatophore. 



