Question

Cytochrome C551 (from Pseudomonas aeruginosa)

Protoporphyrins are related to heme groups.

What can you say about the protein structure illustrated here?

Is this a globular or a fibrous protein?

A residue is shown in yellow. What is the charge on this residue?

Where are the N-terminal and the C-terminal?

Answer 1

• Cytochrome C551 (from Pseudomonas aeruginosa ) proteins have been used as models like folding models. Mitochondrial and bacterial cytochrome C551 have folding mechanism where N- and C- terminal helices interact with each other in folding intermediates. In general it remains in dimeric, trimeric and tetrameric form which were produced from oxidised monomeric Cyt C551 by an addition upto 80% ethanol, subsequent lyophilization and resolvation with buffer. And one of the important point is that there is no oligomer larger than the tetramer formed. But higher order oligomers are formed in horse cytochrome c (another type of cytochrome).
• This particular protein is globular protein. This is due to formation of oligomers and have sometimes quaternery structures.
• 
• The above diagram depicts the monomeric cyt C551 structure (A).
  (B) depicts the dimeric form.
• In these structures the yellow residue shown is the sulfur atoms of haeme axial Met ligand and haeme linked Cys are shown in yellow. The Nitrogen atoms of the haeme axial His ligand are in blue.
• The N - terminal and C- terminal are clearly shown in the picture. Its present in the foldings and that bound the whole protein with each other. The contacts between the N- and C-terminal α-helices are essential for not only protein folding but also domain swapping.

The orange coloured diagram shows the PA cyt C551 ( Pseudomonas aeruginos) .

The helices are depicted as arrows in the secondary structure diagrams. The helices and loops are labeled as H1–H4 and L1–L3, respectively.

Topology diagrams of PA cyt c551 and horse cyt c.

(A) Monomeric PA cyt c551, (B) dimeric PA cyt c551, (C) monomeric horse cyt c, and (D) dimeric horse cyt c. The helices and loops are labeled as H1–H4 and L1–L3, respectively. The helices are depicted as arrows. The hinge loops in the monomers are depicted in pink.