A dimer protein crystal structure of my interest is not having 72 amino acid, but its monomer is having the full amino acid sequence. Is it possible that I can add those missing amino acid in the dimer protein from monomer?
Least-squares-fit two copies of the full-length monomer to the two chains in the dimer. If the additional residues do not clash in the dimer, you get a model of the dimer in this way. Did you check whether the full length protin is indeed monomeric? You might also have a dimer whose symmetry coincides with crystallographic symmetry, therefore showing only a monomer in the asymmetric unit.
@ Annemarie
Protein in its native state only exists as heterodimer (2 chains are there but I want only 1 chain). The crystal structure of this particular chain is missing 72 amino acid at the N-terminal (but still it is in heterodimeric form). There is another structure of the same protein and it is a monomer having full amino acid sequence. I wanted to add these amino acid to the chain present in heterodimeric form.
The same considerations apply - either you can fit your full-length monomer structure against the truncated heterodimer structure without major clashes, or you have to assume that the full-length protein has to undergo a conformational change to form the hetrodimer.
You can add anything to a pdb if you have a good structural superposition program,but you need some additional work (annealing,.MD,energy minimization) to remove bad contacts etc.
There exist several options depending on the software that you have. If you are not familiar with crystallography (i.e. you do not have specific software). You may use you dimer as a template to generate a new dimer from the monomer (you can use the freeware USCF chimera [https://www.cgl.ucsf.edu/chimera/]. However, as other colleagues pointed out, you will have to remove bad contacts.
Another option is to model the full sequence using as template the dimer. Take a look at https://www.swissmodel.expasy.org/interactive (check supported inputs). The server will take care of the minimization.
Good luck
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