https://phys.org/news/2016-10-d-newly-protein-complex-mammalian.html

https://phys.org/news/2016-10-d-newly-protein-complex-mammalian.html

I believe it was myoglobin and hemoglobin solved by the john kendrew and max perutz , and for membrane protein it was rhodopseudomonas viridis

If by protein complex you mean a protein consisting of more than one polypeptide chain (i.e. having quaternary structure), then the first one whose structure was solved is hemoglobin, which consists of 4 polypeptide chains (alpha2beta2).

I don't know what protein was the first to be understood to consist of multiple polypeptide chains. Hemoglobin, insulin, and immunoglobulin are some obvious possibilities. Techniques available to early researchers that could have revealed this feature were analytical ultracentrifugation and electrophoresis in free solution (prior to the introduction of gel electrophoresis). Genetics studies may also have suggested the possibility.

The existence of 2 chains in insulin was demonstrated by Sanger in 1949 based on N-terminal sequencing. You can read the paper here:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1274818/?page=1

I suspect this may be the earliest instance.

Svedberg had a hypothesis in the 1930s that large proteins were made up of multiple identical subunits of particular masses, but I don't know whether he observed separation of the actual subunits of any of the large proteins whose molecular weights he investigated with his ultracentrifuge, such as phycoerythrin and hemocyanin.

Structural investigations into tobacco mosaic virus revealed its subunit nature in 1956.

Dissociation of the chains of gamma-globulin was observed in 1959 by Edelmann

Article Studies on structural units of the -†-globulins

but dissociation of macroglobulin was observed a year earlier.

I recommend the answer of Adam B Shapiro, as i too believe it is hemoglobin.

Wish u good luck.