Protein chemistry

Protein chemistry studies the composition, connectivity, chemical transformations and physical behavior of proteins and peptides, together with methods for isolating and measuring them. Polypeptide sequence, post-translational modification, protein folding and protein denaturation answer different questions about the same material. Chemical modification changes connectivity or chemical groups; changing conformation need not change the encoded sequence.

Measurement itself requires a chain of chemical and physical operations. In a Western blot, separated proteins are transferred from a gel to a receiving sheet before detection. The 1979 study by Harry Towbin, Theophil Staehelin and Julian Gordon used electrophoretic transfer to nitrocellulose and then immunological detection. Transfer of the tested ribosomal proteins from urea-containing gels was quantitative, but the SDS-gel procedure preserved the band pattern without quantitative transfer. A recognizable pattern therefore did not prove complete recovery of every protein.^[ref]

An original measurement model separates abundance from recovery. In a linear detection range, write the observed signal aswhere $Q$ is the amount loaded, $r$ the recovered fraction and $k$ the detection response per recovered amount. If two samples each contain100 units of protein but their recovered fractions are0.8 and0.4, the signals are $80k$ and $40k$. A twofold signal difference occurs without a twofold abundance difference. This simplified model is not a calibration curve for a particular assay.

Western blot controls help test such alternatives, while structural and chemical methods ask other questions. Finding a protein band does not by itself determine its protein tertiary structure or identify every covalent modification. Protein chemistry connects the molecular definition of a sample to the transformations and recovery steps used to observe it.