<P> More recently large - scale mass spectrometry analyses have been used to determine sites of protein phosphorylation . Over the last 4 years, dozens of studies have been published, each identifying thousands of sites, many of which were previously undescribed . Mass spectrometry is ideally suited for such analyses using HCD or ETD fragmentation, as the addition of phosphorylation results in an increase in the mass of the protein and the phosphorylated residue . Advanced, highly accurate mass spectrometers are needed for these studies, limiting the technology to labs with high - end mass spectrometers . However, the analysis of phosphorylated peptides by mass spectrometry is still not as straightforward as for "regular", unmodified peptides . Recently EThcD has been developed combining electron - transfer and higher - energy collision dissociation . Compared to the usual fragmentation methods, EThcD scheme provides more informative MS / MS spectra for unambiguous phosphosite localization . </P> <P> A detailed characterization of the sites of phosphorylation is very difficult, and the quantitation of protein phosphorylation by mass spectrometry requires isotopic internal standard approaches . A relative quantitation can be obtained with a variety of differential isotope labeling technologies . There are also several quantitative protein phosphorylation methods, including fluorescence immunoassays, Microscale thermophoresis, FRET, TRF, fluorescence polarization, fluorescence - quenching, mobility shift, bead - based detection, and cell - based formats . </P> <P> ATP, the "high - energy" exchange medium in the cell, is synthesized in the mitochondrion by addition of a third phosphate group to ADP in a process referred to as oxidative phosphorylation . ATP is also synthesized by substrate - level phosphorylation during glycolysis . ATP is synthesized at the expense of solar energy by photophosphorylation in the chloroplasts of plant cells . </P>

What adds a phosphate group to adp to form atp