Expansion of the genetic code with unnatural amino acids (Uaas) has significantly increased the chemical space available to proteins for exploitation. Due to the inherent limitation of translational machinery and the required compatibility with biological settings, function groups introduced via Uaas to date are restricted to chemically inert, bioorthogonal, or latent bioreactive groups. To break this barrier, here we report a new strategy enabling the specific incorporation of biochemically reactive amino acids into proteins. A latent bioreactive amino acid is genetically encoded at a position proximal to the target natural amino acid; they react via proximity-enabled reactivity, selectively converting the latter into a reactive residue in situ. Using this Genetically Encoded Chemical COnversion (GECCO) strategy and harnessing the sulfur-fluoride exchange (SuFEx) reaction between fluorosulfate-l-tyrosine and serine or threonine, we site-specifically generated the reactive dehydroalanine and dehydrobutyrine into proteins. GECCO works both inter- and intramolecularly, and is compatible with various proteins. We further labeled the resultant dehydroalanine-containing protein with thiol-saccharide to generate glycoprotein mimetics. GECCO represents a new solution for selectively introducing biochemically reactive amino acids into proteins and is expected to open new avenues for exploiting chemistry in live systems for biological research and engineering.