= Fusion protein
A <fusion protein> combines sequences from two or more protein components in one <polypeptide chain>. The joining can arise through a <gene fusion> or be engineered for an experiment. Its covalent continuity distinguishes it from a <protein complex> whose separately made chains associate without becoming one backbone.
A <peptide linker> can connect the components while allowing some relative motion. For an original size calculation, components of 200 and 300 <amino acid residues> joined by a 10-residue <peptide linker> produce a510-residue chain. This arithmetic specifies composition and order; it does not establish that both components retain their original activities.
A structural experiment illustrates both the usefulness and the cost of tethering. Researchers connected <adrenodoxin> to the <cholesterol side-chain cleavage enzyme> to capture a normally transient electron-transfer interaction. Crystallizable <fusion proteins> made with different <peptide linkers> showed a similar interaction interface. Their catalytic activities were lower than those of the nonfused system, and <electron transfer> depended on linker length even when substrate affinity was preserved.<Structural basis for pregnenolone biosynthesis by the mitochondrial monooxygenase system by Natallia Strushkevich and others>[Results: CYP11A1–Adx fusion comparison; Methods, Protein Purification and Crystallization; TableS3 and FiguresS5/S8]{ref}
Tethering can therefore help reveal an interaction without reproducing every property of the freely associating partners. Structural capture, substrate recognition and catalytic turnover remain separate measurements.
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