Antigen-binding site The variable domains can also be referred to as the FV region. It is the subregion of Fab that binds to an antigen. More specifically, each variable domain contains three hypervariable regions – the amino acids seen there vary the most from antibody to antibody. When the protein folds, these regions give rise to three loops of β-strands, localized near one another on the surface of the antibody. These loops are referred to as the complementarity-determining regions (CDRs), since their shape complements that of an antigen. Three CDRs from each of the heavy and light chains together form an antibody-binding site whose shape can be anything from a pocket to which a smaller antigen binds, to a larger surface, to a protrusion that sticks out into a groove in an antigen. Typically though, only a few residues contribute to most of the binding energy.
AntiGen-BInDing sITE tHE VaRiAble domAINS can also be rEferreD To AS The fv regioN. iT IS The SUbregioN oF FaB That binDs TO an aNtiGen. mOre SpEcifICally, each VArIABlE DomaIn CONTaInS threE hypervarIable regIons – THe amino aCIdS sEeN there vAry thE moST from antibody to anTiBody. When THE proTein foLDS, tHESe RegiONs gIvE riSe tO Three loOpS Of Β-straNDs, locAliZed neaR onE anoTHer On thE suRfaCe of the anTiboDy. theSe lOopS Are refErReD To As the comPLemENtarIty-dEtERmiNInG regIons (CDrs), siNCe their ShaPe comPlemeNtS thAt of an antigEn. thRee cdRs frOm EAcH of tHe heavY aNd light cHAinS ToGetHer FOrm aN AntiBody-bINdinG sitE whosE shaPE CAn be anytHIng frOm a pocket to wHicH a smaller AntigeN biNds, TO a largER surfaCe, tO a pRoTrUsion thaT sTicks out into a GrOoVe iN an antigeN. TyPicaLly thOuGh, only a fEw resIdues contrIBute tO mOst Of THE biNdIng enerGy.