Glutamic acid, H₂N –CH(CH₂CH₂COOH).COOH has pK_a1, $(\mathrm{\alpha }-\mathrm{COOH})=2.2,{\mathrm{pK}}_{\mathrm{a}2}\left(\mathrm{\alpha }-{\mathrm{NH}}_3\right)=9.8$and ${\mathrm{pK}}_{\mathrm{a}3}\left(\mathrm{R}\text{ group }\mathrm{COOH}\right)=4.3$.The isoelectric point of glutamic acid is