<p>The D2 protein of Photosystem II has five transmembrane helices (A-E). An extended loop, connecting helices D and E, contributes to the binding environments of the primary quinone electron acceptor Q_A, and that of the bicarbonate bound to the non-heme iron between Q_A and the secondary quinone electron acceptor Q_B. The residues from Ala240 to Thr243 are within a conserved sequence (²⁴⁰AEETYSMVTAN²⁵⁰) that contributes to the stabilization of both Q_A and bicarbonate. We have created the A240D, E241A, E242A, E242D and T243A mutants to study the role of these residues. The mutations in the A240D and E241A strains had little impact on PS II performance, except addition of formate altered chlorophyll <i>a</i> fluorescence decay in the E241A mutant following a single-turnover actinic flash. Measurements of variable chlorophyll <i>a</i> fluorescence and thermoluminescence showed the E242A and E242D mutants had impaired acceptor side electron transport consistent with a reduced redox gap between Q_A and Q_B. In addition, the T243A mutant exhibited a heightened susceptibility to photodamage. These data show that mutations introduced between ²⁴¹Glu–Thr²⁴³ of D2 impair PS II activity but are less detrimental than mutations between the corresponding ²⁴³Glu–Thr²⁴⁵ residues of D1 in the vicinity of the Q_B-binding site.</p>