Residues 338-342 at the C-terminal end of loop 6 in the large subunit β/α barrel structure of Rubisoo influence specificity towards CO₂ and O₂. In Anacystis nidulans Rubisco, replacement of alanine 340 by tyrosine or histidine increased the specificity factor by 12-13%, accompanied by a 25- 33% fall in V(c), the rate of carboxylation, while replacement by asparagine increased the specificity factor by 9% and V(c) by 19%. Other mutations did not significantly alter specificity. Alanine 340 does not interact directly with the bisphosphate substrate, thus replacing it with other residues must have indirect effects on the specificity factor and rate of carboxylation.