Enzyme A decreases the Activation Energy for the reaction (as seen in the graph) by bringing the substrates together in a lock and key model in which the substrate molecules have a geometric shape that fits into the active sites of the enzyme (This is seen in the figure). This stabilizes the intermediate state (called transition state), lowering its energy and thus catalyzing the reaction i.e. reducing the time needed to complete the reaction.
Question 2
When conditions around the enzyme change and affect the shape of the enzyme's active sites, it could decrease its enzymatic activity, reducing the reaction rates, because the mechanism for the catalysis by an enzyme relies on the geometrical fit between the substrate and the enzyme, and changing the shape of the active sites would reduce the ability to couple effectively. This is called Enzymaticinhibition.
