The substrate selectivity of these enzymes is controlled via the flexible orientation of a key glutamine residue’s gamma-amide group in a “glutamine switch” mechanism. The families of these enzymes labeled “1,” “2,” and “3” are “dual-specific,” while families 4 through 9 of them recognize only one substrate. The heart failure medication milrinone inhibits one of these enzymes to decrease (15[1])afterload. In the retina, transducin activates one of these enzymes. These enzymes downregulate the activity of (*) protein kinase A by decreasing the concentration (-5[1])of a second messenger. (10[1])Inhibitors (-5[1])of the “5” (-5[1])isoform (-5[1])of these enzymes cause vasodilation by extending the half-life of cGMP. (10[1])One of these enzymes is inhibited (10[1])by sildenafil to treat erectile dysfunction. (10[1])For 10 points, what enzymes break a bond that forms the “backbone” of (10[1])DNA? ■END■ (10[1]0[3])