7. **Specificity of membrane transporters.** A protein that transports amino acids across the cell membrane was found to bind only a few amino acids efficiently. To find the specificity, many different amino acids and substrate analogs were used as *competitive inhibitors* in transport studies at pH 5.9 with L-histidine (\(K_m = 10 \ \mu M\)). The \(K_i\) values calculated from Lineweaver-Burk plots are shown in the table below. Comparing the structures of L-histidine and the competitive inhibitors (for which most of them, you should know their structures!), **what can you conclude about the characteristics of molecules that this transport protein binds at its active site?**| Amino acid or analog | \(K_i \ (10^{-6} \ M)\) | Structure ||----------------------|-----------------|-----------|| L-Lys | 2 | || L-Arg | 3 | || Gly | 285 | || L-Asp | 450 | || D-His | 340 | || Histamine | 390 | ![Histamine Structure](data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAADIAAAAvCAYAAAB8LgI1AAAACXBIWXMAAA7DAAAOwwHHb6hkAAABJUlEQVRoge2ZQQ3DIAyGOSQszcrKzwME06el3zBYhLBZ6MG2bh1vZbr8HimmY0E/gABzE8E3ri7WrbrZWJ4cjuyBomuY4wUHegptsYMT68KCtHU0HJg0c6MHUE4CWj5hd7GuZgT0gmC2AfyTfXczjy0ArG5Qcl+b0Q9hliHQhe1B9VK3hHQphjDn7WnLF5gEYs/6EUWWfdBy+X5rxBkm7X8phQULBUHC3V57bl6twKYhsfA/4ql75C10TSK4cDaxnTlnJkwwpOWHBgLpyQ+FVlILHKzjOgGJ2nj+0

In competitive inhibition, the inhibitor competes with the substrate for binding at the active site to form an enzyme-inhibitor complex instead of an enzyme-substrate complex. Unlike ES, the EI does not dissociates into free enzyme and product. The structure of the inhibitor is analogous to the substrate and this is why it can bind to the same site as the substrate. The vmax of the enzyme in presence of the competitive inhibitor is unaffected but Km for the substrate is increase, meaning more substrate is required to reach half saturation of the enzyme.The Michaels Menton equation in presence of a competitive inhibition becomes: vo = vmax•[S] / a•Km + [S] Where, vo is the rate of the reaction, vmax is the maximum velocity the enzyme can reach, [S] is the substrate concentration, Km is the Michaelis-Menton constant which denotes enzyme affinity and a is : a = 1 + [i]/Ki Where [i] is inhibitor concentration and Ki in simple terms is a measure of how potent a inhibitor is. Generally a Ki of less than 100 μM is considered an effective or a potent inhibitor while a Ki value of more than 100 μM is considered non potent or ineffective.From the table provided we can see that L-Lysine and L-Arginine are potent inhibitors of the protein transporter. Since other amino acids and the structural analogues have a high Ki, they are non potent inhibitors of the transporter. So it can deduced that the transporter is specific for positively charged amino acids: His, Lys and Arg.