An individual displays impaired glucose tolerance; blood glucose levels remain elevated after a meal for a longer time than is normal, although they do eventually go down to fasting levels. The patient has a normal release of insulin from the pancreas in response to elevated blood glucose levels. Hepatocytes obtained from the patient display normal levels of insulin binding to its receptor, and normal activation of the intrinsic tyrosine kinase activity associated with the insulin receptor. Analysis of glucose 6- phosphate formation within the hepatocytes, however, indicates a much slower rate of formation than in hepatocytes obtained from a normalcontrol. A possible mutation that could lead to these results is which one of the following?