Radiographic studies of a 2 year old child brought to an emergency room reveal a new fracture of the humerus and evidence of multiple old fractures in ribs and long bones of the extremities. A social worker wants to initiate prosecution of the parents for child abuse, but an ale emergency room physician notices that, despite the broken arm, the toddler shows minimal bruising. A very careful, directed, physical examination reveals that the toddler has "peculiar teeth," a blue tinge to the sclera, and unusually mobile joints. The disease that the physician suspects the child has is characterized by an abnormality of which of the following biochemical functions?