A healthy 34-year-old coal mine worker is trapped underground following partial collapse of an access shaft. Rescue efforts are directed toward clearing the obstructed tunnel, but it takes 2 days to reach him. While being taken to the surface, the miner tells rescuers that he feels dizzy and weak. He had an emergency supply of water but has not eaten anything for over 30 hours. Fingerstick blood glucose concentration is 78 mg/dL. Which of the following biochemical reactions is most likely responsible for maintaining this patient's current blood glucose levels?

Question

A healthy 34-year-old coal mine worker is trapped underground following partial collapse of an access shaft.  Rescue efforts are directed toward clearing the obstructed tunnel, but it takes 2 days to reach him.  While being taken to the surface, the miner tells rescuers that he feels dizzy and weak.  He had an emergency supply of water but has not eaten anything for over 30 hours.  Fingerstick blood glucose concentration is 78 mg/dL.  Which of the following biochemical reactions is most likely responsible for maintaining this patient's current blood glucose levels?

Quizplus · Accepted Answer

11ec160b_36a0_c5d5_9249_0f0bcb8c57cd_MD0001_00 The 2 major processes that maintain plasma glucose between meals are glycogenolysis and gluconeogenesis. Glycogenolysis is the primary source of glucose for the first 12-18 hours of fasting. Once hepatic glycogen stores become depleted, gluconeogenesis becomes the major process used by the body to keep blood glucose levels within the normal range. During gluconeogenesis, glucose is formed from lactate, glycerol, and glucogenic amino acids. This process uses many of the enzymes involved in glycolysis. However, hexokinase, phosphofructokinase, and pyruvate kinase are unidirectional and must be bypassed by distinct gluconeogenic enzymes. The first committed step of gluconeogenesis is the biotin-dependent carboxylation of pyruvate to oxaloacetate by mitochondrial pyruvate carboxylase. Oxaloacetate is subsequently converted to malate by malate dehydrogenase to facilitate exit from the mitochondria, and then is converted back to oxaloacetate by cytosolic malate dehydrogenase (malate shuttle). In the cytosol, phosphoenolpyruvate carboxykinase (PEPCK) converts oxaloacetate to phosphoenolpyruvate. Therefore, pyruvate carboxylase and PEPCK work together to bypass pyruvate kinase. The 2 other unique gluconeogenic enzymes are fructose 1,6-bisphosphatase (bypasses phosphofructokinase) and glucose-6-phosphatase (bypasses hexokinase). (Choice A) Conversion of acetoacetyl-CoA to 3-hydroxy-3-methylglutaryl-CoA occurs during the synthesis of cholesterol and ketone bodies. Ketone body synthesis is increased in starvation situations, however, ketone bodies cannot be used to synthesize glucose. (Choice B) Palmitic acid is the first fatty acid produced from acetyl CoA during lipogenesis in the fed state. However, during prolonged fasting, lipolysis predominates and leads to generation of glycerol and fatty acids. (Choice C) Conversion of fructose 6-phosphate to fructose 1,6-bisphosphate occurs during glycolysis and is catalyzed by phosphofructokinase. During starvation, glycolysis is minimized and gluconeogenesis predominates. (Choice D) The first step of glycogenolysis is breakage of 1-4 glycosidic linkage to form glucose-1-phosphate. After 24 hours of fasting, maintenance of blood glucose levels is achieved mostly through gluconeogenesis, not by glycogenolysis. Educational objective: After 12-18 hours of fasting, gluconeogenesis becomes the principal source of blood glucose. Gluconeogenesis uses many glycolytic enzymes, but hexokinase, phosphofructokinase, and pyruvate kinase need to be bypassed as they are unidirectional. The initial steps of gluconeogenesis involve the conversion of pyruvate to oxaloacetate and oxaloacetate to phosphoenolpyruvate by pyruvate carboxylase and phosphoenolpyruvate carboxykinase, respectively.

A Healthy 34-Year-Old Coal Mine Worker Is Trapped Underground Following