Alternative Pathways - Biochemistry

A catabolic reaction is defined as a reaction used to break down a large molecule into smaller subunits. Of the following options, glycogenolysis is the only option where a larger molecule (glycogen) is broken down into smaller subunits (individual glucose molecules).

Glycogen is the storage form of glucose, and is more readily accessible than starches or fats. It is used for short-term supply of glucose and in starvation conditions is used up in a matter of hours. It is mainly stored in the liver and skeletal muscle. Glycogenolysis in the liver results in glucose release into the bloodstream, whereas in the muscle the glucose is immediately used up. The highest demand for the glucose is in the muscle, and that is where most of it is stored.

The non reducing end of a glycogen branch is the end from which glucose units are removed during degradation of glycogen.

Oxaloacetate is the four-carbon molecule that is regenerated by the enzyme malate dehydrogenase in order to continue the cycle to form citrate with acetyl-CoA in the first step of the Krebs cycle. The other answer choices are intermediates of the citric acid cycle, but only oxaloacetate is regenerated, making it a true cycle.

Gluconeogenesis occurs in times of starvation, fasting, and low access to sources of glucose. This is the cells way of creating its own precursor when none is available, albeit at a high energetic cost (one cycle of gluconeogenesis can cost 6 ATP). Thus, it is untrue that gluconeogenesis occurs during high consumption of carbohydrates and fatty acids. Each other selection is true regarding gluconeogenesis.

The Cori cycle comes into play during anaerobic conditions; where lactate from glycolysis travels from the muscle to the liver to be converted into glucose via gluconeogenesis. The glucose is then sent back to the muscle to be used for energy. Note that some of the lactate that is converted into glucose can be stored as glycogen in the liver, but glycogen is not transported from the liver to the skeletal muscle.

Gluconeogenesis occurs in times of starvation, fasting, and low access to sources of glucose. This is the cells way of creating its own precursor when none is available, albeit at a high energetic cost (one cycle of gluconeogenesis can cost 6 ATP). Thus, it is untrue that gluconeogenesis occurs during high consumption of carbohydrates and fatty acids. Each other selection is true regarding gluconeogenesis.

The Cori cycle comes into play during anaerobic conditions; where lactate from glycolysis travels from the muscle to the liver to be converted into glucose via gluconeogenesis. The glucose is then sent back to the muscle to be used for energy. Note that some of the lactate that is converted into glucose can be stored as glycogen in the liver, but glycogen is not transported from the liver to the skeletal muscle.

Oxaloacetate is the four-carbon molecule that is regenerated by the enzyme malate dehydrogenase in order to continue the cycle to form citrate with acetyl-CoA in the first step of the Krebs cycle. The other answer choices are intermediates of the citric acid cycle, but only oxaloacetate is regenerated, making it a true cycle.

Fats are broken down into acetyl-CoA to be used for energy. However, large quantities of alcohol will produce large quantities of acetyl-CoA when metabolized. Acetyl-CoA can be converted into fatty acids when it is in excess. Then fatty acids can assemble into triglycerides and be stored as fat.

In order to break down glycogen into individual glucose-6-phosphate units, all of the above enzymes are required. Each plays a specific role in one of the following activities: degradation of glycogen initially, remodeling of the glycogen so that it can be acted upon by the enzymes, and conversion of glucose-1-phosphate to glucose 6-phosphate.

When glycogen phosphorylase reaches a branching point in glycogen, the bonds switch from being alpha-1,4-glycosidic bonds to alpha-1,6-glycosidic bonds. It is unable to cleave these bonds, and so other enzymes (a transferase and a glucosidase) must come into play.

Glycogen is first debranched and broken down from its non-reducing end by glycogen phosphorylase to give the product G1P, which is then converted into G6P by phosphoglutomutase. Glycogen synthase, glycogen branching enzyme, and UDP-glucose pyrophosphorylase are required for glycogen synthesis.

Glycogen phosphorylase, the rate-limiting enzyme of glycogenolysis does not breaks alpha 1,6 glycosidic bonds. It releases glucose from glycogen by hydrolyzing alpha 1,4 glycosidic bonds until it reaches a branch point in the glycogen molecule. At this time, another enzyme, a debranching alpha 1,6 glycosidase hydrolyzes the alpha 1,6 glycosidic bonds. Glycogen phosphorylase is under regulation by many hormones, including insulin and glucagon, as well as epinephrine.

Glycogen is mostly stored in the liver and skeletal muscle. When it is broken down in the liver, the last enzyme, a phosphatase, removes the last phosphate group to release plain glucose into the bloodstream. In the muscle, there is no need to release the glucose, so glycogen is only broken down as far as glucose-6-phosphate. Skeletal muscle cells lack the last phosphatase required to remove the phosphate from carbon 6. This isn't an obstacle, however, because the glucose-6-phosphate is already on to the second stage of glycolysis.

Insulin is released in response to high blood glucose. It causes a signaling cascade that, in addition to other things, stops glycogenolysis. This is done by converting glycogen phosphorylase from it's active "a" form to its inactive "b" configuration. The "R" state is the active state, so the presence of glucose would not trigger the breakdown of glycogen. 5' AMP would not inhibit an inactive form of an enzyme. High AMP would mean a demand for ATP, so it would convert the enzyme to its "a" form.

Oxaloacetate contributes to fatty acid synthesis, but it’s not a lipid, because, among other reasons, it’s not hydrophobic. This is also why it cannot cross mitochondrial membranes. Glucose can be synthesized from glycerol, but this process occurs via dihydroxyacetone phosphate, and doesn’t involve oxaloacetate. Oxaloacetate is synthesized from pyruvate, which is the end product of glycolysis, so oxaloacetate cannot be an intermediary in glycolysis. However, lactate can be converted to pyruvate, which is the principle substrate for gluconeogenesis (sometimes called “reverse glycolysis”). In gluconeogenesis, oxaloacetate is an intermediary in the conversion of pyruvate to phosphoenolpyruvate, and so that makes it an intermediary, too, in the synthesis of glucose from lactate.

Fats are broken down into acetyl-CoA to be used for energy. However, large quantities of alcohol will produce large quantities of acetyl-CoA when metabolized. Acetyl-CoA can be converted into fatty acids when it is in excess. Then fatty acids can assemble into triglycerides and be stored as fat.

Futile cycling occurs when two metabolic processes occur in opposite directions, and thus result in no net change. This is very wasteful, and not ideal. The only example of the answer choices of metabolic processes occurring in opposite directions is glycolysis and gluconeogenesis occurring simultaneously. Other possible examples could include: glycogenesis and glycogenolysis, beta-oxidation and fatty acid synthesis, etc.

Gluconeogenesis occurs in times of starvation, fasting, and low access to sources of glucose. This is the cells way of creating its own precursor when none is available, albeit at a high energetic cost (one cycle of gluconeogenesis can cost 6 ATP). Thus, it is untrue that gluconeogenesis occurs during high consumption of carbohydrates and fatty acids. Each other selection is true regarding gluconeogenesis.