Question

 

Discuss the physiological events occurring relative to fatty acid metabolism and glucose metabolism when carbohydrate is plentiful and energy levels are high.

Answer 1

 

Fatty acid metabolism consists of catabolic processes that generate energy, and anabolic processes that create biologically important molecules (triglycerides, phospholipids, second messengers, local hormones and ketone bodies.

Fatty acid metabolism is stringently controlled so that synthesis and degradation are highly responsive to physiological needs. Fatty acid synthesis is maximal when carbohydrate and energy are plentiful and when fatty acids are scarce. Acetyl CoA carboxylase plays an essential role in regulating fatty acid synthesis and degradation. Recall that this enzyme catalyzes the committed step in fatty acid synthesis: the production of malonyl CoA (the activated two-carbon donor). The carboxylase is controlled by three global signals—glucagon, epinephrine, and insulin—that correspond to the overall energy status of the organism. Insulin stimulates fatty acid synthesis by activating the carboxylase, whereas glucagon and epinephrine have the reverse effect. The levels of citrate, palmitoyl CoA, and AMP within a cell also exert control. Citrate, a signal that building blocks and energy are abundant, activates the carboxylase. Palmitoyl CoA and AMP, in contrast, lead to the inhibition of the carboxylase.

Glucose metabolism: In aerobic respiration, the main form of cellular respiration used by humans, glucose and oxygen is metabolized to release energy, with carbon dioxide and water as byproducts. Most of the fructose and galactose travel to the liver, where they can be converted to glucose. Glucose is metabolized into pyruvate through glycolysis in the cytoplasm, and pyruvate is completely oxidized to generate ATP through the TCA cycle and oxidative phosphorylation in the mitochondria. In the fed state, glycolytic products are used to synthesize fatty acids through de novo lipogenesis.

Hepatocytes have great flexibility in selecting metabolic fuels (glucose and/or fatty acids). Fuel selection is regulated by both nutrient and hormonal signals. Glycolysis is dominant in the fed state in which glucose is abundant. Glycolytic intermediates and products are used to synthesize lipids, amino acids, and other important molecules, in addition, to be completely oxidized to generate ATP.