The Krebs cycle plays a more important role than simply degrading pyruvate produced during glucose catabolism. Fragments from other organic compounds formed from fat and protein breakdown provide energy during the Krebs cycle.
The “metabolic mill” (Figure 12) depicts the Krebs cycle as the essential "connector" between energy from food macronutrients energy and chemical energy of ATP. The Krebs cycle also serves as a metabolic hub to provide intermediates to synthesize bionutrients for maintenance and growth. For example, excess carbohydrates provide the glycerol and acetyl fragments to synthesize triglyceride. Acetyl–CoA also functions as the starting point for synthesizing cholesterol and many hormones. In contrast, fatty acids do not contribute to glucose synthesis because pyruvate's conversion to acetyl-CoA does not reverse (notice the one-way arrow in Figure 12). Many of the carbon compounds generated in Krebs cycle reactions also provide the organic starting points for synthesizing nonessential amino acids. Amino acids, particularly alanine with carbon skeletons resembling Krebs cycle intermediates after deamination becomes synthesized to glucose.
Fats Burn in a Carbohydrate Flame
Interestingly, fatty acid breakdown depends in part on a continual background level of carbohydrate breakdown. Recall that acetyl–CoA enters the Krebs cycle by combining with oxaloacetate to form citrate. Depleting carbohydrate decreases pyruvate production during glycolysis. Diminished pyruvate further reduces Krebs cycle intermediates, slowing Krebs cycle activity. Fatty acid degradation in the Krebs cycle depends on sufficient oxaloacetate availability to combine with the acetyl-CoA formed during b-oxidation. When carbohydrate level decreases, the oxaloacetate level may become inadequate. In this sense, “fats burn in a carbohydrate flame.”
For Your Information
Excess Protein Accumulates Fat
Athletes and others who believe that taking protein supplements add to muscle beware. Extra protein consumed above what the body requires ends up as body fat. If an athlete desires to become fat, excessive protein intake achieves this end. A protein excess does not contribute to the synthesis of muscle tissue.
Excess energy intake from any fuel source can be counterproductive. Too much of any macronutrient results in accumulation of body fat. Surplus dietary carbohydrate first fills the glycogen reserves. Once these reserves fill, excess carbohydrate converts to triglycerides for storage in adipose tissue. Excess dietary calories as fat move easily into the body’s fat deposits as does any protein excess. Excess amino acids readily convert to fat.
Lecture #5 Workbook
Define Key Terms and Concepts Adenosine triphosphatase
In sprint running, cycling, and swimming, energy output can increase 120 times above resting metabolism. In contrast, during less intense but sustained marathon running, for example, energy requirements still exceed the resting level by 20 to 30 times. This chapter explains how the body’s diverse energy systems interact during rest and different exercise intensities.