The Biochemistry of Energy Transfer

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The carbohydrates, fat, and proteins consumed are oxidized, and the energy that is released in the process is transferred to ATP (A).

The key substance for this energy transfer is acetyl-coenzyme A (acetyl-CoA). Carbohydrates are converted to pyru-vate during glycolysis and then further to acetyl-CoA. The fatty acids resulting from hydrolysis of triglycerides are also broken down into this two-carbon key compound. Amino acids from proteins are either metabolized indirectly through a pyruvate stage or directly into acetyl-CoA. The resulting acetyl-CoA pool can either be used to build amino and fatty acids or enter the citrate cycle where it is oxidized for energy gain. During this process, carbon dioxide (CO2) forms when carbon atoms get oxidized; the coenzyme nicotinamide adenine dinucleotide (NAD+) is reduced to NADH and flavine adenine dinucleotide (FAD) is reduced to FADH2. These are subsequently reoxidized during oxida-tive phosphorylation, and the energy released in the process is stored as ATP energy. Organisms need a sophisticated respiratory apparatus for this purpose alone: oxygen has to be made available in order to oxidize NADH, and the CO2 resulting from oxidation of energy nutrients' carbon atoms needs to be eliminated.

The energy metabolism's major metabolic pathways share mutually interactive control mechanisms without which an efficient and self-regulated interplay of the energy pathways of carbohydrates, lipids, and proteins would be impossible. Energy use functions as an important control value, overall. Many enzymatic pathways of the energy metabolism are inhibited when a cell receives more energy than it needs. The second enzyme of the glycolytic pathway, phosphofructokinase-1, represents such an important regulatory enzyme for an early metabolic step. Its activity is inhibited by the energy-rich end product ATP, as well as by an intermediate, citrate.

A rapid energy transformation process is, therefore, necessitates the removal of the forming ATP through energy use, as well as sufficient supply of substrate and oxygen. Aerobic metabolism prevails when the two latter requirements are met. During physical activity it is unavoidable for the oxygen supply to be occasionally insufficient for the necessary energy transformation. This leads to incomplete performance of the last step, oxidative phosphorylation. Its substrate NADH builds up and in turn inhibits the citrate cycle upstream, leading in turn to a build-up of pyru-vate, which inhibits glycolysis. Thus, the entire energy transformation is halted. The body has one alternative allowing it to extract a small amount of energy-even in this situation—converting pyruvate into lactate. While this is a deadend pathway, it removes pyruvate so that glycolysis can again produce at least a small amount of ATP. This anaerobic metabolism enables sudden, maximal muscle performance without any required preparatory steps.

- A. Transfer of Nutrient Energy to High-Energy Compounds -

Glycogen

Glycogen

Glucose

ATP ADP

Glucose

ATP ADP

Hexokinase Q

Fructose-1,6-bisphosphate

Hexokinase Q

Ç) Phosphofructokinase-1

ADP ATP

Glucose-6-phosphate

O Oxygen O Nitrogen O Carbon O Sulfur

Glucose-6-phosphate

2 ADP

Ç) Phosphofructokinase-1

Phosphoenol-U=YU* pyruvate

2 ADP

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