Central Pathways of Energy Metabolism

Cells have millions of metabolic reactions going on at any given second. If one looks at metabolic charts showing individual reactions, it resembles a gigantic roadmap. Like a roadmap, there are "interstate highways" that serve as common central thoroughfares through which much of the traffic will pass at any given time. These are the central metabolic pathways. Figure 12.1 presents a brief overview of metabolism and categorizes the pathways into three "stages" based on the size of metabolites in it.

Glycolysis - One of the central metabolic pathways is glycolysis, shown schematically in relation to other pathways in Figure 12.2. It is classified as a stage 2 pathway for degradation of carbohydrates, in either aerobic or anaerobic cells. Another schematic view of the pathway is shown in Figure 12.3. Pyruvate, the product of glycolysis, is handled differently by anaerobic (fermentation) and aerobic pathways. Anaerobic pathways lead to a variety of products, including lactate and ethanol, while aerobic pathways lead to acetyl-CoA and ultimately to carbon dioxide in the citric acid cycle (Figure 12.4).

Citric Acid Cycle - The citric acid cycle is the principal stage 3 pathway. It accepts compounds, derived not only from carbohydrate, but also from lipid or protein, and oxidizes them to CO 2. All catabolic pathways converge at the citric acid cycle.

Electron Transport / Oxidative Phosphorylation - Oxidation in the citric acid cycle generates reduced electron carriers whose reoxidation drives ATP biosynthesis, primarily through processes in the mitochondrial respiratory chain-electron transport and oxidative phosphorylation (Figure 12.4).

Fatty Acid and Amino Acid Oxidation - Acetyl-CoA for the citric acid cycle comes not only from pyruvate oxidation in glycolysis, but also from breakdown of fatty acids by oxidation (Figure 12.5) and from some amino acid oxidation pathways (Figure 12.6).

Fatty Acid and Steroid Biosynthesis - Acetyl-CoA can also be used in anabolic pathways (pathways for synthesizing molecules), providing substrates for synthesis of fatty acids and steroids (Figure 12.5).

Gluconeogenesis - An anabolic pathway for making glucose, gluconeogenesis is an important pathway for making carbohydrates.

Photosynthesis - The process by which green plants capture light energy to drive the generation of chemical energy (ATP) and reducing power (NADPH), both of which are used for carbohydrate synthesis.

See also: Biosynthesis and Degradation, Oxidation as a Metabolic Energy Source, ATP as Free Energy Currency, Glucose, Pyruvate, Acetyl-CoA, ATP, NADPH

INTERNET LINKS:

> 1. Glycolysis/Gluconeogenesis

> 2. Citric Acid Cycle

> 3. Oxidative Phosphorylation

> 4. Fatty Acid Metabolism

> 5. Amino Acid Metabolism

> 6. Sterol Biosynthesis

> 7. Carbon Fixation (photosynthesis)