Oxidation of fuel molecules

Oxidation of acetyl CoA The tricarboxylic acid (TCA) cycle (see p. 107) is the final common pathway in the oxidation of fuel molecules such as acetyl CoA. Large amounts of ATP are gener ated as electrons flow from NADH and FADH2 to oxygen via oxidative phosphorylation (see p. 77). 

The Krebs-citric acid cycle is the final common pathway for the oxidation of fuel molecules amino acids, fatty acids and carbohydrates. Most fuel molecules enter the cycle as a breakdown product, acetyl coenzyme A (acetyl CoA), which reacts with oxaloacetate (a four-carbon compound) to produce citrate (a six-carbon compound), which is then converted in a series of enzyme-catalysed steps back to oxaloacetate. In the process, two molecules of carbon dioxide and four energy-rich molecules are given off, and these latter are the precursors of the energy-rich molecule ATP, which is subsequently formed and which acts as the fuel source for all aerobic organisms. 

Activated carriers of electrons for fuel oxidation. In aerobic organisms, the ultimate electron acceptor in the oxidation of fuel molecules is O2. However, electrons are not transferred directly to O2. Instead, fuel molecules transfer electrons to special carriers, which are either pyridine nucleotides or flavins. The reduced forms of these carriers then transfer their highpotential electrons to O2. 

Nicotinamide adenine dinucleotide is a major electron carrier in the oxidation of fuel molecules (Figure 14.13). The reactive part of NAD+ is its nicotinamide ring, a pyridine derivative synthesized from the vitamin niacin. In the oxidation... 

The other major electron carrier in the oxidation of fuel molecules is the coenzyme flavin adenine dinucleotide (Figure 14.14). The abbreviations for the oxidized and reduced forms of this carrier are FAD and FADH2, respectively. FAD is the electron acceptor in reactions of the type... 

The citric acid cycle is the final common pathway for the aerobic oxidation of fuel molecules. Moreover, as we will see shortly (Section 17.3) and repeatedly elsewhere in our study of biochemistry, the cycle is an important source of building blocks for a host of important biomolecules. As befits its role as the metabolic hub of the cell, entry into the cycle and the rate of the cycle itself are controlled at several stages. 

The citric acid cycle is the final common pathway for the oxidation of fuel molecules. It also serves as a source of building blocks for biosyntheses. Most fuel molecules enter the cycle as acetyl CoA. The link between glycolysis and the citric acid cycle is the oxidative decarboxylation of pyruvate to form acetyl CoA. In eukaryotes, this reaction and those of the cycle take place inside mitochondria, in contrast with glycolysis, which takes place in the cytosol. 

In the third stage, ATP is produced from the complete oxidation of the acetyl unit of acetyl CoA. The third stage consists of the citric acid cycle and oxidative phosphorylation, which are the final common pathways in the oxidation of fuel molecules. 

Nicotinamide adenine dinudeotide is a major electron carrier in the oxidation of fuel molecules (Figure 15.13). The reactive part of NAD is its nicotinamide ring, a pyridine derivative synthesized from the vitamin niacin. In the oxidation of a substrate, the nicotinamide ring of NAD a hydrogen ion and two electrons, which are equivalent to a hydride ion (H ). The reduced form of this carrier is called NADH, In the oxidized form, the nitrogen atom carries a positive charge, as indicated by NAD. the electron acceptor in many reactions of the type... 

The citric acid cycle is the final common pathway for the oxidation of fuel molecules. It also serves as a source of building blocks for biosyntheses. 

The electron transport system passes electrons harvested during oxidation of fuel molecules to molecular oxygen. At three sites protons are pumped from the mitochondrial matrix into the intermembrane compartment. Thus, the electron transport system builds the high-energy H+ reservoir that provides energy for ATP synthesis. 

In the third stage, ATP is produced from the complete oxidation of the acetyl unit of acetyl CoA. The third stage consists of the citric acid cycle and oxidative phosphorylation, which are the final common pathways in the oxidation of fuel molecules. Acetyl CoA brings acetyl units into the citric acid cycle [also called the tricarboxylic acid (TCA) cycle or Krebs cycle], where they are completely oxidized to CO2. Four pairs of electrons are transferred (three to NAD and one to FAD) for each acetyl group that is oxidized. Then, a proton gradient is generated as electrons flow from the reduced forms of these carriers to O2, and this gradient is used to synthesize ATP. 

Chemotrophs derive free energy from the oxidation of fuel molecules, such as glucose and fatty acids. Which compound, glucose or a saturated fatty acid containing 18 carbons, would yield more free energy per carbon atom when subjected to oxidation in the cell See Figure 14.10 on page 381 for a comparison. 

Pyridine nucleotides, such as NADH, serve as acceptors and donors of electrons in many metabolic reactions, including those that generate energy for the cell. Because the absolute number of pyridine nucleotides in the cell is low, the cycle of oxidation and reduction for these compounds must occur rapidly for the oxidation of fuel molecules to proceed at a sufficient rate. 

Interconnection between Energy Expenditure and Oxidation of Fuel Molecules... 

The coordination of the oxidation of fuel molecules and the synthesis of ATP can be understood by drawing on the following concepts ... 

The electron carriers in the electron transport chain are limited in number. The concentration of ADP in the cell is low relative to ATP (Sec. 10.3), so once ADP is phosphorylated, no more ATP can be synthesized. Similarly, when NAD+ is reduced to NADH, it is no longer available for further oxidation of fuel molecules. The individual components of the electron transport chain must also continually pass on then-electrons to the next carrier otherwise, they remain fully reduced and are imable to accept electrons from the preceding electron carrier. Even compounds that participate in the metabolic pathways that generate NADH are in limited supply (recall moiety conservation in Sec. 10.3), and they too can become limiting unless they are regenerated. 

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