Aerobic Energy System

Overview of Aerobic Metabolism | Mitochondria | Kreb's Cycle (Citric Acid Cycle)

Electron Transport Chain (Oxidation Phosphorylation) | Lipid Metabolism

Overview of Aerobic Metabolism

Mitochondria

• cellular oxidation of CHO, fats, or proteins takes place entirely in the mitochondria
• fatty acids and glucose are transferred into the mitochondria and formed into acetyl CoA
• the mitochondria are not the bean-shaped organelle depicted in most texts, are rather a long branching reticulum
• mitochondria are distributed within two areas of a muscle: beneath the sarcolemma (subsarcolemma mitochondria) and among the contractile elements (intermyofibrillar mitochondria)
• mitochondria contains two membranes: the outer membrane and the inner membrane which is the location of the oxidative phosphorylation; enzymes of the Kreb's cycle are found in matrix of the mitochondria

Kreb's Cycle (Citric Acid Cycle)

• first step after pyruvate entry is to combine with Coenzyme A and form acetyl Coenzyme A which releases one CO₂ and reduces one NAD⁺
• primary purpose is to completely oxidize the two-carbon acetyl CoA substrate into 2 CO₂ and to reduce NAD⁺ and FADH
• net reaction of the Kreb's cycle:

acetyl CoA + 3 NAD^+ + FAD + ADP + Pi + 2 H2O

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2 CO2 + 3 NADH + FADH2 + ATP + 2 H^+ + CoA

Electron Transport Chain - Oxidative Phosphorylation

In aerobic metabolism, O₂ is the ultimate electron acceptor, however, electrons are not transferred directly but rather transferred by special carriers. The reduced forms of these carriers then transfer their high-potential electrons to O₂ by means of an electron transport chain located in the inner membrane of mitochondria. ATP is formed from ADP and Pi because of energy release from this flow of electrons.

Nicotinamide adenine dinucleotide (NAD⁺) is the major electron acceptor in the oxidation of substrates and is a derivative of niacin, one of the B vitamins. In the oxidation of a substrate, two H⁺s and electrons are removed. NAD⁺ accepts one of the H⁺ and the two electrons while the second H⁺ is carried in the solvent. Thus, the reduced form is properly expressed as NADH + H⁺. The other major electron carrier is flavin adenine dinucleotide (FAD) and is derived from riboflavin (vitamin B2). FAD, however, accepts both of the hydrogen ions and electrons and is reduced to FADH₂.

The purpose of these reducing equivalents is to capture most of the energy from the substrates through the process of reduction (gaining electrons) and, later, giving off the electrons in a series of reactions in which the final acceptor of the electrons is oxygen. During this process, the reducing equivalents become oxidized, and energy released from this process is used to phosphorylate ADP. Thus, the term oxidative phosphorylation is really two separate processes that take place, but are usually linked, or coupled, together.

The general functioning of the ETC begins with the removal of two H⁺ and two electrons from the reducing equivalents (NADH + H⁺ and FADH₂). The electrons are passed down a series of different electron carriers eventually being accepted by oxygen. In these series of oxidation reactions, three pairs of H+ are pumped out of the mitochondrial matrix into the intermembrane space for each NADH + H⁺ entering the ETC while only two pairs of H⁺ are pumped out for each FADH₂. This creates a potential gradient which ultimately supplies the energy to phosphorylate ADP. The H⁺ are returned back to the mitochondrial matrix via a proton pump located in the F0 stalk of the F complex. The F1 component contains a mitochondrial ATP synthase which catalyzes the phosphorylation of ADP.

Mitochondria are impermeable to NADH + H⁺ and NAD⁺. Thus, the NADH + H⁺ reduced during glycolysis are unable to enter the ETC. Rather, there are two different shuttling systems that transfer the electrons from NADH + H⁺ outside the mitochondria to another reducing equivalent inside. In FT fibers, the glycerol phosphate shuttle is the primary shuttle system while the malate-aspartate shuttle predominates in the ST fibers. The glycerol phosphate shuttle transfers electrons to FAD and to NAD⁺ in the malate-aspartate system. Thus, only two ATP are formed for each NAD⁺ reduced during glycolysis in FT fibers while three ATP are formed in ST fibers.

 We can calculate the energy yield derived from the oxidation of glucose, a six-carbon molecule. In FT fibers, 36 ATP are produced while 38 are produced in ST fibers. However, if the carbohydrate was derived from muscle glycogen, add one ATP.

Lipid Metabolism

Lipids are a major source of energy during rest and exercise. Approximately half of the lipids-stored as triglycerides-that are used for energy come from adipose tissue with the other half from intramuscular stores. There are several steps in the mitochondrial oxidation of lipids that begin with the mobilization of the triglycerides.

1. Initially, triglycerides must first be mobilized into free fatty acids (FFA) and glycerol by the hormone-sensitive lipase.
2. FFA from the adipose tissues diffuse into the blood and are transported to the tissues bound to the major blood protein, albumin. As blood flows through skeletal muscle, approximately 50% of the FFA are taken up by the tissues. Thus, the rate of FFA uptake is controlled by blood flow, the concentration of FFA in plasma, and the capacity to transport FFA into the muscle.
3. Next, while in the sarcoplasm, FFA from adipose tissue or intramuscular stores are prepared for transport into the mitochondria.
4. Once inside the mitochondria, the fatty acid undergoes a process called ß-oxidation. Two-carbon molecules are cleaved from the fatty acid chain to form one acetyl CoA with each turn of the cycle. Each turn results in the reduction of one NAD⁺ and one FAD.
5. In the same manner as CHO metabolism, acetyl CoA from ß-oxidation enters the Kreb's cycle.

We can calculate the energy yield derived from the oxidation of a fatty acid. Palmitate, a typical fatty acid, is 16 carbons in length, has a net synthesis of 129 ATP.