12/24/2022 0 Comments Cellular respiration an overview![]() ![]() GTP is equivalent to ATP in terms of energy charge. The Krebs cycle summarizes a circular series of reactions in the mitochondria to metabolize AcCoA to two molecules of CO 2 with resultant generation of one molecule of GTP, three molecules of NADH, and one molecule of FADH 2. Two molecules of AcCoA may condense to form acetyl acetate, which can subsequently be metabolized to β-hydroxy butyrate and acetone, all of which may be used as energy substrate by the heart, brain, and skeletal muscle during fasting after a depletion of glycogen stores. As noted above, AcCoA is also generated within the mitochondria by β-oxidation of fatty acids. This key reaction is regulated by a family of pyruvate dehydrogenase kinases. Thiamin, lipoic acid, magnesium, and coenzyme A serve as cofactors for this reaction, which represents the first irreversible step in terms of mitochondrial oxidation of pyruvate. Pyruvate is metabolized in the mitochondria to AcCoA and CO 2 via pyruvate dehydrogenase, a large polyhedral protein complex with molecular weight of 10 × 10 6 Da. ATP and GTP for these series of reactions are provided by β-oxidation of fatty acids. Alternatively, amino acids can undergo gluconeogenesis, a costly process that basically requires four ATP molecules plus two GTP molecules and two NADH molecules to regenerate one molecule of glucose from two molecules of pyruvate (see Chapter 77). ![]() ![]() Accordingly, amino acids can be mobilized for energy production as well as de novo protein synthesis. All amino acids may be catabolized to either AcCoA or some Krebs cycle intermediate. Particularly as an aspect of the metabolic stress response mediated by cortisol, catecholamines, and interleukins 6 and 2, protein degradation can occur with release of amino acids. It should be appreciated that generation of AcCoA by fatty acid β-oxidation occurs independent of pyruvate dehydrogenase that can be rate limiting for complete glucose metabolism. These reactions occur in the mitochondria after fatty acid transport by a carnitine transport system. Pyruvate transamination yields alanine, whereas pyruvate carboxylase generates oxylacetate ( Figure 74-6).Ĭatabolism of fatty acids by β-oxidation generates one molecule of AcCoA and one molecule each of FADH 2 and NADH for each two-carbon fatty acid fragment cycle. Alternatively, pyruvate is shuttled to the mitochondria, where it is further metabolized to carbon dioxide (CO 2) and AcCoA. Under anaerobic conditions, pyruvate may be reduced by NADH to lactate to regenerate NAD +. These initial series of reactions ultimately generate two net molecules of ATP, two molecules of NADH, and two molecules of pyruvate. Ten enzymatic reactions within the cell cytoplasm define the metabolic pathway, termed glycolysis. Glucose is transported into cells via glucose transporter (GLUT) receptors and osmotic gradients (see Chapter 77). Shuttling of electrons generated from reducing equivalents along the mitochondrial electron transport chainĪcetate coupled to coenzyme A (AcCoA) is derived from carbohydrates, lipids, and proteins. Metabolism of acetate to carbon dioxide in the Krebs cycle with generation of additional reducing equivalents 3. Cellular respiration consists of three related series of biochemical reactions: 1.ĭegradative reactions resulting in the formation of acetyl coenzyme A and reducing equivalents 2. As previously indicated, cellular respiration allows controlled release of free energy from carbohydrate, fat, and protein energy substrate. ![]()
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