29: The Organic Chemistry of Metabolic Pathways
- Page ID
- 448876
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)In this chapter, we’ll look at some of the pathways by which organisms carry out their chemistry, focusing primarily on how they metabolize fats and carbohydrates. The treatment will be far from complete, but it should give you an idea of the kinds of processes that occur.
- 29.0: Why This Chapter?
- Biochemical reactions are not mysterious. Even though the biological reactions that take place in living organisms often appear complicated, they follow the same rules of reactivity as laboratory reactions and they operate by the same mechanisms.
- 29.1: An Overview of Metabolism and Biochemical Energy
- The many reactions that occur in the cells of living organisms are collectively called metabolism. The pathways that break down larger molecules into smaller ones are called catabolism, and the pathways that synthesize larger biomolecules from smaller ones are known as anabolism. Catabolic reaction pathways are usually exergonic and release energy, while anabolic pathways are often endergonic and absorb energy.
- 29.2: Catabolism of Triacylglycerols - The Fate of Glycerol
- The metabolic breakdown of triacylglycerols begins with their hydrolysis in the stomach and small intestine to yield glycerol plus fatty acids. The reaction is catalyzed by a lipase. The active site of the enzyme contains a catalytic triad of aspartic acid, histidine, and serine residues, which act cooperatively to provide the necessary acid and base catalysis for the individual steps.
- 29.3: Catabolism of Triacylglycerols - β-Oxidation
- he fatty acids that result from triacylglycerol hydrolysis are converted into thioesters with coenzyme A and then catabolized by a repetitive four-step sequence of reactions called the β-oxidation pathway. Each passage along the pathway results in the cleavage of an acetyl group from the end of the fatty-acid chain, until the entire molecule is ultimately degraded. As each acetyl group is produced, it enters the citric acid cycle and is further degraded to CO₂.
- 29.4: Biosynthesis of Fatty Acids
- One of the most striking features of the common fatty acids is that they have an even number of carbon atoms. This occurs because all fatty acids are derived biosynthetically from acetyl CoA by sequential addition of two-carbon units to a growing chain. The acetyl CoA, in turn, arises primarily from the metabolic breakdown of carbohydrates in the glycolysis pathway. Thus, dietary carbohydrates consumed in excess of immediate energy needs are turned into fats for storage.
- 29.5: Catabolism of Carbohydrates- Glycolysis
- Glucose is the body’s primary short-term energy source. Its catabolism begins with glycolysis, a series of ten enzyme-catalyzed reactions that break down glucose into 2 equivalents of pyruvate. The steps of glycolysis, also called the Embden–Meyerhoff pathway after its discoverers.
- 29.6: Conversion of Pyruvate to Acetyl CoA
- Pyruvate, produced by catabolism of glucose (and by degradation of several amino acids), can undergo several further transformations depending on the conditions and on the organism.
- 29.7: The Citric Acid Cycle
- The initial stages of catabolism result in the conversion of both fats and carbohydrates into acetyl groups that are bonded through a thioester link to coenzyme A. Acetyl CoA then enters the next stage of catabolism—the citric acid cycle, also called the tricarboxylic acid (TCA) cycle, or Krebs cycle. The overall result of the cycle is the conversion of an acetyl group into two molecules of CO2 plus reduced coenzymes by the eight-step reaction sequence.
- 29.8: Carbohydrate Biosynthesis- Gluconeogenesis
- Glucose is the body’s primary fuel when food is plentiful, but in times of fasting or prolonged exercise, glucose stores can become depleted. Most tissues then begin metabolizing fats as their source of acetyl CoA, but the brain is different. The brain relies almost entirely on glucose for fuel and is dependent on receiving a continuous supply in the blood. When the supply of glucose fails, even for a brief time, irreversible damage can occur. Thus, a pathway for synthesizing glucose from simple
- 29.9: Catabolism of Proteins- Deamination
- The catabolism of proteins is much more complex than that of fats and carbohydrates because each of the 20 α-amino acids is degraded through its own unique pathway. The general idea, however, is that (1) the α amino group is first removed as ammonia by a deamination process, (2) the ammonia is converted into urea, and (3) the remaining amino acid carbon skeleton (usually an α-keto acid) is converted into a compound that enters the citric acid cycle.
- 29.10: Some Conclusions about Biological Chemistry
- This section discusses the importance of understanding metabolic pathways in biological chemistry. It highlights how these pathways are interconnected and regulated, emphasizing their complexity and efficiency. The synthesis and breakdown of biomolecules are crucial for maintaining cellular functions and homeostasis. Additionally, it notes the role of enzymes in catalyzing reactions and the significance of metabolic flexibility in adapting to different environmental conditions.
- 29.11: Chemistry Matters—Statin Drugs
- Coronary heart disease—the buildup of cholesterol-containing plaques on the walls of heart arteries—is the leading cause of death for people older than 20 in industrialized countries. It’s estimated that up to one-third of women and one-half of men will develop the disease at some point in their lives.
- 29.13: Summary
- This section provides a concise overview of metabolic pathways, emphasizing the intricate web of chemical reactions within cells that sustain life. It summarizes how enzymes, cofactors, and metabolites work together to drive energy production, biosynthesis, and regulation processes. The content also reflects on key biochemical concepts and their relevance to broader biological functions.