Chapter Capsule

A. Metabolism is the sum of cellular chemical and physical activities; it involves chemical changes to reactants and the release of products using well-established pathways.

B. Metabolism is a complementary process consisting of anabolism, synthetic reactions that convert small molecules into large molecules, and catabolism, in which large molecules are degraded. Together, they generate thousands of intermediate molecular states, called metabolites, which are regulated at many levels.

A. Metabolism is made possible by organic catalysts, or enzymes, that speed up reactions by lowering the energy of activation. Enzymes are not consumed and can be reused. Each enzyme acts specifically upon its assigned metabolite, called the substrate.

B. Enzyme Structure: Depending upon its composition, an enzyme is either conjugated or simple. A conjugated enzyme consists of a protein component called the apoenzyme and one or more activators called cofactors. Some cofactors are organic molecules called coenzymes, and others are inorganic elements, typically metal ions. To function, a conjugated enzyme must be a complete holoenzyme with all its parts. Simple enzymes are composed solely of protein.

C. Enzyme Specificity: Substrate attachment occurs in the special pocket called the active, or catalytic, site. In order to fit, a substrate must conform to the active site of the enzyme. This three-dimensional state is determined by the amino acid content, sequence, and folding of the apoenzyme. Thus, enzymes are usually substrate-specific.

D. Cofactors: Metallic cofactors impart greater reactivity to the enzyme-substrate complex. Coenzymes such as NAD (nicotinamide adenine dinucleotide) are transfer agents that pass functional groups from one substrate to another. Coenzymes usually contain vitamins.

E. Enzyme Classification: Enzyme names consist of a prefix derived from the type of reaction or the substrate and the ending -ase. By convention, enzymes may also be classified according to their location of action. Thus, an exoenzyme is secreted, but an endoenzyme is not. Moreover, a constitutive enzyme is regularly found in a cell, whereas an induced enzyme is synthesized only if its substrate is present.

F.Types of Enzyme Function: Metabolic reactions vary. The release of water that comes with formation of new covalent bonds is a condensation reaction. Hydrolysis reactions involve addition of water to break bonds. Functional groups may be added, removed, or traded in many reactions. Coupled redox reactions transfer electrons and protons (H¹) from one substrate to another. Compounds yielding electrons are another. Compounds yielding electrons are oxidized, whereas those gaining electrons are reduced.

G. Enzyme Sensitivity: Enzymes are labile (unstable) and function only within narrow operating ranges of temperature and pH, and they are especially vulnerable to denaturation. Enzymes are vital metabolic links and thus constitute easy targets for many harmful physical and chemical agents.

A. Regulatory controls can act on enzymes directly or on the process that gives rise to the enzymes.

1. A substance that resembles the normal substrate and can occupy the same active site is said to exert competitive inhibition.

2. In feedback (end product) control, the concentration of the product at the end of a pathway blocks the action of a key enzyme by feedback inhibition. This is found in allosteric enzymes, which have a regulatory site different from the active site. When a product attaches here, the enzyme’s action is blocked.

3. Another mechanism, feedback repression, inhibits at the genetic level by controlling the synthesis of key enzymes. Enzyme induction conserves cell resources by producing enzymes only when the appropriate substrate is present.

A. The Production and Use of Energy:

1. Energy is the capacity of a system to perform work. It is consumed in endergonic reactions and is released in exergonic reactions. The freed energy is associated with electrons that can be temporarily captured and transferred to high-energy molecules.

2. Extracting energy requires a series of electron carriers arrayed in a downhill redox chain between electron donors and electron acceptors. In oxidative phosphorylation, energy is transferred to inorganic phosphate, causing it to form high-energy compounds such as ATP.

B. Principal Pathways in Oxidation of Glucose: Carbohydrates, such as glucose, are energy-rich because they can yield a large number of electrons per molecule. Glucose is dismantled in two stages.

1. Glycolysis is a pathway that degrades glucose to pyruvic acid in the absence of oxygen.

2. Important intermediates in glycolysis are glucose-6-phosphate, fructose-1,6-bisphosphate, glyceraldehyde-3-phosphate, bisphosphoglyceric acid, phosphoglyceric acids, phosphoenolpyruvate, and pyruvic acid.

C. Fate of Pyruvic Acid in TCA and Electron Transport:

1. Pyruvic acid is processed in aerobic respiration via the tricarboxylic acid (TCA) cycle and its associated electron transport chain.

2. Acetyl coenzyme A is a product of pyruvic acid processing. This compound undergoes further oxidation and decarboxylation in the TCA cycle, which generates ATP, CO₂, and H₂O.

3. Important intermediary metabolites are pyruvate, oxaloacetate, citrate, isocitrate, a-ketoglutarate, succinate, fumarate, and malate.

4. The respiratory chain completes energy extraction. Important redox carriers of the electron transport system are NAD, FAD (flavin adenine dinucleotide), coenzyme Q, and cytochromes.

5. The chemiosmotic hypothesis is a conceptual model that explains the origin and maintenance of electropotential gradients across a membrane that leads to ATP synthesis, by ATP synthase.

6. The final electron acceptor in aerobic respiration is oxygen. In anaerobic respiration, sulfate, nitrate, or nitrite serve this function.

D. Fermentation is anaerobic respiration in which both the electron donor and final electron acceptors are organic compounds.

1. Fermentation enables anaerobic and facultative microbes to survive in environments devoid of oxygen. Production of alcohol, vinegar, and certain industrial solvents relies upon fermentation. Fermentation products also play a part in identifying some bacteria.

2. The phosphogluconate pathway is an alternative anaerobic pathway for hexose oxidation that also provides for the synthesis of NADPH and pentoses.

E. Versatility of Glycolysis and TCA Cycle: Many pathways of metabolism are bidirectional, or amphibolic, pathways that can be adapted to serving several functions.

1. Metabolites of these pathways double as building blocks and sources of energy. Intermediates such as pyruvic acid are convertible into amino acids through amination. Amino acids can be deaminated and used as energy sources. Components for purines and pyrimidines are derived from amino acid pathways.

2. Two-carbon acetyl molecules from pyruvate decarboxylation can be used in fatty acid synthesis. Combined with glyceraldehyde-3-phosphate, these fatty acids yield triglyceride, a typical storage fat. Alternately, fats can be broken down and fed into the respiratory pathways by beta oxidation.