Chapter 04 Cellular Metabolism
4.1 Introduction
A living cell is the site of enzyme-catalyzed metabolic reactions that maintain the life of the cell and the body.
Enzymes allow metabolic reactions to proceed quickly enough to maintain life.
A constant supply of energy is needed for the body’s metabolism.
Information from DNA is required to construct proteins, including the enzymes necessary for reactions.
Cellular Metabolism is the group of chemical reactions that acquire, store, and release energy in the cells of the body.
Energy is derived from chemical bonds in nutrient molecules obtained in the diet.
4.2 Metabolic Reactions
Metabolic reactions are of two types:
Catabolism: larger molecules are broken down into smaller ones; releases energy (40%) and heat (60%).
Anabolism: larger molecules are synthesized from smaller ones; requires energy released in catabolism.
The reactions of metabolism are often reversible.
Anabolism + Catabolism = Metabolism.
Anabolism and catabolism are typically coupled; the energy released in catabolism fuels anabolism.
Dehydration synthesis is a key mechanism in anabolism (removal of water to join two smaller molecules).
Anabolism provides the biochemicals for growth and repair.
Dehydration synthesis examples:
To synthesize a polysaccharide, many simple sugars (monosaccharides) bind together: many monosaccharides form a polysaccharide with a net loss of water molecules.
To form triglycerides (fats), glycerol + 3 fatty acids bind together.
Two bound amino acids form a dipeptide, while many join to form a polypeptide; the bond between two amino acids in a protein is called a peptide bond.
Nucleotides are joined together to synthesize nucleic acids.
Hydrolysis is the reverse: a molecule of water is split, and its parts are inserted into a larger molecule to break chemical bonds. It is responsible for the digestion of dietary nutrients (carbohydrates, lipids, and proteins).
Example of hydrolysis:
Note: The reactions shown under anabolism are reversible; proceeding to the left represents catabolic reactions.
4.3 Control of Metabolic Reactions
Enzyme Action
Enzymes control the rates of all the metabolic reactions of the cell.
This is necessary because the temperature in cells is often too low for the reactions to run fast enough to support life processes.
Enzymes are complex proteins that function to lower the activation energy of a reaction, so it may begin and proceed more rapidly; enzymes are therefore classified as catalysts.
Enzymes work in small quantities and are not used up in the chemical reactions they catalyze, so they are recycled and reused by the cell.
Enzyme action details:
Each enzyme is specific, acting on only one kind of molecule, called the substrate.
Enzymes recognize their substrates by their complementary shapes; the three-dimensional structure of an enzyme that gives it specificity is called its conformation.
Active sites on the enzyme combine with a special portion of the substrate, creating an enzyme-substrate complex.
The binding of an enzyme and its substrate changes the shape of the substrate, so that the activation energy is lowered, and a reaction occurs.
The speed of enzymatic reactions depends on the number of enzyme and substrate molecules available.
At the end of the reaction, the enzyme is released and can be used again.
A diagrammatic sequence shows substrate molecules → enzyme → enzyme–substrate complex → product molecule.
Enzymes and Metabolic Pathways
Different enzymes control each chemical reaction in the sequence of cellular metabolism.
The enzymes controlling either an anabolic or catabolic sequence must act in a specific order.
A sequence of enzyme-controlled reactions, both for synthesis and breakdown, is called a metabolic pathway.
The rate of a metabolic pathway is determined by a regulatory enzyme responsible for one of its steps, by limiting the number of enzyme molecules available.
A rate-limiting enzyme is a regulatory enzyme that controls the whole metabolic pathway; usually, it is the first enzyme in the series of reactions.
Figure 4.6 illustrates an enzyme-controlled metabolic pathway; the rate-limiting enzyme is typically the bottleneck.
Factors That Alter Enzymes
Some enzymes only become active when they combine with a nonprotein component called a cofactor.
Cofactors may be ions of an element.
Small organic cofactors are called coenzymes; these are often vitamins or derived from a vitamin.
Enzymes (proteins) can be denatured by heat, extreme pH, chemicals, electricity, radiation, some poisons, and other causes that render them nonfunctional.
Enzyme denaturation is irreversible.
4.4 Energy for Metabolic Reactions
Basic Concepts
Energy is the capacity to change something or to do work.
Common types of energy include heat, light, sound, electrical energy, mechanical energy, and chemical energy.
Most metabolic reactions use chemical energy.
Release of Chemical Energy
Chemical energy is held in the chemical bonds of molecules and is released when bonds break.
Release of chemical energy in the cell often occurs through the oxidation of glucose, in a process called cellular respiration.
Cellular respiration reactions release chemical bond energy when nutrient molecules are broken down.
This energy cannot be used directly, so it is stored in a molecule called adenosine triphosphate (ATP).
About 0.40 of this energy is captured, and transferred to high-energy electrons that help to synthesize ATP; the other 0.60 is released as heat, which is used to maintain body temperature.
Adenosine Triphosphate (ATP) Molecules
ATP molecules contain three phosphates in a chain.
ATP is the main energy-carrying molecule in the cell.
A cell uses ATP for many functions, including active transport and the synthesis of various compounds.
The end (terminal) phosphate bond of ATP, and the second phosphate bond, are called high-energy bonds, since they both store a large amount of energy.
Energy is stored while converting adenosine diphosphate (ADP) to ATP; when ATP releases its terminal phosphate to become ADP, stored energy is released.
ADP can be converted back into ATP.
ATP and ADP are interconverted between cellular respiration (which releases energy) and reactions in the cell (which utilize the energy).
Key reactions and relationships:
ATP hydrolysis: \text{ATP} \rightarrow \text{ADP} + \text{P}_{\text{i}}
Phosphate synthesis (recharging): \text{ADP} + \text{P}_{\text{i}} \rightleftharpoons \text{ATP}
The energy stored in ATP drives active transport and biosynthetic reactions, linking cellular respiration to metabolic work.
Figure 4.7 depicts ATP structure and high-energy phosphate bonds.