MODERN BOTANY - P. RAVEN - 1990

SECTION II. ENERGY AND LIVING CELLS

CHAPTER 5. THE FLOW OF ENERGY

Oxidation-Reduction

Energy interconversions occur As a result of Chemical Reactions in which energy stored in certain chemical bonds is transferred to newly formed chemical bonds. In such reactions, electrons move from one energy level to another. In many reactions, electrons are transferred from one atom or molecule to another. These reactions are known as oxidation-reduction (redox) reactions and are of paramount importance to living systems. The loss of an electron is The process of oxidation, and the atom or molecule that loses the electron is oxidized. Oxygen often participates in oxidation processes because it has a high affinity for electrons and frequently acts as their electron acceptor.

Reduction is the gain of an electron. Oxidation and reduction occur simultaneously; the electron lost by the oxidized atom is picked up by another atom, which is thereby reduced.

Generally speaking, oxidation-reduction reactions may involve single electrons: for example, sodium loses an electron and is oxidized to Na+, while chlorine gains an electron and is reduced to Cl-. However, The oxidation of organic molecules involves the loss of both electrons and protons, whereas their reduction involves the gain of both electrons and protons.

For instance, the oxidation of a glucose molecule results in the loss of both electrons and hydrogen ions, which are taken up by oxygen:

С₆Н₁₂O₆ + 6O₂ —> 6СO₂ + 6Н₂O + energy.

The electrons move to a lower energy level, and energy is released.

Conversely, during Photosynthesis, electrons and hydrogen ions are transferred from Water to carbon dioxide; that is, Carbon dioxide is reduced to form glucose:

6СO₂ + 6Н₂O + energy —> С₆Н₁₂O₆ + 6O₂.

In this case, the electrons move to a higher energy level, and an input of Energy is required for this reaction to take place.

In living systems, oxidation-reduction reactions include photosynthesis (where Energy is stored), as well as Glycolysis and Respiration (where energy is released). As previously mentioned, the Complete oxidation of one mole of glucose yields 686 kcal of Free energy. (Conversely, the reduction of carbon dioxide to form one mole of glucose stores 686 kcal of free energy in the chemical bonds of glucose.) Moreover, if the oxidation of glucose were to release energy rapidly, most of that energy would be dissipated as heat. Naturally, this would not only fail to benefit The Cell, but would also cause a lethal increase in its Temperature. However, living systems possess mechanisms that regulate numerous chemical reactions in such a way that energy is stored in specific chemical bonds and can then be released gradually, as needed.

Enzymes and Living Systems

Thousands of different chemical reactions occur within any living system, many of them taking place simultaneously. The sum of all these reactions is called METABOLISM (from the Greek metabole, meaning "change"). Simply listing all the individual chemical reactions would make it difficult to grasp the overall picture of cellular metabolism. Fortunately, there are several fundamental principles that help navigate the labyrinth of cellular metabolism. First, Enzymes—the catalysts and regulators of metabolic processes in living systems—participate in all chemical reactions within the cell. Second, biochemists group these reactions into discrete stages that form pathways. Each pathway may consist of a dozen or more sequential reactions and serves a specific purpose in the vital activity of a cell or Organism. Furthermore, certain pathways share common reaction "steps," such as those involved in the synthesis of various Amino Acids or nitrogenous bases. Some pathways are similar; for example, fat Hydrolysis and glucose oxidation both proceed with the release of energy (Fig. 5-4).

Class="center">Fig. 5-4. A modern highway and the winding old roads to its right represent alternative routes through the mountains. Similarly, identical chemical reactions can proceed via alternative pathways, leading to significant differences in reaction rates

Many living systems possess unique metabolic pathways. Plant Cells expend energy to build Cellulose walls; such synthetic reactions do not occur in animal cells. Red Blood Cells specialize in the synthesis of Hemoglobin; they are found exclusively in animals and humans. It is hardly surprising that the distinguishing features of cells and organisms are tied not only to their Structure AND Functions, but also to their biochemical processes. Yet it is remarkable how similar metabolism is across a vast array of diverse organisms; the metabolic differences between, say, a human, an oak tree, a fungus, and a jellyfish are quite subtle. Certain pathways are found in virtually all living systems.

The sheer scale of chemical work performed within a cell is illustrated by the fact that the vast majority of the thousands of different molecules found in a cell are also synthesized within it. The sum of the chemical reactions involved in their synthesis is called anabolism. Anabolic reactions typically decrease Entropy and almost always require an input of energy (endergonic reactions).

Cells also constantly break down large molecules; this collective breakdown process is termed Catabolism. Catabolic reactions increase entropy and generally proceed with the release of energy (exergonic reactions). Catabolic reactions serve two main purposes: (1) they release energy for anabolism or other cellular work, and (2) they supply "raw Materials" for anabolic reactions. Consequently, both aspects of metabolism—anabolism and catabolism—are essential for the normal functioning of the cell.

Living systems carry out numerous chemical reactions under remarkably challenging conditions. Most of these reactions take place inside the cell, where thousands of different molecules mingle together. The temperature cannot be high, otherwise many fragile structures essential to life would be destroyed. How is this complex chemical work accomplished? The answer can be given in a single word: enzymes. Without enzymes, biochemical reactions would proceed so slowly (if at all) that life would cease.

Enzymes as Catalysts

Enzymes are catalysts of biological reactions. They differ from other catalysts in being exceptionally selective in their action. Some enzymes catalyze reactions involving only a single set of reactant compounds. In enzyme-catalyzed reactions, the compound with which the enzyme interacts is called the substrate. The selectivity of an enzyme is manifested in its choice of substrate and is known as Specificity. On the other hand, enzymes are similar to other catalysts in that they are not consumed during the reaction and can be used repeatedly.

Enzymes dramatically increase reaction rates. For example, the reaction between carbon dioxide and water

СО₂ + Н₂О ⇄ Н₂СО₃

can occur spontaneously, particularly in oceans. In The Human Body, this reaction is catalyzed by the enzyme Carbonic anhydrase (enzyme names typically end with the suffix "ase"). Carbonic anhydrase is one of the most efficient enzymes known, with each molecule catalyzing The formation of 10⁵ molecules of carbonic acid per second. The catalyzed reaction proceeds 10⁷ times faster than the uncatalyzed one. In animals, it plays a vital role in transporting carbon dioxide from the cells where it is produced into the bloodstream for subsequent transport to the Lungs.

Active Site

Enzymes are complex Globular Proteins consisting of one or more polypeptide chains. These chains are folded in such a way as to form a "groove" or "pocket" where substrate molecules bind and the reaction takes place. This specific region of the enzyme is called the Active Site.

The active site is formed as a result of the folding of the polypeptide chain. The substrate fits the active site much like a key fits a lock (Fig. 5-5). Not only does the active site possess a precise three-dimensional structure, but it also matches the substrate in charge distribution, the arrangement of atomic groups, and hydrophilicity or Hydrophobicity. For instance, if a specific region of the substrate bears a negative charge, the corresponding zone of the active site carries a positive charge, and so on. Thus, the active site not only holds the substrate molecule in place but also orients it properly.

Fig. 5-5. Illustration of the lock-and-key hypothesis of enzyme action. A sucrose molecule is hydrolyzed into one molecule of glucose and one molecule of fructose. The enzyme catalyzing this reaction is strictly specific; its active site configuration complements The surface of the sucrose molecule.

The amino acids that form the active site are not necessarily adjacent to one another within the same polypeptide chain. Indeed, in an enzyme with a quaternary structure, they may even belong to different chains, as shown in Fig. 5-6. Amino acids constitute the active site as a result of the specific folding of polypeptide chains within the enzyme molecule.

Fig. 5-6. Model of an enzyme. This particular enzyme (the digestive enzyme Chymotrypsin) consists of three polypeptide chains. The amino (-NН₂) and carboxyl (-СООН) groups are indicated. Numbers designate the positions of specific amino acids within the chains and the Disulfide Bonds linking them. The three-dimensional structure is formed through a combination of disulfide bonds and interactions between protein chains, as well as between the chains and surrounding molecules

water. This interaction is driven by differences in the positive and negative charges (i.e., polarity) of various amino acids. As a result of the folding and twisting of The polypeptide chains, specific Amino acids are brought together into a unique structure that forms the enzyme's active site. Two amino acids involved in forming the active site are highlighted in brown in the figure

The Induced-Fit Hypothesis of Enzyme Active Site Conformation

Recent studies on Enzyme Structure suggest that substrate binding induces a conformational change in the enzyme, thereby achieving a more precise fit between the active site and the substrate. It is believed that this induced conformation may exert a certain amount of strain on the reacting molecules, thereby accelerating the reaction.