Energy Flow in the Life of a Cell

1. 6 Energy Flow in the Life of a Cell 0

2. Chapter 6 At a Glance • 6.1 What Is Energy? • 6.2 How Is Energy Transformed During Chemical Reactions? • 6.3 How Is Energy Transported Within Cells? • 6.4 How Do Enzymes Promote Biochemical Reactions? • 6.5 How Are Enzymes Regulated?

3. 6.4 How Do Enzymes Promote Biochemical Reactions? • Catalysts reduce the energy required to start a reaction • Enzymes, like all catalysts, lower activation energy • Enzymes are biological catalysts

4. 6.4 How Do Enzymes Promote Biochemical Reactions? • Catalysts reduce energy required to start a reaction (continued) • Catalysts are molecules that speed up the rate of a chemical reaction without themselves being used up or permanently altered • Catalytic converters in cars facilitate the conversion of carbon monoxide (CO) to carbon dioxide (CO2) • 2 CO  O2 2 CO2 heat energy

5. 6.4 How Do Enzymes Promote Biochemical Reactions? • Catalysts reduce energy required to start a reaction (continued) • All catalysts have three important properties 1. They speed up reactions by lowering the activation energy required for the reaction to begin 2. They speed up only exergonic reactions 3. They are not consumed or changed by the reactions they promote

6. Figure 6-10 Catalysts such as enzymes lower activation energy high Activation energy without catalyst Activation energy with catalyst energy content of molecules reactants products low progress of reaction

7. 6.4 How Do Enzymes Promote Biochemical Reactions? • Enzymes are biological catalysts • Enzymes are employed to catalyze (speed up) chemical reactions in cells by lowering the activation energy needed to start the reaction • Enzymes are biological catalysts and regulate all the reactions in living cells

8. 6.4 How Do Enzymes Promote Biochemical Reactions? • Enzymes are biological catalysts (continued) • Enzymes (proteins) have two attributes that set them apart from nonbiological catalysts 1. Enzymes are very specific for the reactions they catalyze 2. Enzyme activity is regulated

9. 6.4 How Do Enzymes Promote Biochemical Reactions? • Enzyme structures allow them to catalyze specific reactions • Each enzyme has a pocket called an active site into which one or more reactant molecules, called substrates, can enter • The amino acid sequence of the enzyme protein and the way the protein chains are folded create in the active site a distinctive shape and distribution of electrical charge • The distinctive shape of the active site is both complementary and specific to the substrate • Active site amino acids bind to the substrate and distort bonds to facilitate a reaction

10. 6.4 How Do Enzymes Promote Biochemical Reactions? • Enzyme structures allow them to catalyze specific reactions (continued) • There are three steps of enzyme catalysis • Both the shape and the charge of the active site allow substrates to enter the enzyme only in specific orientations • Upon binding, the substrates and active site change shape to promote a reaction • When the reaction between the substrates is finished, the product(s) no longer properly fit(s) into the active site and diffuse(s) away • The enzyme reverts back to its original configuration and is ready to accept more substrates

11. Animation: Enzymes and Substrates

12. Figure 6-11 The cycle of enzyme–substrate interactions substrates active site of enzyme enzyme product Substrates enter the active site in a specific orientation The substrates and active site change shape, promoting a reaction between the substrates The substrates, bonded together, leave the enzyme; the enzyme is ready for a new set of substrates

13. 6.4 How Do Enzymes Promote Biochemical Reactions? • Enzymes, like all catalysts, lower activation energy • The breakdown or synthesis of a molecule within a cell usually occurs in many small steps, each catalyzed by a different enzyme • Each of the enzymes lowers the activation energy for its particular reaction, allowing the reaction to occur readily at body temperature

14. 6.5 How Are Enzymes Regulated? • The sum of all the chemical reactions inside a cell is its metabolism • Many cellular reactions are linked through metabolic pathways • In metabolic pathways, an initial reactant molecule is modified by an enzyme, creating a slightly different intermediate molecule, which is modified by another enzyme, and so on, until a final product is produced

15. Figure 6-12 Simplified metabolic pathways End products Initial reactant Intermediates PATHWAY 1 enzyme 2 enzyme 3 enzyme 4 enzyme 1 PATHWAY 2 enzyme 5 enzyme 6

16. 6.5 How Are Enzymes Regulated? • Cells regulate metabolic pathways by controlling enzyme synthesis and activity • For a given amount of enzyme, as substrate levels increase, the reaction rate will increase until the active sites of all the enzyme molecules are being continuously occupied by new substrate molecules • Metabolic pathways are controlled in several ways • Control of enzyme synthesis, which regulates availability • Control of enzyme activity

17. 6.5 How Are Enzymes Regulated? • Cells regulate metabolic pathways by controlling enzyme synthesis and activity (continued) • Genes that code for enzymes may be turned on or off • Genes that code for specific proteins are turned on and off according to the cell’s changing need • An increase in substrate can trigger increased enzyme production, leading to decreased substrate levels

18. 6.5 How Are Enzymes Regulated? • Cells regulate metabolic pathways by controlling enzyme synthesis and activity (continued) • Some enzymes are regulated by being synthesized only during specific stages in the life of an organism • This regulation can be altered by mutation

19. 6.5 How Are Enzymes Regulated? • Cells regulate metabolic pathways by controlling enzyme synthesis and activity (continued) • Some enzymes are synthesized in inactive form • For example, the protein-digesting enzymes pepsin and trypsin are inactive when synthesized, but become activated in the stomach under acidic conditions (pepsin) or in the small intestine under alkaline conditions (trypsin)

20. 6.5 How Are Enzymes Regulated? • Cells regulate metabolic pathways by controlling enzyme synthesis and activity (continued) • Enzyme activity may be controlled by competitive or noncompetitive inhibition • In competitive inhibition, a substance that is not the enzyme’s normal substrate binds to the active site of the enzyme, competing with the substrate for the active site • In noncompetitive inhibition, a molecule binds to a site on the enzyme distinct from the active site

21. Figure 6-13a A substrate binding to an enzyme substrate active site enzyme noncompetitive inhibitor site A substrate binding to an enzyme

22. Figure 6-13b Competitive inhibition A competitive inhibitor molecule occupies the active site and blocks entry of the substrate Competitive inhibition

23. Figure 6-13c Noncompetitive inhibition The active site changes shape so the substrate no longer fits when a noncompetitive inhibitor molecule binds the enzyme noncompetitive inhibitor molecule Noncompetitive inhibition

24. 6.5 How Are Enzymes Regulated? • Cells regulate metabolic pathways by controlling enzyme synthesis and activity (continued) • Some enzymes are controlled by allosteric regulation • Allosteric enzymes are enzymes that participate in metabolic pathways. • Small regulator molecules can bind to enzymes and enhance or inhibit activity by allosteric regulation, which can either activate or inhibit the enzyme

25. 6.5 How Are Enzymes Regulated? • Cells regulate metabolic pathways by controlling enzyme synthesis and activity (continued) • Some enzymes are controlled by allosteric regulation • Enzymes that undergo allosteric regulation have a special regulatory binding site on the enzyme that is distinct from the enzyme’s active site and similar to a noncompetitive inhibitor site • Allosteric regulation can either increase or decrease enzyme activity, whereas noncompetitive inhibition only reduces activity

26. 6.5 How Are Enzymes Regulated? • Cells regulate metabolic pathways by controlling enzyme synthesis and activity (continued) • Feedback inhibition is a negative feedback type of allosteric inhibition that causes a metabolic pathway to stop producing its product when quantities reach an optimum level • An enzyme near the beginning of a metabolic pathway is allosterically inhibited by the end product of the pathway

27. Figure 6-14 Allosteric regulation of an enzyme by feedback inhibition intermediates enzyme 5 enzyme 3 enzyme 1 enzyme 4 enzyme 2 As levels of isoleucine rise, isoleucine binds to the regulatory site on enzyme 1, inhibiting it threonine (initial reactant) enzyme 1 isoleucine (end product) isoleucine

28. 6.5 How Are Enzymes Regulated? • Poisons, drugs, and environmental conditions influence enzyme activity • Drugs and poisons often inhibit enzymes by competing with the natural substrate for the active site • This process occurs either by competitive or by noncompetitive inhibition • Environmental conditions can denature enzymes, distorting the three-dimensional structure crucial for their function

29. 6.5 How Are Enzymes Regulated? • Poisons, drugs, and environmental conditions influence enzyme activity (continued) • Some poisons and drugs are competitive or noncompetitive inhibitors of enzymes • Competitive inhibitors of enzymes, including some nerve gases and insecticides, permanently block the active site of acetylcholinesterase • Arsenic, mercury, and lead bind permanently to the nonactive sites of various enzymes, inactivating them

30. 6.5 How Are Enzymes Regulated? • Poisons, drugs, and environmental conditions influence enzyme activity (continued) • The activity of an enzyme is influenced by the environment • The three-dimensional structure of an enzyme is sensitive to pH, salts, temperature, and the presence of coenzymes

31. 6.5 How Are Enzymes Regulated? • The activity of an enzyme is influenced by the environment (continued) • Enzyme structure is distorted (denatured) and function is destroyed when pH is too high or low • Salts in an enzyme’s environment can also destroy function by altering structure • Salt ions can bind with key amino acids in enzymes, influencing three-dimensional structure and destroying function

32. 6.5 How Are Enzymes Regulated? • The activity of an enzyme is influenced by the environment (continued) • Temperature also affects enzyme activity • Low temperatures slow down molecular movement • High temperatures cause enzyme shape to be altered, destroying function • Most enzymes function optimally only within a very narrow range of these conditions

33. Figure 6-15a Effect of pH on enzyme activity For trypsin, maximum activity occurs at about pH 8 For pepsin, maximum activity occurs at about pH 2 fast For most cellular enzymes, maximum activity occurs at about pH 7.4 rate of reaction slow 7 0 1 10 2 3 4 5 6 8 9 pH Effect of pH on enzyme activity

34. Figure 6-15b Effect of temperature on enzyme activity fast For most human enzymes, maximum activity occurs at about 98.6F (37C) rate of reaction slow 104 32 68 140 (F) 0 20 40 60 (C) temperature Effect of temperature on enzyme activity