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An Introduction to Metabolism

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1 An Introduction to Metabolism
6 An Introduction to Metabolism

2 Do now: (think back to most recent lab)
Write down the reaction for the decomposition of hydrogen peroxide. Identify the reactants and the products. What was the enzyme used to facilitate this reaction? Why was this enzyme necessary? What was the purpose of the guaiacol? What effect do you think pH would have on the enzyme?

3 Overview: The Energy of Life
The living cell is a miniature chemical factory where thousands of reactions occur The cell extracts energy and applies energy to perform work Some organisms even convert energy to light, as in bioluminescence © 2014 Pearson Education, Inc. 3

4 Concept 6.1: An organism’s metabolism transforms matter and energy
Metabolism is the totality of an organism’s chemical reactions Energy is the capacity to cause change Catabolic pathways release energy by breaking down complex molecules into simpler compounds Ex: cellular respiration, hydrolysis Anabolic pathways consume energy to build complex molecules from simpler ones Dehydration synthesis of lipids, carbohydrates, proteins © 2014 Pearson Education, Inc. 4

5 Free-Energy Change (G), Stability, and Equilibrium
A living system’s free energy is energy that can do work when temperature and pressure are uniform, as in a living cell The change in free energy (∆G) during a chemical reaction is the difference between the free energy of the final state and the free energy of the initial state ∆G = Gfinal state – Ginitial state Only processes with a negative ∆G are spontaneous © 2014 Pearson Education, Inc. 5

6 Free Energy and Metabolism
The concept of free energy can be applied to the chemistry of life’s processes © 2014 Pearson Education, Inc. 6

7 An exergonic reaction proceeds with a net release of free energy and is spontaneous; ∆G is negative
© 2014 Pearson Education, Inc. 7

8 An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous; ∆G is positive In biological systems the energy released from exergonic reactions are used to drive endergonic reactions © 2014 Pearson Education, Inc. 8

9 The Activation Energy Barrier
Every chemical reaction between molecules involves bond breaking and bond forming The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA) Activation energy is often supplied in the form of thermal energy that the reactant molecules absorb from their surroundings Why is obtaining thermal energy not an effective strategy for biological reactions? © 2014 Pearson Education, Inc. 9

10 Progress of the reaction
Figure 6.12 A B C D Transition state A B EA Free energy C D Reactants A B Figure 6.12 Energy profile of an exergonic reaction G  0 C D Products Progress of the reaction 10

11 Enzymes A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction An enzyme is biological catalyst. (proteins) What reaction is shown below? Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction © 2014 Pearson Education, Inc. 11

12 How Enzymes Speed Up Reactions
Enzymes catalyze reactions by lowering the EA barrier Enzymes do not affect the change in free energy (∆G); instead, they hasten reactions that would occur eventually Animation: How Enzymes Work © 2014 Pearson Education, Inc. 12

13 Progress of the reaction
Figure 6.13 Reactants Free energy G is unaffected by enzyme Figure 6.13 The effect of an enzyme on activation energy Products Progress of the reaction 13

14 Progress of the reaction
Figure 6.13 Course of reaction without enzyme EA without enzyme EA with enzyme is lower Reactants Free energy Course of reaction with enzyme G is unaffected by enzyme Figure 6.13 The effect of an enzyme on activation energy Products Progress of the reaction 14

15 Label: substrate, enzyme, active site, enzyme substrate complex
Figure 6.14 Induced fit between an enzyme and its substrate Enzyme Enzyme-substrate complex 15

16 Substrate Specificity of Enzymes
The reactant that an enzyme acts on is called the enzyme’s substrate The enzyme binds to its substrate, forming an enzyme-substrate complex The active site is the region on the enzyme where the substrate binds Enzyme specificity results from the complementary fit between the shape of its active site and the substrate shape © 2014 Pearson Education, Inc. 16

17 Enzymes change shape due to chemical interactions with the substrate
What is the difference between the lock and key model vs the induced fit model? This induced fit of the enzyme to the substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction Video: Enzyme Induced Fit © 2014 Pearson Education, Inc. 17

18 In case you were curious….
In an enzymatic reaction, the substrate binds to the active site of the enzyme The active site can lower an EA barrier by Orienting substrates correctly Straining substrate bonds Providing a favorable microenvironment Covalently bonding to the substrate © 2014 Pearson Education, Inc. 18

19 Substrates are 2 Substrates enter 1 held in active site by
Figure 2 Substrates are held in active site by weak interactions. 1 Substrates enter active site. Substrates Enzyme-substrate complex Figure The active site and catalytic cycle of an enzyme (steps 1-2) 19

20 Substrates are 2 Substrates enter 1 held in active site by
Figure 2 Substrates are held in active site by weak interactions. 1 Substrates enter active site. Substrates Enzyme-substrate complex Figure The active site and catalytic cycle of an enzyme (step 3) 3 Substrates are converted to products. 20

21 2 Substrates are held in active site by weak interactions. 1
Figure 2 Substrates are held in active site by weak interactions. 1 Substrates enter active site. Substrates Enzyme-substrate complex Figure The active site and catalytic cycle of an enzyme (step 4) 4 Products are released. 3 Substrates are converted to products. Products 21

22 2 Substrates are held in active site by weak interactions. 1
Figure 2 Substrates are held in active site by weak interactions. 1 Substrates enter active site. Substrates Enzyme-substrate complex Active site is available for new substrates. 5 Figure The active site and catalytic cycle of an enzyme (step 5) Enzyme Products are released. 4 3 Substrates are converted to products. Products 22

23 Effects of Local Conditions on Enzyme Activity
An enzyme’s activity can be affected by General environmental factors, such as temperature and pH Chemicals that specifically influence the enzyme © 2014 Pearson Education, Inc. 23

24 Each enzyme has an optimal temperature in which it can function
Optimal temperature for typical human enzyme (37C) Optimal temperature for enzyme of thermophilic (heat-tolerant) bacteria (77C) Rate of reaction Figure 6.16a Environmental factors affecting enzyme activity (part 1: temperature) 20 40 60 80 100 120 Temperature (C) (a) Optimal temperature for two enzymes 24

25 Each enzyme has an optimal pH in which it can function
Optimal pH for pepsin (stomach enzyme) Optimal pH for trypsin (intestinal enzyme) Rate of reaction Figure 6.16b Environmental factors affecting enzyme activity (part 2: pH) 1 2 3 4 5 6 7 8 9 10 pH (b) Optimal pH for two enzymes 25

26 Activating an enzyme Cofactors Inorganic
Ex: heme in hemoglobin Coenzymes- special cofactors which are organic. include vitamins © 2014 Pearson Education, Inc. 26

27 Inhibiting an enzyme (a) Normal binding (b) Competitive inhibition
(c) Noncompetitive inhibition Substrate Active site Competitive inhibitor Enzyme Figure 6.17 Inhibition of enzyme activity Noncompetitive inhibitor 27

28 Enzyme Inhibitors Competitive inhibitors bind to the active site of an enzyme, competing with the substrate Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective Examples of inhibitors include toxins, poisons, pesticides, and antibiotics © 2014 Pearson Education, Inc. 28

29 Regulation of Enzymes Sometimes a cell needs a reaction to occur, sometimes it needs it to stop. Analogous to when a car needs to break, and when it needs to accelerate. How does it regulate? Allosteric regulation may either inhibit or stimulate an enzyme’s activity Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site © 2014 Pearson Education, Inc. 29

30 Allosteric Activation and Inhibition
Each enzyme has active and inactive forms The binding of an activator stabilizes the active form of the enzyme The binding of an inhibitor stabilizes the inactive form of the enzyme © 2014 Pearson Education, Inc. 30

31 Cooperativity Cooperativity is a form of allosteric regulation that can amplify enzyme activity One substrate molecule primes an enzyme to act on additional substrate molecules more readily Is this positive or negative feedback? © 2014 Pearson Education, Inc. 31

32 Enzyme Feedback– Positive or Negative?
feedback inhibition, the end product of a metabolic pathway shuts down the pathway Advantage?? prevents a cell from wasting chemical resources by synthesizing more product than is needed

33 Specific Localization of Enzymes Within the Cell
Structures within the cell help bring order to metabolic pathways Some enzymes act as structural components of membranes In eukaryotic cells, enzymes for cellular respiration are located in mitochondria © 2014 Pearson Education, Inc. 33

34 some enzymes reside in specific organelles
Figure 6.20 Mitochondria The matrix contains enzymes in solution that are involved in one stage of cellular respiration. some enzymes reside in specific organelles Enzymes for another stage of cellular respiration are embedded in the inner membrane. Figure 6.20 Organelles and structural order in metabolism 1 m 34


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