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Metabolic Pathways and Enzymes Cellular reactions are usually part of a metabolic pathway, a series of linked reactions Illustrated as follows: E 1 E 2.

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Presentation on theme: "Metabolic Pathways and Enzymes Cellular reactions are usually part of a metabolic pathway, a series of linked reactions Illustrated as follows: E 1 E 2."— Presentation transcript:

1 Metabolic Pathways and Enzymes Cellular reactions are usually part of a metabolic pathway, a series of linked reactions Illustrated as follows: E 1 E 2 E 3 E 4 E 5 E 6 A → B → C → D → E → F → G Letters A-F are reactants or substrates, B-G are the products in the various reactions, and E 1 -E 6 are enzymes.

2 Enzymes – speed up the rate of chemical reactions Substrates – molecules which react with enzymes Only one small part of an enzyme, called the active site, reacts with the substrate(s). Active site may undergo a slight change in shape in order to fit with the substrate The enzyme is not changed by the reaction (active site returns to its original state), and it is free to act again. E1 E2 E3 E4 E5 E6 A → B → C → D → E → F → G

3 Induced Fit Model Because the enzyme must undergo a slight change in shape to fit with the substrate, this is known as the induced fit model.

4 Energy of activation (Ea) - the energy that must be added to cause molecules to react with one another Enzyme lowers the amount of energy required for reaction to occur Enzymes allow reactions to take place at lower temperatures – otherwise, reactions would not be able to occur at normal body temperatures Activation Energy

5 Energy of activation (Ea) When no enzyme is present – more energy required When an enzyme is added – less energy required

6 Enzymatic Reaction Substrate is broken down into smaller products Substrates are combined into a larger product

7 Every reaction in a cell requires a specific enzyme. Enzymes are named for their substrates: SubstrateEnzyme LipidLipase UreasUrease MaltoseMaltase Ribonucleic acidRibonuclease activity.html Enzyme Names

8 Factors Affecting Enzymatic Speed Temperature and pH Substrate concentration Enzyme concentration

9 Temperature and pH: As the temperature rises, enzyme activity increases. If the temperature is too high, enzyme activity declines rapidly because the enzyme is denatured. When enzyme is denatured, its shape changes and it can no longer attach to the substrate. Each enzyme has an ideal temperature and pH at which the rate of reaction is highest. Change in pH can alter the structure of the enzyme, and can eventually cause enzyme to denature.

10 Rate of an enzymatic reaction as a function of temperature and pH

11 Rates and concentration: Reaction rate depends on the number of enzyme-substrate complexes that can be formed. When all available enzymes and active sites are filled, the rate of activity cannot increase further.

12 Substrate concentration Enzyme activity increases as substrate concentration increases because there are more collisions between substrate molecules and the enzyme. Enzyme concentration Enzyme activity increases as enzyme concentration increases because there are more collisions between substrate molecules and the enzyme.

13 Overview of Cellular Respiration Makes ATP (potential energy) from glucose (chemical energy) Releases energy in 4 reactions Glycolysis, Transition reaction, Citric acid cycle (Kreb’s cycle), and Electron transport system An aerobic process that requires O 2 If oxygen is not available (anaerobic), glycolysis is followed by fermentation Coupled Reaction

14 The four phases of complete glucose breakdown

15 Where does each step occur? Outside the mitochondria Step 1 - Glycolysis Inside the mitochondria Step 2 - Transition reaction (matrix) Step 3 – Citric acid cycle (matrix) Step 4 – Electron transport system (cristae)

16 Structure of mitochondria: Has a double membrane, with an intermembrane space between the two layers. Cristae are folds of inner membrane The matrix, the innermost compartment, which is filled with a gel-like fluid.

17 It is an oxidation-reduction reaction, or redox reaction for short. Oxidation is the loss of electrons; hydrogen atoms are removed from glucose. Reduction is the gain of electrons; oxygen atoms gain electrons. Remember OIL RIG (oxidation is loss, reduction is gain) Reaction that Occurs in Cellular Respiration

18 Enzymes involved: NAD + Nicotinamide adenine dinucleotide Accepts H + to become NADH FAD Flavin adenine dinucleotide (sometimes used instead of NAD +) Accepts 2H + to become FADH 2

19 The NAD + cycle

20 Step 1. Glycolysis Occurs in the cytoplasm (outside the mitochondria) Glucose  2 pyruvate molecules. Universally found in all organisms Does not require oxygen (anaerobic). Main energy source for prokaryotes colysis.html

21 Glycolysis Summary Inputs: Glucose 2 NAD+ 2 ATP 4 ADP + 2 P Outputs: 2 pyruvate 2 NADH 2 ADP 2 ATP (net gain) When oxygen is available, pyruvate enters the mitochondria, where it is further broken down If oxygen is not available, fermentation occurs

22 Occurs in the matrix of the mitochondria Is the transition between glycolysis and the citric acid cycle. Pyruvate (made during glycolysis) is converted to acetyl CoA, and CO 2 is released NAD + is converted to NADH + H + The transition reaction occurs twice per glucose molecule. Step 2 - Transition Reaction

23 Transition reaction inputs and outputs per glucose molecule Inputs: 2 pyruvate 2 NAD + Outputs: 2 acetyl groups 2 CO 2 2 NADH ml

24 Step 3 - Citric Acid Cycle (aka Kreb’s Cycle) Occurs in the matrix of the mitochondria. C 2 acetyl group is converted to a C 6 citrate. Each acetyl group gives off 2 CO 2 molecules. NAD+ accepts electrons 3 times FAD accepts electrons once. Results in a gain of one ATP per every turn of the cycle; there are two cycles per glucose, so a net of 2 ATP are produced. The citric acid cycle produces four CO 2 per molecule of glucose.

25 Citric acid cycle

26 Citric acid cycle inputs and outputs per glucose molecule Inputs: 2 acetyl groups 6 NAD + 2 FAD 2 ADP + 2 P Outputs: 4 CO 2 6 NADH 2 FADH 2 2 ATP

27 Step 4 - Electron Transport System (ETS) Requires oxygen (aerobic) Located in the cristae of mitochondria NADH and FADH 2 carry electrons picked up during glycolysis, transition reaction, & citric acid cycle and enter the ETS. The ETS consists of: –protein complexes that pump H + –mobile carriers that transport electrons –ATP synthase complex - H + flow through it, making ATP H+ flow through from high to low concentration For every 3 H + that flow through, one ATP is made

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29 Overview of the electron transport system

30 hill.com/sites/ /student_view0/chapter9/animations.ht ml

31 Energy Yield from Electron Transport Chain Per glucose molecule: –10 NADH take electrons to the ETS  3 ATP from each –2 FADH 2 take electrons to the ETS  2 ATP from each Electrons carried by NADH produced during glycolysis are shuttled to the electron transport chain by an organic molecule (mechanism of delivery may vary # of ATP produced by ETS).

32 Accounting of energy yield per glucose molecule breakdown

33 Fermentation Occurs when oxygen is not available. During fermentation, the pyruvate formed by glycolysis is reduced to lactic acid. Fermentation uses NADH and regenerates NAD+. Occurs in anaerobic bacteria, fungus, & human muscle cells. LYSIS.HTML

34 Advantages and Disadvantages of Fermentation Fermentation can provide a rapid burst of ATP in muscle cells, even when oxygen is in limited supply. Lactate, however, is toxic to cells. Initially, blood carries away lactate as it forms; eventually lactate builds up, lowering cell pH, and causing muscles to fatigue. Oxygen debt occurs, and the liver must reconvert lactate to pyruvate.

35 Fermentation inputs and outputs per glucose molecule Inputs: Glucose 2 ATP 4 ADP + 2 P Outputs: 2 lactate or 2 alcohol & 2 CO 2 2 ADP 2 ATP (net gain)


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