Chapters 3 and 7: Enzymes and Cellular Respiration

Metabolic Pathways and Enzymes Cellular reactions are usually part of a metabolic pathway, a series of linked reactions Many reactions have molecules in common Energy can be released in small amounts rather than all at once 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.

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

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

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

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.

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

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

Factors Affecting Enzymatic Speed Temperature and pH Substrate concentration Enzyme concentration

Temperature and pH: As the temperature rises, enzyme activity increases. If the temperature is too high, enzyme activity slows 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.

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

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.

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.

Overview of Cellular Respiration Makes ATP molecules 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

The four phases of complete glucose breakdown

ATP (adenosine triphosphate) The energy currency of cells. A nucleotide made of the following: Adenine (a base) Ribose (a sugar) Three phosphate groups Constantly regenerated from ADP (adenosine diphosphate) after energy is expended by the cell. Pneumonic devices – ATP – a triple phosphate - ADP – a double phosphate atpsyn2.swf

The ATP cycle

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)

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

The NAD + cycle

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.

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)

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

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

Step 2 - Transition Reaction Requires oxygen (aerobic) Occurs in the matrix of the mitochondria Pyruvate (made during glycolysis) is converted to acetyl CoA, and CO 2 is released NAD + is converted to NADH The transition reaction occurs twice per glucose molecule.

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

Step 3 - Citric Acid Cycle (aka Kreb’s Cycle) Occurs in the matrix of the mitochondria. Requires oxygen (aerobic) Cycle occurs twice (each of the following occurs twice) C 2 acetyl group is converted to a C 6 citrate. NAD + accepts electrons 3 times FAD accepts electrons once. Produces four CO 2 Results in a gain of one ATP

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 ml

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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Energy Yield from Glucose Metabolism 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

Accounting of energy yield per glucose molecule breakdown

Fermentation Occurs when oxygen is not available. Follows glycolysis Pyruvate formed by glycolysis is reduced to alcohol and CO 2, or to lactate. Fermentation uses NADH and regenerates NAD +, which can be used during glycolysis. Occurs in anaerobic bacteria, fungus, & human muscle cells. LYSIS.HTML

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

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.

Efficiency of Fermentation Two ATP produced during fermentation are equivalent to 14.6 kcal; complete oxidation of glucose to CO 2 and H 2 O represents a yield of 686 kcal per molecule of glucose. Thus, fermentation is only 2.1% efficient compared to cellular respiration (which is 39% efficient). (14.6/686) x 100 = 2.1%