Energy and Enzymes

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Forms of Energy These forms of energy are important to life: chemical radiant (examples: heat, light) mechanical electrical Energy can be transformed from one form to another. Chemical energy is the energy contained in the chemical bonds of molecules. Radiant energy travels in waves and is sometimes called electromagnetic energy. An example is visible light. Photosynthesis converts light energy to chemical energy. Energy that is stored is called potential energy.

Laws of Thermodynamics 1st law: Energy cannot be created or destroyed. Energy can be converted from one form to another. The sum of the energy before the conversion is equal to the sum of the energy after the conversion. Example: A light bulb converts electrical energy to light energy and heat energy. Fluorescent bulbs produce more light energy than incandescent bulbs because they produce less heat. 2nd law: Some usable energy dissipates during transformations and is lost. During changes from one form of energy to another, some usable energy dissipates, usually as heat. The amount of usable energy therefore decreases.

Energy is required to form bonds. Atoms or molecules Energy + Energy Larger molecule The energy that was used to form the bonds is now stored in this molecule. 8

Energy is released when bonds are broken. 8 Menu

Energy is released when bonds are broken. When bonds break, energy is released. It may be in a form such as heat or light or it may be transferred to another molecule. 8 Menu

ATP (Simplified Drawing) 3 phosphate groups Base (adenine) A Sugar (ribose) 11

ATP Stores Energy Energy ATP ADP + Pi + Energy Breaking the bonds releases the energy. The phosphate bonds are high-energy bonds. A ATP

ATP is Recycled ATP Energy Energy ADP + Pi ATP (Adenosine Triphosphate) is an energy-containing molecule used to supply the cell with energy. The energy used to produce ATP comes from glucose or other high-energy compounds. ATP is continuously produced and consumed as illustrated below. ADP + Pi + Energy  ATP + H2O (Note: Pi = phosphate group) ATP Energy Energy (from glucose or other high-energy compounds) ADP + Pi 13

In this diagram, energy from breaking bonds in this molecule is used to form ATP. ADP + Pi ATP Energy 8 Menu

Menu The energy in ATP can be used to form bonds in other molecules. ADP + Pi Energy The energy in ATP can be used to form bonds in other molecules. ATP 8 Menu

ATP (Adenosine Triphosphate) NH2 Base (adenine) C N N C CH HC C N O- O- O- N O P O P O P O CH2 - O C C O O O H H C C H C OH OH 3 phosphate groups Ribose

Phosphorylation ATP is synthesized from ADP + Pi. The process of synthesizing ATP is called phosphorylation. Two kinds of phosphorylation are illustrated on the next several slides. Substrate-Level Phosphorylation Chemiosmotic Phosphorylation

Substrate-Level Phosphorylation A high-energy molecule (substrate) is used to transfer a phosphate group to ADP to form ATP. High-energy molecule ADP

Substrate-Level Phosphorylation A high-energy molecule (substrate) is used to transfer a phosphate group to ADP to form ATP. High-energy molecule ADP This bond will be broken, releasing energy.

Substrate-Level Phosphorylation A high-energy molecule (substrate) is used to transfer a phosphate group to ADP to form ATP. High-energy molecule ADP The energy released will be used to bond the phosphate group to ADP, forming ATP.

Substrate-Level Phosphorylation A high-energy molecule (substrate) is used to transfer a phosphate group to ADP to form ATP. High-energy molecule ADP Enzyme An enzyme is needed.

Substrate-Level Phosphorylation Breaking this bond will release energy.

Substrate-Level Phosphorylation The energy will be used to form this bond.

Substrate-Level Phosphorylation The energy has been transferred from the high-energy molecule to ADP to produce ATP. Low-energy molecule ATP

Mitochondrion Structure This drawing shows a mitochondrion cut lengthwise to reveal its internal membrane. Intermembrane Space Cristae Matrix

Chemiosmotic Phosphorylation This drawing shows a close-up of a section of a mitochondrion. Chemiosmotic Phosphorylation H+ H+ Matrix (inside) H+ H+ Outside Intermembrane Space Matrix H+ H+ H+ H+ H+ H+ H+ H+ H+ 33 Menu

Chemiosmotic Phosphorylation Pumps within the membrane pump hydrogen ions from the matrix to the intermembrane space creating a concentration gradient. Chemiosmotic Phosphorylation H+ H+ Matrix (inside) H+ H+ Outside Intermembrane Space Matrix H+ H+ H+ H+ H+ H+ H+ H+ H+ 33 Menu

Chemiosmotic Phosphorylation This process requires energy and will be discussed in the chapter on cellular respiration. Chemiosmotic Phosphorylation H+ H+ Matrix (inside) H+ H+ Outside Intermembrane Space Matrix H+ H+ H+ H+ H+ H+ H+ H+ H+ 33 Menu

Chemiosmotic Phosphorylation A high concentration of hydrogen ions in the intermembrane space creates osmotic pressure. Chemiosmotic Phosphorylation H+ H+ Matrix (inside) H+ H+ Outside Intermembrane Space Matrix H+ H+ H+ H+ H+ H+ H+ H+ H+ 33 Menu

Chemiosmotic Phosphorylation Osmotic pressure forces the hydrogen ions through this protein (called ATP synthase) as they return to the matrix. Chemiosmotic Phosphorylation H+ H+ Matrix (inside) H+ H+ Outside Intermembrane Space Matrix H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ 33 Menu

Chemiosmotic Phosphorylation Osmotic pressure forces the hydrogen ions through this protein (ATP synthase) as they return to the matrix. Chemiosmotic Phosphorylation H+ H+ Matrix (inside) H+ H+ Outside Intermembrane Space Matrix H+ H+ H+ ADP + Pi H+ H+ H+ ATP ATP synthase produces ATP by phosphorylating ADP. The energy needed to produce ATP comes from hydrogen ions forcing their way into the matrix as they pass through the ATP synthase (due to osmotic pressure). H+ H+ H+ H+ 33 Menu

Chemiosmotic Phosphorylation Chemiosmotic phosphorylation (previous slide) is used by the mitochondrion to produce ATP. The energy needed to initially pump H+ ions into the intermembrane space comes from glucose. The entire process is called cellular respiration and will be discussed in a later chapter. The chloroplast also produces ATP by chemiosmotic phosphorylation. The energy needed to produce ATP comes from sunlight.

Chloroplast Structure The chloroplast is surrounded by a double membrane. Molecules that absorb light energy (photosynthetic pigments) are located on disk-shaped structures called thylakoids. The interior portion is the stroma. Stroma Double membrane Thylakoids

A Thylakoid (see previous slide) In order to synthesize ATP, hydrogen ions must first be pumped into the thylakoid. This process requires energy. H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ 72

A Thylakoid H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ A concentration gradient of hydrogen ions is established. The osmotic pressure from this gradient can be used as an energy source for producing ATP. H+ 72

Chemiosmotic Phosphorylation Osmotic pressure forces hydrogen ions through this protein (ATP synthase) as they return to the stroma. H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ ADP + Pi ATP H+ H+ ATP synthase produces ATP by phosphorylating ADP. The energy comes from hydrogen ions forcing their way into the stroma as they pass through the ATP synthase under pressure (osmotic pressure). H+ H+ 72

Phosphorylation We have just discussed two different forms of phosphorylation: Substrate-level phosphorylation Chemiosmotic phosphorylation We saw that chemiosmotic phosphorylation occurred in both the mitochondria (during cellular respiration) and in the chloroplast (during photosynthesis). These two processes are sometimes given separate names: Oxidative phosphorylation (in mitochondria) Photophosphorylation (in chloroplast)

Catabolic and Anabolic Reactions The energy-producing reactions within cells generally involve the breakdown of complex organic compounds to simpler compounds. These reactions release energy and are called catabolic reactions. Anabolic reactions are those that consume energy while synthesizing compounds. ATP produced by catabolic reactions provides the energy for anabolic reactions. Anabolic and catabolic reactions are therefore coupled (they work together) through the use of ATP. Diagram: next slide

Catabolic and Anabolic Reactions An anabolic reaction A catabolic reaction ATP ADP + Pi Energy Catabolic and Anabolic Reactions 8 Menu

Substrates (Reactants) Anabolic Reactions Products Anabolic reactions consume energy. Energy Supplied Substrates (Reactants) Energy Released 9 Menu

Catabolic Reactions Energy Supplied Substrate (Reactant) Catabolic reactions release energy. Energy Released When bonds are broken, energy is released. 10 Menu

Activation Energy Energy Supplied Activation Energy Energy Released In either kind of reaction, additional energy must be supplied to start the reaction. This energy is called activation energy. Energy Supplied Activation Energy Energy Released 21 Menu

Activation Energy Energy Supplied Activation Energy Energy Released An example of activation energy is the spark needed to ignite gasoline. Energy Supplied Activation Energy Energy Released 21 Menu

Enzymes Lower Activation Energy Enzymes lower the amount of activation energy needed for a reaction. Enzymes Lower Activation Energy Activation energy without enzyme Energy Supplied Activation energy with enzyme Energy Released 22 Menu

1 3 2 Enzymes Menu Enzymes are organic catalysts. Substrate Active Site Enzyme Product Enzyme-Substrate Complex 3 2 Enzyme 15 Menu

Enzymes Catalysts are substances that speed up chemical reactions.  Organic catalysts (contain carbon) are called enzymes. Enzymes are specific for one particular reaction or group of related reactions. Many reactions cannot occur without the correct enzyme present. They are often named by adding "ase" to the name of the substrate. Example: Dehydrogenases are enzymes that remove hydrogen.

Induced Fit Theory An enzyme-substrate complex forms when the enzyme’s active site binds with the substrate like a key fitting a lock. The substrate molecule does not fit exactly in the active site. This induces a change in the enzymes conformation (shape) to make a closer fit. After the reaction, the products are released and the enzyme returns to its normal shape. Only a small amount of enzyme is needed because they can be used repeatedly.

Rate of Reaction Reactions with enzymes are up to 10 billion times faster than those without enzymes. Enzymes typically react with between 1 and 10,000 molecules per second. Fast enzymes catalyze up to 500,000 molecules per second. Substrate concentration, enzyme concentration, Temperature, and pH  affect the rate of enzyme reactions.

Substrate Concentration At lower concentrations, the active sites on most of the enzyme molecules are not filled because there is not much substrate.  Higher concentrations cause more collisions between the molecules.  With more molecules and collisions, enzymes are more likely to encounter molecules of reactant. The maximum velocity of a reaction is reached when the active sites are almost continuously filled. Increased substrate concentration after this point will not increase the rate.  Reaction rate therefore increases as substrate concentration is increased but it levels off. Rate of Reaction Substrate Concentration

Enzyme Concentration If there is insufficient enzyme present, the reaction will not proceed as fast as it otherwise would because there is not enough enzyme for all of the reactant molecules. As the amount of enzyme is increased, the rate of reaction increases. If there are more enzyme molecules than are needed, adding additional enzyme will not increase the rate. Reaction rate therefore increases as enzyme concentration increases but then it levels off. Rate of Reaction Enzyme Concentration

Effect of Temperature on Enzyme Activity Rate of Reaction 30 40 50 Temperature 23

Effect of Temperature on Enzyme Activity Increasing the temperature causes more collisions between substrate and enzyme molecules. The rate of reaction therefore increases as temperature increases. Effect of Temperature on Enzyme Activity Rate of Reaction 30 40 50 Temperature 23

Effect of Temperature on Enzyme Activity Enzymes denature when the temperature gets too high. The rate of reaction decreases as the enzyme becomes nonfunctional. Rate of Reaction 30 40 50 Temperature 23

Temperature Higher temperature causes more collisions between the atoms, ions, molecules, etc. It therefore increases the rate of a reaction. More collisions increase the likelihood that substrate will collide with the active site of the enzyme. Above a certain temperature, activity begins to decline because the enzyme begins to denature (unfold). The rate of chemical reactions therefore increases with temperature but then decreases. Rate of Reaction 30 40 50 Temperature

Denaturation If the hydrogen bonds within an enzyme are broken, the enzyme may unfold or take on a different shape. The enzyme is denatured. A denatured enzyme will not function properly because the shape of the active site has changed. If the denaturation is not severe, the enzyme may regain its original shape and become functional. The following will cause denaturation: Heat Changes in pH Heavy-metal ions (lead, arsenic, mercury) Alcohol UV radiation

Effect of pH on Enzyme Activity Each enzyme has its own optimum pH. Pepsin Trypsin Rate of Reaction 2 3 4 5 6 7 8 9 pH 24

pH Each enzyme has an optimal pH. Pepsin, an enzyme found in the stomach, functions best at a low pH. Trypsin, found in the intestine, functions best at a neutral pH. A change in pH can alter the ionization of the R groups of the amino acids. When the charges on the amino acids change, hydrogen bonding within the protein molecule change and the molecule changes shape. The new shape may not be effective. The diagram shows that pepsin functions best in an acid environment. This makes sense because pepsin is an enzyme that is normally found in the stomach where the pH is low due to the presence of hydrochloric acid. Trypsin is found in the duodenum (small intestine), and therefore, its optimum pH is in the neutral range to match the pH of the duodenum. Pepsin Trypsin Rate of Reaction 2 3 4 5 6 7 8 9 pH

Metabolic Pathways Metabolism refers to the chemical reactions that occur within cells. Reactions occur in a sequence and a specific enzyme catalyzes each step.

Metabolic Pathways A B C D E F enzyme 1 enzyme 2 enzyme 3 enzyme 4 Notice that C can produce either D or F. This substrate has two different enzymes that work on it. Metabolic Pathways A B C D E enzyme 1 enzyme 2 enzyme 3 enzyme 4 Enzymes are very specific. In this case enzyme 1 will catalyze the conversion of A to B only. F enzyme 5 4

A Cyclic Metabolic Pathway In this pathway, substrate “A” enters the reaction. After several steps, product “E” is produced. A B A + F  B B  C  D D  F + E F C D E 7

Regulation of Enzymes The next several slides illustrate how cells regulate enzymes. For example, it may be necessary to decrease the activity of certain enzymes if the cell no longer needs the product produced by the enzymes.

Regulation of Enzymes genetic regulation of enzymes regulation already produced Enzymes are proteins. Recall the central dogma: DNA mRNA Proteins Proteins can be regulated by making more or less of them as needed. The topic of regulating protein synthesis (manufacture) is deferred to a later chapter. 25

(illustrated on the next slide) Regulation of Enzymes genetic regulation regulation of enzymes already produced competitive Inhibition (illustrated on the next slide) 26

Competitive Inhibition In competitive inhibition, a similar-shaped molecule competes with the substrate for active sites. 27

Competitive Inhibition 28

Regulation of Enzymes genetic regulation regulation of enzymes already produced competitive inhibition noncompetitive Inhibition (next slide) 29

Noncompetitive Inhibition Active site Inhibitor Altered active site Enzyme Another form of inhibition involves an inhibitor that binds to an allosteric site of an enzyme.  An allosteric site is a different location than the active site. The binding of an inhibitor to the allosteric site alters the shape of the enzyme, resulting in a distorted active site that does not function properly.

Noncompetitive Inhibition The binding of an inhibitor to an allosteric site is usually temporary.   Poisons are inhibitors that bind irreversibly. For example, penicillin inhibits an enzyme needed by bacteria to build the cell wall. Bacteria growing (reproducing) without producing cell walls eventually rupture.

Regulation of Enzymes genetic regulation regulation of enzymes already produced competitive inhibition noncompetitive inhibition feedback inhibition (next slide) 33

Example of Feedback Inhibition In this example of feedback inhibition, heat (the product) inhibits its production. This keeps the temperature constant. Cold Thermostat Heater Heat inhibits

A B C D Feedback Inhibition The goal of this hypothetical metabolic pathway is to produce chemical D from A. A B C D enzyme 1 enzyme 2 enzyme 3 B and C are intermediates. The next several slides will show how feedback inhibition regulates the amount of D produced. Enzyme regulation by negative feedback inhibition is similar to the thermostat example. As an enzyme's product accumulates, it turns off the enzyme just as heat causes a thermostat to turn off the production of heat. 34

X X A B C D X Feedback Inhibition C and D will decrease because B is needed to produce C and C is needed to produce D. X The amount of B in the cell will decrease if enzyme 1 is inhibited. A B C D X enzyme 1 enzyme 2 enzyme 3 Enzyme 1 is structured in a way that causes it to interact with D. When the amount of D increases, the enzyme stops functioning. 35

A B C D B X X X C D Feedback Inhibition B, C, and D can now be synthesized. A B C D B X X X C D enzyme 1 enzyme 2 enzyme 3 When the amount of D drops, enzyme 1 will no longer be inhibited by it. 39

A B C D X Feedback Inhibition enzyme 1 enzyme 2 enzyme 3 As D begins to increase, it inhibits enzyme 1 again and the cycle repeats itself. 42

Ribozymes Ribozymes are molecules of RNA that function like enzymes, that is, they have an active site and increase the rate of specific chemical reactions.

Energy is transferred with electrons. Oxidized atom Electron is donated Energy is donated Reduced atom Electron is received Energy is received 49

Energy is transferred with electrons. Oxidized atom Electron is donated Energy is donated Reduced atom Electron is received Energy is received 49

Energy is transferred with electrons. This atom served as an energy carrier. It picked up an electron from the atom on the left and gave it to the one on the right. Oxidized atom Electron is donated Energy is donated Reduced atom Electron is received Energy is received 49

Oxidation and Reduction Oxidation is the loss of electrons or hydrogen atoms.  Oxidation reactions release energy. Reduction is gain of electrons or hydrogen atoms and is associated with a gain of energy. Oxidation and reduction occur together. When a molecule is oxidized, another must be reduced. These coupled reactions are called oxidation-reduction or redox reactions. Food is highly reduced (has many hydrogens). The chemical pathways in cells that produce energy for the cell oxidize the food (remove hydrogens), producing ATP.

Cofactors Many enzymes require a cofactor to assist in the reaction.  These "assistants" are nonprotein and may be metal ions such as magnesium (Mg++), potassium (K+), and calcium (Ca++).  The cofactors bind to the enzyme and participate in the reaction by removing electrons, protons , or chemical groups from the substrate.

Coenzymes Cofactors that are organic molecules are coenzymes. In oxidation-reduction reactions, coenzymes often remove electrons from the substrate and pass them to different enzymes. In this way, coenzymes serve to carry energy in the form of electrons (or hydrogen atoms) from one compound to another.

Coenzymes Coenzyme Coenzymes are cofactors that are not protein. They bind to the enzyme and also participate in the reaction by carrying electrons or hydrogen atoms.

Vitamins are Coenzymes Vitamin Coenzyme Name Niacin NAD+ B2 (riboflavin) FAD B1 (thiamine) Thiamine pyrophosphate Pantothenic acid Coenzyme A (CoA) B12 Cobamide coenzymes 46

Electron Carriers Electron carriers function in photosynthesis and cellular respiration. Three major electron carriers are listed below. Respiration NAD+ FAD Photosynthesis NADP+ 52

NAD+ (Nicotinamide Adenine Dinucleotide) Organic Molecule NAD+ NAD+ + NAD+ + 2H  NADH + H+ NAD+ functions in cellular respiration by carrying two electrons. With two electrons, it becomes NADH. NAD+ oxidizes its substrate by removing two hydrogen atoms. One of the hydrogen atoms bonds to the NAD+. The electron from the other hydrogen atom remains with the NADH molecule but the proton (H+) is released.  NAD+ + 2H ® NADH + H+ NADH can donate two electrons (one of them is a hydrogen atom) to another molecule. Menu

NAD+ + 2H  NADH + H+ NADH + H+ Energy + 2H Energy + 2H NAD+ 53

FAD + 2H  FADH2 FADH2 Energy + 2H Energy + 2H FAD 53

FAD (flavin adenine dinucleotide) FAD is reduced to FADH2. It can transfer two electrons to another molecule. FAD + 2H ® FADH2

NADP+ + 2H  NADPH + H+ NADPH + H+ Energy + 2H Energy + 2H NADP+ 53

NADP+ (Nicotinamide Adenine Dinucleotide Phosphate) NADP+ + 2H ® NADPH + H+ NADP+ is similar to NAD+ in that it can carry two electrons, one of them in a hydrogen atom, the other one comes from a hydrogen that is released as a hydrogen ion. (Click here to review NAD+.) Electrons carried by NADPH in photosynthesis are ultimately used to reduce CO2 to carbohydrate.

The End