Today is Tuesday, November 3 rd, 2015

Transcription

1 In This Lesson: Metabolism and Enzymes (Lesson 1 of 3) Today is Tuesday, November 3 rd, 2015 Pre-Class: List as many things as you can about enzymes. What do you remember? Think: What do we call the molecules it works with? Where on the enzyme does all the action happen? What can break an enzyme? Of what are they made? Oh yeah, get a [small] paper towel too.

2 Today s Agenda Chemical reactions with respect to energy changes. Gibbs Free Energy. Enzymes. Enzyme functions and mechanisms. Where is this in my book? Chapter 8.

3 By the end of this lesson You should be able to distinguish between endergonic and exergonic reactions. You should be able to describe the features of an enzyme. You should be able to use an enzyme, as in, understand how to turn it on and turn it off.

4 Let s not get ahead of ourselves Challenge questions!

5 The Circle of? The Lion King had the Circle of Life:

6 The Circle of? South Park had the Circle of Poo:

7 The Circle of? In reality, it s all a Circle of Energy. Kind of. The sun s energy is converted to ATP and Organic Molecules by plants which are converted to ATP and Organic Molecules by herbivores which are converted to ATP and Organic Molecules by carnivores Simba?

8 Overview Metabolism is really just the chemical reactions of life. Anabolism Forming bonds between molecules. Dehydration synthesis, synthesis of polymers. Fun Fact: Anabolic steroids get their name from this. Catabolism Breaking bonds between molecules. Hydrolysis, digestion, breakdown of molecules. Metabolism can also be considered to include regulation of enzymes as well, even if there s no real product associated with that part of the process.

9 REMINDER Dehydration synthesis: H 2 O Hydrolysis/digestion: H 2 O

10 REMINDER Dehydration synthesis: Hydrolysis

11 Energy Release/Absorption Reactions can further be classified by whether they have a net release or absorption of energy. Exergonic reactions have a net release of energy and are associated with digestion (breaking down) of molecules: -ΔG

12 Energy Release/Absorption Endergonic reactions have a net absorption of energy and are associated with synthesizing (building) molecules: +ΔG

13 Wait ΔG? ΔG is equal to the Gibbs free energy of the reaction. Think of free energy as the ability to do work. When there is an endergonic reaction, energy is put into the molecules that can later be used to do work. Exergonic reactions release energy that can be used elsewhere. You ve already seen this with ATP: Using ATP requires the breaking off of a phosphate group, releasing energy. Rebuilding ATP from ADP requires the addition of a phosphate group, requiring energy.

14 Gibbs Free Energy Gibbs free energy is a product of thermodynamics. Most relevant to biology: First Law: Energy is constant in the universe and reactions. Second Law: Spontaneous reactions increase the entropy (disorder) of the universe. For a basic example, consider that it s far more likely for an egg to break into a bunch of pieces than it is for a bunch of pieces to form an egg. See also: Humpty Dumpty. Obviously what cells do is an exception. Side Note: Don t confuse entropy (disorder) with enthalpy (heat energy change).

15 Gibbs Free Energy Equation ΔG = ΔH T ΔS ΔG = Gibbs Free Energy ΔH = Change in enthalpy (heat) T = Temperature (in Kelvin) ΔS = Change in entropy (disorder)

16 Gibbs Free Energy Equation Units: ΔG and ΔH are given in heat units: calories (cal) or kilocalories (kcal) 1000 cal = 1 kcal ΔS is given as heat/kelvin. Okay, practice problem time!

17 Gibbs Free Energy Example If the change in free energy is kcal for a reaction occurring at 22 C and the change in entropy is 100 cal/k, what is the change in enthalpy? Is the reaction endergonic or exergonic? 22 C = 295 K ΔG = ΔH T ΔS kcal = ΔH (295 K)(100 cal/k) kcal = ΔH cal kcal = ΔH 29.5 kcal ΔH = kcal Which means this is exergonic since free energy decreases. +ΔG = endergonic; -ΔG = exergonic (+ΔH = endothermic; -ΔH = exothermic)

18 Time to Practice Independently Gibbs Free Energy Practice Problems worksheet

19 Thermodynamic Concepts Not incredibly important for Biology but here if you d like to know. ΔG = ΔH T ΔS When you think about it, the free energy equation is really just a relationship between enthalpy and entropy (adjusted for temperature). The T ΔS part of the equation (entropy) needs to be larger than the ΔH part of the equation (enthalpy) to make the reaction exergonic. An exothermic reaction (-ΔH) is not necessarily exergonic, but it s somewhat energetically favorable. If ΔH is negative and ΔS is positive (increase in entropy), the equation is VERY thermodynamically favorable.

20 About P i Did you catch the P i in the first problem? The hydrolysis of ATP goes by this reaction: ATP ADP + P i P i is the notation for a free phosphate group. The i stands for inorganic. Inorganic just means that it s not being pulled off some other molecule.

21 Coupling Reactions Because organisms need to, you know, live, they must couple endergonic reactions with exergonic reactions. To put it another way, think of what we do every day: eating. You eat food which is then digested and broken down to provide your body with energy. That energy is then used to help you grow and do things that require energy. Coupled!

22 Coupling Reactions In other words: + + Energy + +

23 Coupling Reactions One more example. Take a look at the inner membrane of the mitochondria for a great look at coupled reactions. More on this to come later:

24 The Big Picture Keep in mind that big organic compounds with lots of chemical bonds (especially giant hydrocarbons) contain a lot of bonding energy. This explains why fat is such a good source of energy. Remember triglycerides? They re among the most energy-containing molecules out there:

25 Spontaneity Reactions don t just happen spontaneously, however. Imagine immediately losing starch molecules to spontaneous digestion. Consider, for example, the thermite reaction: Thermite is a mix of aluminum powder and iron oxide powder (essentially rust). It was originally used for welding railroad ties together out in the wilderness. However, it s important to note that just mixing the substances does not actually set off the reaction. In the video, watch for the input of energy to kick-start things. Thermite video

26 Activation Energy The use of the sparkler in the video provided the activation energy necessary to start the reaction. Activation energy is the amount of energy needed to destabilize a molecule s bonds. Exergonic reactions can happen spontaneously, but activation energy makes them process sloooooooooowly. Too sloooooooooowly for living things. G is not a part of activation energy. In graph form:

27 Activation Energy Catalysts are substances (not necessarily organic substances) that help lower activation energy. Enzymes are proteins that lower activation energy.

28 Important Distinction Many people realize that enzymes speed up reactions. It s important to realize, however, that they do not increase the movement of the particles involved. Remember, enzymes lower activation energy. Here s a conceptual example:

29 Enzyme Function Example Imagine you earn an allowance of $1 per week. You want to buy a video game system that costs $349. How many weeks do you need? 349. Enjoy that. If the game system goes on sale for $50, how many weeks would you need? 50. So you can get the system sooner, but are you earning money at a faster rate? No. This is how enzymes work they lower the threshold.

30 Enzyme Details Enzymes are biological catalysts: Made of protein or RNA (RNA enzyme = ribozyme). Facilitate chemical reactions by: Lowering activation energy to increase reaction rate. Not being consumed in reactions. A single enzyme can catalyze thousands of reactions per second. Not changing G released or required. Required for most biological reactions. Reactions would take too long otherwise. Highly specific (thousands of different kinds in each cell)

31 Enzymes Enzymes come with their own vocabulary: The reactant which binds to an enzyme is called the substrate. Once bound, they are temporarily called the enzyme-substrate complex. Products are the substrates after the reaction. Exactly where on the enzyme molecule a substrate binds is called the active site.

32 Specificity What s this about being specific? Enzymes fit their substrates and only their substrates through something known as the lock and key model. Quite like a key fitting into a lock, only with hydrogen bonds. This is a functional but simplistic model.

33 Specificity More accurate is the induced fit model: Just like the lock and key model, except the binding of the substrate causes a conformational change in the enzyme that leads to an even closer fit. Functional groups become closer together for catalysis.

34 What s the difference? Imagine a constrictor snake killing its prey: As the prey exhales, the snake coils more tightly, preventing the prey from inhaling again. In the same way, in the induced fit model, the substrate fits the enzyme like a key into a lock, but the binding of the substrate causes it to fit even more tightly.

35 What s in a name? Enzymes also have friendly names, usually: Sucrase breaks down sucrose. Proteases break down proteins. Lipases break down lipids. DNA Polymerase polymerizes DNA. ATP Synthase synthesizes ATP. Pepsin breaks down polypeptides. In other words, enzymes are named for their reactions.

36 Enzyme Mechanisms Without going into tremendous detail, how do enzymes actually do their jobs? Here are a couple of examples: During synthesis reactions, the active site of the enzyme orients the substrates in a correct position so as to bring them closer together. Like a kid that puts two dolls together and says, And now they kiss! During digestion reactions, the active site puts stress on the bonds that must be broken to facilitate breakage.

37 Whew. Okay, that was a lot Time for, you guessed it, a POGIL! Yes! I LOVE POGILS! Note that this one will take you through a review of enzymes and then help you explore some new concepts about them. We ll be exploring those new concepts afterward, so if you don t know a question, leave it blank for now. Mainly these are questions concerning the factors that affect enzyme function.

38 Enzyme Contest I need ten volunteers. Five of you are going to play the role of an enzyme called Splint Splittase. Guess what it does. It splits splints (twice each, into four total pieces). I ll also need five people to act as official judges. You ll count how many splints are split. Remember, one splint needs to be split into four for it to count as one.

39 Enzyme Contest There is a catch, however. To explore the limiting factors of enzymes, we re going to add some details: One of you is a control group. One of you is going to cross your fingers. One of you will have a lot of splints. One of you will have only a few splints total. One of you is going to put your hands in ice water for a while before we start.

40 Factors Affecting Enzyme Function Summary Slide Enzyme Concentration Substrate Concentration Temperature ph Salinity Activators Inhibitors

41 Enzyme Concentration Keep in mind, enzymes aren t intelligent beings. No offense if your friends or family members are enzymes. They need to rely on chance collisions with substrates to be effective, so: As enzyme concentration goes up, so does reaction rate. More enzymes means more collisions with substrates.

42 Reaction Rate Enzyme Concentration: Graph Form Unlimited substrate Limited substrate Enzyme Concentration Whoa. Why the leveling-off point? Eventually, we reach a point at which the substrate concentration limits the enzyme s ability to work. Simulated by the Splint Splittase with too few splints. If, however, we assume that the substrate is unlimited, a different pattern emerges.

43 Substrate Concentration In contrast, if substrate concentrations get very high, the enzyme reaches a point at which reaction rate is maximized. So, as substrate concentration increases, reaction rate increases (until saturation). At saturation, all enzymes active sites are occupied. This was symbolized by the Splint Splittase with a lot of splints. I could have added a truckload more and the rate would not have increased.

44 Reaction Rate Substrate Concentration: Graph Form Saturation reached Substrate Concentration

45 Temperature All enzymes have an optimal temperature (or range of temperatures) for maximizing molecular collisions. Remember that increased temperature is just increased molecular kinetic energy (motion). Human enzymes work best between 35 C and 40 C. Human body temperature = 37 C average. This partly explains why hyperthermia (fevers) and hypothermia can be so dangerous. Heat can denature enzymes by breaking ionic and hydrogen bonds, changing their shape. Lack of heat causes molecules to move too slowly for enzymes to function properly. Symbolized by the Splint Splittase in ice water (since I couldn t start any fires).

46 Reaction Rate Temperature: Graph Form Human Enzyme Thermophilic Bacteria 37 C (98.6 F) Temperature 70 C (158 F)

47 ph Changes in ph can disrupt bonds and thus denature proteins just like heat. Heat just used kinetic energy to destabilize bonds. For ph, the addition or removal of H + ions disrupts the bonds by changing attraction between charged amino acids. Most human enzymes (but not all) work between ph 6 and ph 8. Some others: Pepsin works in the stomach (ph 2-3) Trypsin works in the small intestine (ph 8). This was symbolized by the Splint Splittase with crossed fingers (since I couldn t dump acid everywhere).

48 Reaction Rate ph: Graph Form Pepsin Trypsin 2.5 ph 8

49 Salinity Salinity changes alter the concentrations of cations (positively charged ions) and anions (negatively charged ions). This is any salt, not just NaCl, by the way. This leads to denaturation too. The Dead Sea is dead for a good reason. Today, biologists have come to know a number of extremophiles organisms (usually bacteria or archaea) that live in extremely salty, hot, or acidic/basic environments. Also symbolized by the Splint Splittase with crossed fingers (since I can t dump salt water everywhere).

50 Salinity Reminder: Dissociation Bound ions in component ions out. Ca Cl Cl Ca 2+ Cl - Cl -

51 Reaction Rate Salinity: Graph Form Salinity

52 Activators There are three main types of activators out there: Cofactors are small, inorganic non-proteins that bind to and activate the enzyme. Like how Fe is a part of hemoglobin or Mg is in chlorophyll. Mg, K, Ca, Zn, and Cu also are common cofactors. Coenzymes are small, organic non-proteins that bind temporarily or permanently near an enzyme s active site. Many vitamins are coenzymes: Coenzyme A, NAD (vitamin B 3 : niacin), FAD (vitamin B 2 : riboflavin) Cooperators are substrates that act as activators by changing enzyme shapes. Often the substrate changes the shape of a multi-subunit enzyme to kick-start the rest of it.

53 Inhibitors Summary Slide Broadly, inhibitors work against enzyme activity. Types of inhibition: Competitive inhibition Noncompetitive (allosteric) inhibition Additional inhibition effects/details: Irreversible inhibition Feedback inhibition

54 Inhibition: Competitive Competitive Inhibition: When there s another molecule that isn t a substrate, yet can bind to the active site of an enzyme. Penicillin, for example, blocks an enzyme used by bacteria to make cell walls. Antabuse (disulfiram) is a commercial drug that treats alcoholism by inhibiting the enzyme that breaks down alcohol, leading to severe hangover and illness within 5-10 minutes after drinking. Competitive inhibition can be overcome by increasing substrate concentration. In other words, substrates can outcompete inhibitors. Analogy: Competitive inhibition is like someone sitting in your seat.

55 Inhibition: Noncompetitive Noncompetitive (Allosteric) Inhibition: When an inhibitor binds somewhere other than the active site and causes a conformational change that prevents binding with the normal substrate. This is known as an allosteric inhibitor more to come. Some anti-cancer drugs inhibit enzymes that copy DNA, preventing new cell growth. Cyanide poison permanently inhibits Cytochrome C, an enzyme in respiration, preventing cells from making ATP. Analogy: Noncompetitive inhibition is like someone sitting in the row behind you putting their feet or coat on your chair.

56 Effects/Details of Inhibition Irreversible inhibition, as with cyanide, is exactly what it sounds like. For irreversible competitive inhibition, the inhibitor binds permanently to the active site. For irreversible noncompetitive inhibition, the inhibitor binds permanently to the allosteric site. Allo- meaning other and -steric meaning shape-related. These inhibitors permanently change enzyme shape, as in many nerve gases and sarin gas, and in insecticides.

57 Allosteric Regulation In addition to allosteric inhibition, there is also allosteric activation. In other words, shape changes can activate or deactivate an enzyme. Activators and inhibitors stabilize these shape changes.

58 Feedback Inhibition One last dynamic associated with inhibition is feedback inhibition. This is when the final product of a metabolic pathway (next slide) inhibits the earlier step(s). This inhibition prevents unnecessary accumulation of a product. Substance A Substance Substance Substance B C D Substance E Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 1 Substance E Inhibits Enzyme 1

59 Feedback Inhibition Example The amino acid isoleucine is made from another amino acid, threonine. Isoleucine inhibits the first step in the pathway, shutting off its own production. Isoleucine collides with the enzyme more than the initial substrate does.

60 Metabolic Pathways? We ll be exploring this extensively, but now s a good time to mention that metabolic pathways are just various series of reactions associated with metabolism. Like we saw with feedback inhibition, pathways allow for control/regulation of reactions as well as efficiency. Yay evolution!

61 Closure BBC Bitesize Enzymes video