Enzymes

Objectives  Flow of energy through life.  Metabolism meaning.  Chemical reactions & energy.  Activation energy meaning.  Catalyst meaning.  What Are Enzymes?  Function of enzymes.  How do enzymes Work?  Properties of enzymes.  Mechanisms of Enzymes.  Factors affecting enzyme function. 2

Flow of energy through life Life is built on chemical reactions. – transforming energy from one form to another. organic molecules  ATP & organic molecules sun solar energy  ATP & organic molecules

Metabolism Metabolism can be described as a set of biochemical reactions on which the entire life of living organisms depend. Chemical reactions of life:  forming bonds between molecules dehydration synthesis synthesis anabolic reactions  breaking bonds between molecules hydrolysis digestion catabolic reactions

Chemical reactions & energy  Some chemical reactions release energy. – exergonic – digesting polymers – hydrolysis = catabolism  Some chemical reactions require input of energy. – endergonic – building polymers – dehydration synthesis = anabolism digesting molecules= LESS organization= lower energy state building molecules= MORE organization= higher energy state

Energy & life  Organisms require energy to live – where does that energy come from? Coupling exergonic reactions (releasing energy) with endergonic reactions (needing energy). ++ energy + + digestion synthesis

Activation energy To start any chemical reaction, energy is required. The minimum amount of energy required to start a reaction is known as activation energy.

Catalyst A substance that lowers activation energy of a reaction so the reaction occurs more quickly but is NOT used up by the reaction.

What Are Enzymes? Proteins.  Most enzymes are Proteins.  Kind of catalysts found only in living things.  Act as biological Catalysts.

Enzymes vocabulary  Enzyme: helper protein molecule.  Substrate: reactant which binds to enzyme. enzyme-substrate complex. Temporary association. substrate enzyme products active site

Enzymes vocabulary  Product: end result of reaction.  Active site part of enzyme that substrate molecule fits into. enzyme’s catalytic site. substrate enzyme products active site

Types of enzymes 1.Intracellular enzymes: Enzymes that are produced in certain cells and remain to react in the cell (work inside the cells). These enzymes could exist either in cytoplasm (in organelles) or nucleus. 2.Extracellular enzymes: Some enzymes are produced by cell but then transported out of the cell for action outside the cell. Enzymes that are produced inside cells but work outside them.

Function of enzymes  Increase the rate of chemical reaction by lowering activation energy.  Enzymes change only the speed of the reaction.  They help the reactions to occur, but they are not changed or used up in the reaction, so they can be used over and over again (Reusable).  Enzymes allow digestion to occur faster; otherwise the hamburger you ate last week might still be in your stomach!  By using enzymes to break chemical bonds in food molecules, organisms release energy for life processes.

How do enzymes Work? Free Energy Progress of the reaction Reactants Products activation energy Without Enzyme With Enzyme Enzymes work by weakening bonds which lowers activation energy.

Enzyme (sucrase) Active site Substrate (sucrose) Enzyme available with empty active site Substrate binds to enzyme Substrate is converted to products 4 Products are released GlucoseFructose The enzyme is unchanged and can repeat the process How an enzyme works? Substrate (sucrose)

Enzymes are recycled and reused many times for the same type of reaction (never used up). Example: Sucrase converts sucrose  glucose + fructose » Sucrase remains unchanged How an enzyme works?

Enzymes are specific for the reaction they catalyze. Each enzyme is the specific helper to a specific reaction – each enzyme needs to be the right shape for the job – enzymes are named for the reaction they catalyze. sucrase breaks down sucrose proteases breakdown proteins lipases breakdown lipids DNA polymerase builds DNA adds nucleotides to DNA strand Oh, I get it! They end in -ase Properties of enzymes

 Reaction specific: – Each enzyme works with a specific substrate Chemical fit between active site & substrate: – H bonds & ionic bonds. – The enzymes are usually bigger than the molecules that they act upon.  Not consumed in reaction – Single enzyme molecule can catalyze thousands or more reactions per second. Enzymes unaffected by the reaction.  Affected by cellular conditions – Any condition that affects protein structure.

Which is the enzyme and which is the substrate? How do You know? What kind of reaction is this?  The enzyme does not get changed in this reaction - therefore it is the green one. Another clue is that it is bigger than the substrate.  Hydrolysis reaction because the substrate is being broken down. Learning check

Hydrolysis with enzyme action

Dehydration synthesis of a product with enzyme

Mechanisms of Enzymes There are 2 main hypothesis explaining the mechanism of enzyme action : The lock-and-key hypothesis The induced-fit hypothesis

Lock-and-key hypothesis The hypothesis proposed that the active site and substrate are exactly complementary. The shape of the substrate (‘key’) fits into the active site of the enzyme (‘lock’) forming an enzyme-substrate complex. Reaction takes place and products are formed and released. A lock has a specific shape. Only one specific key will open that lock.

 More accurate model of enzyme action.  The hypothesis suggested that active site is flexible and is not exactly complementary to the shape of the substrate.  The substrate binds to the active site.  The binding induces the enzyme to change shape leading to a tighter fit. Induced-fit hypothesis

Factors affecting enzyme function  Enzyme concentration  Substrate concentration  Temperature  pH  Activators  Inhibitors

Enzyme concentration The rate of an enzyme-catalyzed reaction is directly proportional to the concentration of the enzymes if substrates are present in excess concentration and no other factors are limiting. – as  enzyme =  reaction rate more enzymes = more frequently collide with substrate enzyme concentration reaction rate

Enzyme concentration enzyme concentration reaction rate After a while the rate of reaction becomes constant. At this point, the rate of the reaction can be increased only by increasing the concentration of the substrate. Reaction rate levels off substrate becomes limiting factor not all enzyme molecules can find substrate.

Substrate concentration If enzyme molecules are available in a reaction, increase in substrate concentration causes increase in the rate of reaction. – as  substrate =  reaction rate more substrate = more frequently collide with enzyme

 If the ratio of enzymes is less than the ratio of substrates then the enzymes enter a state called saturation of active sites.  So there are no vacant active sites available for reaction so instead of increased rate of reaction, the rate remains stagnant. reaction rate levels off: all enzymes have active sites saturated. Faster reaction but it reaches a saturation point when all the enzyme molecules are occupied Substrate concentration

 Optimum temperature for maximum enzyme function is usually about 35-40C.  Reactions proceed slowly below optimal temperatures.  Above 45 C most enzymes are denatured (change in their shape so the enzyme active site no longer fits with the substrate and the enzyme can't function). Temperature

The effect of pH Each enzyme has a pH range within which it can work best. This is called for that particular enzyme. Most enzymes act at a pH in range of 5 – 9, and reactions most efficiently at a pH of 7. There are exceptions for certain enzymes such as: pepsin (stomach)= pH 2-3 trypsin (small intestines)= pH 8

7 The effect of pH pH reaction rate pepsintrypsin pepsin trypsin

Activators  Compounds which help enzymes.  They increase enzyme activity.  Cofactors can be either inorganic or organic compounds.  There are three kinds of cofactors: a)Prosthetic group: Organic molecule that is permanently attached to an enzyme. (tightly bound) b) Coenzymes: organic molecules are not permanently attached to the enzyme molecule. (bound and easily released) (vitamins) c) Metal cofactors: Inorganic metal ions. (Mg, K, Ca, Zn, Fe, Cu)

Inhibitors  Molecules that reduce enzyme activity.  They block the enzyme but they do not usually destroy it.  Many drugs and poisons are inhibitors of enzymes in the nervous system. Competitive inhibitors Noncompetitive inhibitors

Enzyme Inhibitors Competitive inhibitors: are chemicals that resemble an enzyme’s normal substrate and compete with it for the active site. (b) Competitive inhibition A competitive inhibitor mimics the substrate, competing for the active site. Competitive inhibitor A substrate can bind normally to the active site of an enzyme. Substrate Active site Enzyme (a) Normal binding

Competitive inhibition Succinate Fumarate + 2H + + 2e - Succinate dehydrogenase CH 2 COOH CHCOOH COOH CH 2 Malonate

Enzyme Inhibitors A noncompetitive inhibitor binds to the enzyme away from the active site, altering the conformation of the enzyme so that its active site no longer functions. Noncompetitive inhibitor (c) Noncompetitive inhibition Noncompetitive inhibitors: Bind to another part of the enzyme causing the enzyme to change its shape, which in turn alters the active site.

Classification of Enzymes Enzymes are classified according to the type of reaction they catalyze: Class Reactions catalyzed  Oxidoreductases Oxidation-reduction  Transferases Transfer of functional groups  Hydrolases Hydrolysis  Lyases -Formation of double bonds by removing chemical groups from a substrate without hydrolysis -Addition of chemical groups to double bond  Isomerases Rearrange atoms  Ligases Use ATP to combine molecules

Classification of Enzymes

Home Work The digestion of Carbohydrates starts in the mouth but it stops in the stomach, Why?