Concept of Enzymes - Big Lecture with Questions and Answers

University:
Kansas State University
Course:
BIOCH 765 | Biochemistry II
Academic year:

2014
Views:

137

Pages:

79
Author:

Jackdams

Enzymes Objectives: Concept for enzymes. Mechanism of enzyme action. Factors affect rate of enzyme action. Enzyme specificity. Enzyme kinetics ( Km & Vmax ). Enzyme inhibition. Regulation of enzyme activity. Clinical uses of enzymes in diagnosis and prognosis of different diseases. • Classes of enzymes. • Coenzymes. 3 Catalytic Proteins 1: Enzymes 4 Enzymes: “The biological catalysts” • They are organic thermo-labile catalysts that increase the chemical reaction without change. • They accelerate the rate of chemical reaction without being consumed in the reaction. Chemical Nature of Enzymes • All enzymes are protein in nature except ribozymes (RNA in nature). What is the difference between an enzyme and a protein? Protein Enzymes RNA •All enzymes are proteins except some RNAs • not all proteins are enzymes Chemical Nature of Enzymes • Protein enzymes are classified into 2 types: • 1- Simple Protein enzymes: They are formed of protein only. 2- Complex (conjugated) Protein : They are formed of protein part and non protein part. 2- Complex (conjugated) Protein : enzymes formed of two parts: 1) Protein part: called apoenzyme 2) Non- protein: called cofactor • The whole enzyme is called holoenzyme. Nature of Enzyme The cofactor may be coenzyme or prosthetic group • Coenzyme: Is organic, thermo-labile , loosely attached to enzyme. • They are mainly vitamin B derivatives e.g. FAD, NAD. • Prosthetic group: Is inorganic, thermo-stable, firmly attached to enzyme. • They are usually metal ions e.g. Ca, Zn Enzymes vocabulary substrate reactant which binds to enzyme enzyme-substrate complex: temporary association product end result of reaction active site enzyme’s catalytic site; substrate fits into active site active site substrate enzyme AP Biology products Active Site(catalytic site) • A restricted region of an enzyme molecule which binds to the substrate. Active Site • It is formed from Substrate Enzyme Amino acids sequences in the polypeptide chain . 13 Mechanism of enzyme action 1- The substrate (S) binds to the enzyme (E) at its active catalytic site to form activated intermediate enzyme substrate complex (ES). 2- The activated complex (ES) cleaved to the products (P) and the original enzyme (E) Enzymatic reaction steps Substrate approaches active site Enzyme-substrate complex forms Substrate transformed into products Products released Enzyme recycled In conjugated protein enzymes, the coenzyme acts as an acceptor for one of the products helping the cleavage of the enzyme substrate complex. Theories of enzyme substrate binding (Two theories) 1- Lock and key theory: “key fits into lock” • The catalytic site of the enzyme has a shape that is complementary (fit) to the shape of the substrate. • The substrate fits in this catalytic site in a similar way to lock and key. The key will only fits its own lock . Key and lock model Size doesn’t matter… Shape matters! 22 Lock and Key model Proposed by Fischer in 1894 In this model, the active sites of the unbound enzyme is complementary in shape to the substrate Key and look 2- Induced fit theory: • It is a more flexible model, where the catalytic site is not fully formed. • The catalytic site of the enzyme is not complementary to the substrate. • Binding of the substrate to the enzyme induces changes in the shape of the catalytic site making it more fit for substrate. Induced- fit model 26 Induced-fit model Proposed by Koshland in 1958 In this model, the enzyme changes shape on substrate binding Enzyme action 1- Enzymes increase the rate of reaction by decreasing the activation energy of reaction. 2- The activation energy is the energy barrier between reactants and products.  So what’s a cell got to do to reduce activation energy? get help! … chemical help… ENZYMES Call in the ENZYMES!AP Biology Enzymes increase the rate of reaction by: • It decreases the energy needed for activation (activation energy). • It decreases the energy barrier between reactants and end products. Catalysts Work by Reducing the Activation Energy of a Reaction Pheeew… that takes a lot less energy! 33 Reaction profile for enzymatic and non-enzymatic reactions Enzymes Lower a Reaction’s Activation Energy Mechanism of Enzyme Activity A substrate(s) fits into a binding site on the enzyme. The enzyme lowers the energy required to reach the transition state. The product no longer fits the binding site and is released. Enzyme Nomenclature • The word enzyme is formed from two Greek words: en means inside and zyme, which means yeast i.e., the word enzyme means inside yeast. • There are many methods for naming enzymes: 1- The old trivial name as pepsin and trypsin. 2- The name of substrate and the suffix – ase added to it as lactase acting on lactose and sucrase acting on sucrose. 3- Two words, one for the substrate and the other for the type of reaction e.g. succinate dehydrogenase, pyruvate decarboxylase and glutamine synthetase. • Enzyme Code (EC): Each enzyme has a numerical code which is formed of four digits separated by dots: The first digit denotes the class (reaction type) of the enzyme. The second digit denotes the functional group upon which the enzyme acts. The third digit denotes the coenzyme. The fourth digit denotes the substrate. For example 1.1.1.1 enzyme, 1 means oxidoreductase, 1.1 means that the functional group is hydroxyl group (-OH), 1.1.1 means NAD is the coenzyme and 1.1.1.1 means alcohol. So, 1.1.1.1 means alcohol dehydrogenase enzyme. Got any Questions?! AP Biology 2007-2008 Activity 1- Prosthetic group is : A) organic. B) thermostable. C) losely attached to the enzyme D) called apoenzyme. 2- The catalytic activity of an enzyme is restricted to its small portion called (A) Active site (B) Passive site (C) Allosteric site (D) All Choices are correct Activity  3-An activated enzyme made of polypeptide chain and a co-factor is (A) Coenzyme (B) Substrate (C) Apoenzyme (D) Holoenzyme 3- Enzymes are largely ……………… in their chemical nature. (A) Lipids (B) Steroids (C) Protein (D) All A, B and C Activity 5-The “lock and key” model of enzyme action illustrates that a particular enzyme molecule (A) forms a permanent enzyme-substrate complex (B) may be destroyed and resynthesized several times (C) interacts with a specific type of substrate molecule which is complementary to its shape (D) reacts at identical rates under all conditions  1- Discuss : (A) Theories of enzyme action (B) Enzyme code 2- Give an account on the chemical nature of the enzymes. 3- In a table ,give 3 differences between: Coenzyme & prosthetic group. 46 Thank you Factors Affect Rate of Enzyme Action AP Biology 2007-2008 Factors Affect Rate of Enzyme Action Enzyme concentration Substrate concentration Temperature pH Concentration of coenzymes Concentration of ion activators Time Inhibitors AP Biology catalase Factors affecting the rate of enzyme action 1- Effect of enzyme concentration The rate of enzyme action is directly proportional to the concentration of enzyme provided that there are sufficient supply of substrate & constant conditions. 52 2- Effect of substrate concentration -The rate of reaction increases as the substrate concentration increases up to certain point at which the reaction rate is maximal (Vmax.) At Vmax, the enzyme is completely saturated with the substrate any increase in substrate concentration doesn't affect the reaction rate. Substrate concentration reaction rate What’s happening here?! substrate concentration AP Biology Effect of substrate concentration on the velocity of an enzyme- catalyzed reaction What’s happening here?! 56 Michaelis constant (Km) - It is the substrate concentration that produces half maximum velocity of enzyme • Enzymes with low Km: have high affinity to the substrate i.e. they act at maximal velocity at low substrate concentration • E.g. Hexokinase acts on glucose at low concentration (fasting state) • Enzymes with high Km: they have low affinity to the substrate i.e. they act at maximal velocity at high substrate concentration • E.g. Glucokinase enzyme acts on glucose at high concentration (fed state) Concentration of substrate Km 3- Effects of temperature - Rate of reaction increases gradually with the rise in temperature until reach a maximum at a certain temperature, called optimum temperature - The optimum temperature is 37-40 C in humans Temperature reaction rate What’s happening here?! 37° temperature AP Biology The effect of temperature on reaction rate is due to: 1- Increase of temperature increase the initial energy of substrate and thus decrease the activation energy 2- Increase of collision of molecules: more molecules become in the bond forming or bond breaking distance. After the optimum temperature, the rate of reaction decrease due to denaturation of the enzyme (60-65 C). Effect of temperature on the velocity of an enzymecatalyzed reaction . 66 4- Effect of PH - Each enzyme has an optimum PH at which its activity is maximal • E.g. Optimum PH of pepsin = 1.5 - 2 • Optimum PH of pancreatic lipase = 7.5 - 8 • Optimum PH of salivary amylase = 6.8 pH What’s happening here?! 14 Change of PH above or below optimum PH decrease rate of enzyme action due to: 1- The enzyme activity depends on the ionization state of both enzyme and substrate which is affected by PH. 2- Marked change in PH will cause denaturation of enzyme. Effect of pH on enzyme activity 70 71 5- Concentration of coenzymes: In the conjugated enzymes that need coenzymes, the increase in the coenzyme concentration will increase the reaction rate. 6- Concentration of ion activators: The increase in metal ion activator increase the reaction rate Enzymes are activated by ions: 1- Chloride ion activate salivary amylase 2- Calcium ion activate thromobokinase enzyme 7- Effect of time: • In an enzymatic reaction, the rate of reaction is decreased by time. • This is due to: 1- The decrease in substrate concentration. 2- The accumulation of the end products. 3- The change in PH than optimum PH. 8- Presence of enzymes inhibitor: presence of enzyme inhibitor decreases or stops the enzyme activity. Enzyme inhibitors may be: 1- Competitive inhibitors. 2- Non competitive inhibitors. Don’t be inhibited! Ask Questions! 2007-2008 Activity 1- High Km enzyme: A) has high affinity to its substrate. B) Need high conc. of its substrate to reach its Vmax . C) has high max. velocity . D) like hexokinase. 2- Optimum pH: A) it is the same for all enzymes B) it is acidic for pepsin enzyme. C) at which the enzyme act at lowest rate D) the enzyme is stable under its marked changes 77  1- Enumerate the factors affecting enzyme action and discuss one of them . 2- Discuss the effect of substrate conc. on enzyme action. 3- Optimum temp. & optimum pH ( def. of each & explain how can they affect the enzyme action . 78 Thank you