Lecture #5 V2 - Enzymes

Ogan Gurel, MD Biology 301 Lecture #5 Roosevelt University

Biology 301 Cellular and Molecular Biology Spring 2002

Lecture # 5: Enzymes

Ogan Gurel, MD Biology 301 Lecture #5 Roosevelt University

Overall outline  Principles of Chemical Kinetics  Enzyme Kinetics  Enzyme Regulation

Ogan Gurel, MD Biology 301 Lecture #5 Roosevelt University

Principles of Chemical Kinetics  Rate Laws  Reaction profile  Catalysis

Rate Laws The rate law is a function of 1. the rate constant and 2. the concentrations of the reactants.

Ogan Gurel, MD Biology 301 Lecture #5 Roosevelt University

Reaction profile

Ogan Gurel, MD Biology 301 Lecture #5 Roosevelt University

Catalysis

Ogan Gurel, MD Biology 301 Lecture #5 Roosevelt University

Ogan Gurel, MD Biology 301 Lecture #5 Roosevelt University

Enzyme Kinetics  Michaelis-Menten Model  Enzyme-subtrate interactions  Enzyme reaction cycle  External effects on enzyme efficiency

Michaelis-Menten Model

Ogan Gurel, MD Biology 301 Lecture #5 Roosevelt University

 Enzymes are used for the catalysis and control of cell biochemistry.  The Michaelis-Menten model is a general description of simple enzyme kinetics.  At low [S] the rate is first-order with respect to substrate; at high [S] the rate is zero-order (saturated kinetics).

  Using some simple algebra and approximations show how Michaelis­Menten kinetics reduces  to first­order at low substrate concentrations and zero­order at high substrate concentrations. 

Enzyme-subtrate interactions

Ogan Gurel, MD Biology 301 Lecture #5 Roosevelt University

  Hexokinase catalyzes the first step in glycolysis.  It is found in all cells including the liver; its  Km is about 30 µ M.  Glucokinase catalyzes the same reaction, is found exclusively in the liver  and its Km is 10mM. How are the two enzymes different and why are there two of them? 

Enzyme reaction cycle

Ogan Gurel, MD Biology 301 Lecture #5 Roosevelt University

External effects on enzyme efficiency

Ogan Gurel, MD Biology 301 Lecture #5 Roosevelt University

Ogan Gurel, MD Biology 301 Lecture #5 Roosevelt University

Enzyme Regulation  Feedback (negative) inhibition  Types of inhibition  Inhibition kinetics

Feedback (negative) inhibition

Ogan Gurel, MD Biology 301 Lecture #5 Roosevelt University

 Feedback inhibition is a very common regulatory mechanism in biology.  Negative feedback systems are stable systems.

  Negative feedback systems are not just confined to enzyme pathways.  Show how some of the  pituitary­endocrine  feedback  loops  follow  the  same  paradigm.    Another  example  is  the  relationship  between  acid  secretion  and  gastrin  secretion  by  parietal  cells  and  antral  cells  in  the stomach. 

Types of inhibition

Ogan Gurel, MD Biology 301 Lecture #5 Roosevelt University

Inhibition kinetics

Ogan Gurel, MD Biology 301 Lecture #5 Roosevelt University

 Inhibitors can alter the Km or Vmax of an enzyme.  Competitive inhibition changes the Km  Noncompetitive inhibition changes the Vmax.  Inhibitors can act either noncovalently or covalently.  Positive regulation is also possible.

  1.    Show  how  the  Lineweaver­Burke  plot  converts  the  hyperbolic  Michaelis­Menten  graph  into a linear one.  Why would one use these linear plots?   

2.  How does aspirin work?  What enzyme does it act on?  Is it an example of covalent or non­ covalent modification?  Does it work via competitive or non­competitive inhibition?  Describe  its effects on platelet function; what are the clinical implications of its mode of action? 

 

3.  How does the  antibiotic  penicillin work?  Is it a  competitive  or non­competitive inhibitor?   Why is penicillin specific to bacteria?