1. Enzymes- biological catalysts Protein catalysts that can accelerate reaction rates as much as 10 17 – typical acceleration is 10 7 /10 8 over uncatalyzed reactions Protein catalysts that can accelerate reaction rates as much as 10 17 – typical acceleration is 10 7 /10 8 over uncatalyzed reactions RNA (ribozymes) can also catalyze self-splicing reactions – 1989 Chemistry Nobel Prize Sydney Altman and Tom Cech (CU-Boulder )

2. How much difference can an enzyme make ? Urea is one of the major breakdown products of proteins and one of the main ingredients of urine. The enzyme urease enhances the rate of urea hydrolysis at pH 8 and 20 o C by a factor of 10 14. If a given quantity of urease can completely hydrolyze a given quantity of urea in 5 minutes, how long, in years, would the reaction take in the absence of the urease enzyme? ANSWER: Almost a billion years (9.51 x 10 8 years) – that’s a long time to have to wait to pee!

3. Importance of Enzymes

4. IMPORTANCE OF ENZYMES Metabolic Regulation - Virtually all important reactions in cell are controlled by enzymes; Thus essentially all of the regulation of metabolism by the cell is based on controlling the concentration and activity levels of enzyme

5. IMPORTANCE OF ENZYMES Theoretical – some enzymes approach catalytic perfection, e.g. rxn rate is determined only by how fast substrates encounter enzymes; model for how to make reactions go fast. Perfect enzymes are rare and catalyze very important reactions in a cell such this key step in glycolysis – the main energy pathway in the cell. Chemists study for perfect enzymes to learn how to make the most efficient catalysts.

6. Drugs are often enzyme inhibitors: Penicillin inhibits enzyme responsible for catalyzing the formation of cell walls in bacteria

7. The Nature of Enzyme Catalysis ● An Enzyme provides a catalytic surface in a cleft or groove on the surface of an enzyme called the Active site.. B B A Active site A Juang RH (2004) BCbasics Active site – pocket or groove on the surface on the enzyme where catalysis occurs

8. Stereo Specificity These two triangles are not identical A The tetrahedral structure of carbon orbital has rigid steric strain which makes the basic building unit of protein conformation Juang RH (2004) BCbasics sp 3 Enzyme surface Enzymes are Highly specific –typically catalyze 1 reaction in the cell andcan distinguish between stereoisomer substrates;

9. Link to Intro Enzyme animation Link to Intro Enzyme animation Link to Lew Port Enzyme animations Link to Lew Port Enzyme animations Link to Lew Port Enzyme animations Link to Lew Port Enzyme animations

10. Figure 6.15 The catalytic cycle of an enzyme

11. Models of Enzyme Action LOCK AND KEY Enzyme structure is rigid. Substrate is exact complement to active site shape of enzyme INDUCED FIT Enzyme structure changes upon binding of substrate - Enzyme structure is flexible- currently accepted model. - Enzyme binds substrate loosely, transition state tightly

12. Conformational changes in yeast hexokinase on binding glucose.

13. Figure 6.14 The induced fit between an enzyme and its substrate

14. Link to enzyme animation of induced fit Link to induced fit 2 animation Link to hexokinase induced fit

15. Specificity – How does the enzyme discriminate between different substrates?  Consider the example of serine proteases, a family of enzymes that have a serine in their active site and specifically cleave proteins at particular amino acid sequences. What are the features of the active sites make one enzyme specific for each different sequence?

16. Specificity of Ser-Protease Family COO - C Asp COO - C Asp Active Site TrypsinChymotrypsinElastase cut at Lys, Argcut at Trp, Phe, Tyrcut at Ala, Gly Non-polar pocket Deep and negatively charged pocket Shallow and non-polar pocket O O –C–N–C–C–N– C NH 3 + O O –C–N–C–C–N– C O O –C–N–C–C–N– Juang RH (2004) BCbasics

17. Specificity – How does the enzyme discriminate between different substrates? In other words, how does the active site only bind the correct substrates? In other words, how does the active site only bind the correct substrates? Key: Active site is complementary to its substrate in size, shape and distribution of charge Key: Active site is complementary to its substrate in size, shape and distribution of charge Link to Interactive Biochemistry Link to Interactive Biochemistry Link to Interactive Biochemistry Link to Interactive Biochemistry