Enzymes L. Scheffler 1

Enzymes Enzymes are catalysts. They increase the speed of a chemical reaction without themselves undergoing any permanent chemical change. Enzymes are neither used up in the reaction, nor do they appear as reaction products. 2

Enzymes Enzymes are protein molecules that catalyze biochemical reactions The substance on which the enzyme acts is known as the substrate 3 Models of some proteins – tertiary and quaternary structure.

Discovery of Enzymes 1825 Jon Jakob Berzelius discovered the catalytic effect of enzymes James Sumner isolated the first enzyme in pure form Northrup and Stanley together with Sumner were awarded the Nobel prize for the isolation of the enzyme pepsin. 4 Berzelius Sumner Northrup Stanley

Enzyme Characteristics High molecular weight proteins with masses ranging from 10,000 to as much as 2,000,000 grams per mole Substrate specific catalysts Highly efficient, increasing reaction rates by a factor as high as

Enzyme Nomenclature The earliest enzymes that were discovered have common names: i.e. Pepsin, Renin, Trypsin, Pancreatin i.e. Pepsin, Renin, Trypsin, Pancreatin The enzyme name for most other enzymes ends in “ase” The enzyme name indicates the substrate acted upon and the type of reaction that it catalyzes 6

Enzyme Names Examples of Enzyme Names Glutamic Oxaloacetic Transaminase (GOT) L-aspartate: 2-oxoglutarate aminotransferase. Enzyme names tend to be long and complicated. They are often abbreviated with acronyms Enzyme names tend to be long and complicated. They are often abbreviated with acronyms 7

Types of Enzyme Specificity Enzyme specificity may be characterized as 1. Absolute: The enzyme catalyzes on one reaction 2. Group Specific: The enzyme acts only on molecules having a particular functional group 3. Linkage Specific: The enzyme acts on a particular type of chemical bond 4. Stereo-chemical Specific: The enzyme acts on a particular stereo or optical isomer 8

Enzyme Specificity The action of an enzyme depends primarily on the tertiary and quaternary structure of the protein that constitutes the enzyme. The part of the enzyme structure that acts on the substrate is called the active site. The active site is a groove or pocket in the enzyme structure where the substrate can bind. 9

Cofactors Cofactors are other compounds or ions that enzymes require before their catalytic activity can occur. The protein portion of the enzyme is referred to as the apoenzyme. The enzyme plus the cofactor is known as a holoenzyme. 10

Cofactors Cofactors may be one of three types 1. Coenzyme: A non protein organic substance that is loosely attached to the enzyme 2. Prosthetic Group: A non protein organic substance that is firmly attached to the enzyme 3. Metal ion activators: K +, Fe 2+, Fe 3+, Cu 2+, Co 2+, Zn 2+, Mn 2+, Mg 2+, Ca 2+, or Mo 2+, 11

Types of Cofactors Enzymes have varying degrees of specificity. One cofactor may serve many different enzymes. 12

Enzymes and Cofactors 13

Enzyme Mechanisms Enzymes lower the activation energy for reactions and shorten the path from reactants to products Enzymes lower the activation energy for reactions and shorten the path from reactants to products 14

Enzyme Mechanisms The basic enzyme reaction can be represented as follows: The basic enzyme reaction can be represented as follows: E + S  ES  E + P Enzyme Substrate Enzyme substrate Enzyme Product(s) complex The enzyme binds with the substrate to form the Enzyme-Substrate Complex. Then the substrate is released as the product(s). 15

Enzyme Mechanisms Diagram of the action of the enzyme sucrase on sucrose. E+S  ES  E+P 16

Enzyme Mechanics An enzyme-substrate complex forms when the enzyme’s active site binds with the substrate like a key fitting a lock. An enzyme-substrate complex forms when the enzyme’s active site binds with the substrate like a key fitting a lock. The shape of the enzyme must match the shape of the substrate. Enzymes are therefore very specific; they will only function correctly if the shape of the substrate matches the active site. 17

Induced Fit Theory 18

Induced Fit Theory The substrate molecule normally does not fit exactly in the active site. This induces a change in the enzymes conformation (shape) to make a closer fit. In reactions that involve breaking bonds, the inexact fit puts stress on certain bonds of the substrate. This lowers the amount of energy needed to break them. 19

Induced Fit Theory The enzyme does not actually form a chemical bond with the substrate. After the reaction, the products are released and the enzyme returns to its normal shape. Because the enzyme does not form chemical bonds with the substrate, it remains unchanged. The enzyme molecule can be reused repeatedly Only a small amount of enzyme is needed 20

Enzymes and Reaction Rates Factors that influence reaction rates of Enzyme catalyzed reactions include Factors that influence reaction rates of Enzyme catalyzed reactions include 1. Enzyme and substrate concentrations 2. Temperature 3. pH 21

Enzymes and Reaction Rates At low concentrations, an increase in substrate concentration increases the rate because there are many active sites available to be occupied At high substrate concentrations the reaction rate levels off because most of the active sites are occupied 22

Substrate concentration The maximum velocity of a reaction is reached when the active sites are almost continuously filled. Increased substrate concentration after this point will not increase the rate. Increased substrate concentration after this point will not increase the rate. V max is the maximum reaction rate 23

Substrate concentration V max is the maximum reaction rate The Michaelis- Menton constant, K m is the substrate concentration when the rate is ½ V max K m for a particular enzyme with a particular substrate is always the same K m for a particular enzyme with a particular substrate is always the same 24

Effect of Temperature Higher temperature increases the number of effective collisions and therefore increases the rate of a reaction. Above a certain temperature, the rate begins to decline because the enzyme protein begins to denature 25

Effect of pH Each enzyme has an optimal pH at which it is most efficient A change in pH can alter the ionization of the R groups of the amino acids. When the charges on the amino acids change, hydrogen bonding within the protein molecule change and the molecule changes shape. The new shape may not be effective. Pepsin is most efficient at pH2.5-3 while Trypsin is efficient at a much higher pH 26

Inhibitors Enzyme inhibitors are substances which alter the catalytic action of the enzyme and consequently slow down or stop catalysis. There are three common types of enzyme inhibition 1. Competitive inhibitors 2. Non-competitive inhibitors 3. Substrate inhibition. 27

Competitive Inhibitors Competitive inhibition occurs when the substrate and a substance resembling the substrate are both added to the enzyme. The inhibitor blocks the active site on the enzyme stopping its catalytic action 28

Non-competitive Inhibitors Non-competitive inhibitors deactivate the active site of the enzyme. They alter the enzyme so that it can no longer bind to the substrate 29

Effect of inhibitors on the Reaction Rate For non-competitive inhibitors Vmax is lower but K m is the same. For competitive inhibitors, V max is the same but K m is increased. 30

Substrate Inhibitors Substrate inhibition occurs when excessive amounts of substrate are present. Additional substrate sometimes interferes with the ability of substrate molecules to find active sites on enzymes In these cases the reaction velocity decreases after the maximum velocity has been reached. 31