Lock-and-Key Theory of Enzyme-Substrate Interaction - Essay Example

Discuss lock-and-key theory of enzyme-substrate interaction giving specific example to illustrate theory. Include the effects of substrate concentration, pH changes, temperature changes, and competitive inhibition. Enzymes are organic catalysts that speed up a chemical reaction that is occurring slowly. This they do by lowering the activation energy and once the reaction starts going, it runs on its own.  This is facilitated by the formation of enzyme-substrate complex. The main theory for the formation of enzyme-substrate-complex is called lock and key hypothesis which states that the substrate fits into an active site in the enzyme perfectly rather like a lock and key (Refer to Figure.2) Enzymes have active sites which interact with the substrate. The structure of the active site is unique for that respective substrate. Just as a uniquely shaped key will only fit in to and open a matching lock, so it is with enzymes and their substrates. The wrong key may fit in to the lock, but nothing can happen because the match of shapes is not correct. This fit is so specific that the change in a single hydrogen atom in a molecule makes it lose its specificity to a particular enzyme. This means that it may not bind to the specific site and even if it does, the enzyme will be unable to do anything chemically to it. The substrate always fits into the enzymes active site and the active site is always a fold or groove in the enzyme. Enzymes are always larger than the substrate and they are flexible so that they can move and fold around the substrate (Refer to Figure.1). This is facilitated by the weak bonds that hold the enzyme in its functional shape. The union between an enzyme and its substrate is called the enzyme -substrate complex. When a substrate is bound to the active site, particular chemical bonds of the substrate are weakened and the substrate bends. This lowers the activation energy to the point where the heat in the environment is sufficient to supply the activation energy to initiate the reaction (Chapter 7, Metabolism and Biochemistry). Effects of substrate concentration on reaction: If the amount of the enzyme is kept constant and the substrate concentration is then gradually increased, the reaction velocity will increase until it reaches a maximum. After this point, increases in substrate concentration will not increase the velocity. This means that when this maximum velocity had been reached, all of the available enzyme has been converted to the enzyme-substrate complex (Refer to Figure.3). Michaelis developed a set of mathematical expressions to calculate enzyme activity in terms of reaction speed from measurable laboratory data. The Michaelis constant Km is defined as the substrate concentration at 1/2 the maximum velocity. Michaelis constants have been determined for many of the commonly used enzymes. The size of Km tells us several things about a particular enzyme. A small Km indicates that the enzyme requires only a small amount of substrate to become saturated. Hence, the maximum velocity is reached at relatively low substrate concentrations. A large Km indicates the need for high substrate concentrations to achieve maximum reaction velocity. The substrate with the lowest Km upon which the enzyme acts as a catalyst is frequently assumed to be enzymes natural substrate, though this is not true for all enzymes (Worthington Biochemical Corporation). Effects of pH:   Each enzyme is effective at a certain range of pH specific to that enzyme beyond which the enzyme becomes unstable. The most favorable pH value, the point where the enzyme is most active is known as the optimum pH (Refer to Figure. 4). Enzymes are affected by even trivial changes in pH. Extremely high or low pH values result in complete loss of activity for most enzymes. For example, the optimum pH of stomach lipase is 4-5, that of pepsin is 1.6-3.2, whereas the optimum pH of trypsin is 7.8 - 8.7 (Worthington Biochemical Corporation). Effect of temperature: The rate of an enzyme-catalyzed reaction increases as the temperature is raised. A ten degree Centigrade rise in temperature will increase the activity of most enzymes by 50 to 100%.  In some cases, variations in reaction temperature as small as 1 or 2 degrees also may increase the activity of enzymes by 10 to 20%. But beyond a certain degree of temperature, the enzymes become denatured and hence the reaction rate decreases. In most cases, the reaction rate decreases after 40 degrees C (Refer to figure. 5). Over a period of time, enzymes get denatured even at moderate temperatures. Also, some enzymes lose their activity when frozen (Advanced Biology). Effect of inhibitors. Enzyme inhibitors are substances which alter the catalytic action of the enzyme and consequently slow down the reaction. In some cases, they may even stop catalysis. There are three types of enzyme inhibitors - competitive, non-competitive and substrate inhibitors. Competitive inhibition occurs when the substrate and a substance resembling the substrate are both added to the enzyme. The inhibitor occupies the active site instead of the substrate because it resembles the substrate. But it can not take the reaction forward and hence the reaction slows down or stops. Non-competitive inhibitors are substances which when added to the enzyme alter the enzyme in a way that it cannot accept the substrate. Substrate inhibition will sometimes occur when excessive amounts of substrate are present. This is thought to be due to the fact that there are so many substrate molecules competing for the active sites on the enzyme surfaces that they block the sites and prevent any other substrate molecules from occupying them. This causes the reaction rate to drop since the entire enzyme present is not being used (Advanced Biology). Figure. 1: The size of the enzyme is much larger than the substrate. (Adapted from Chapter 7, Metabolism and Biochemistry, Microbiology 101/102 Internet Text, Updated on 20th sep. 2001) Figure. 2: Lock and key theory of enzyme substrate interaction. (Adapted from Enzymes, Advanced Biology, Updated on 30th Dec. 2006) Figure. 3: Effect of substrate concentration on the rate of reaction. (Adapted from Enzymes, Advanced Biology, Updated on 30th Dec. 2006) Figure. 4: Effect of pH on the rate of reaction. (Adapted from Enzymes, Advanced Biology, Updated on 30th Dec. 2006) Figure. 5: Effect of temperature on the rate of reaction. (Adapted from Enzymes, Advanced Biology, Updated on 30th Dec. 2006) References Chapter 7, Metabolism and Biochemistry, Microbiology 101/102 Internet Text, Updated on 20th sep. 2001. Accessed on 25th July 2007. http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap7.html#LockKey Enzymes, Advanced Biology, Updated on 30th Dec. 2006. Accessed on 25th July 2007. http://advancedbiology.co.uk/index.php?option=com_content&task=view&id=20. Introduction to enzymes. Introduction to Biochemistry. Worthington Biochemical corporation. 24th July 2007. http://www.worthington- biochem.com/introBiochem/introEnzymes.html Read More