Chapter 7 Energy and Metabolism - Lebanese International University - EENG304 - PDF

Lebanese International University EENG304 CHAPTER SEVEN ENERGY AND METABOLISM Page 162  Enzymes Are Biological catalysts that increase the speed of a chemical reaction without being consumed by the reaction.(speed the metabolic reaction). Most enzymes ar...

Lebanese International University EENG304 CHAPTER SEVEN ENERGY AND METABOLISM Page 162  Enzymes Are Biological catalysts that increase the speed of a chemical reaction without being consumed by the reaction.(speed the metabolic reaction). Most enzymes are of protein nature Each enzyme acts on a specific substrate  An enzyme work by forming an enzyme-substrate complex The enzyme accomplishes its function by forming an unstable intermediate complex with the substrate, the substance on which it acts. When the enzyme–substrate complex, or ES complex, breaks up, the product is released; the original enzyme molecule is regenerated and is free to form a new ES complex.  An enzyme work by forming an enzyme-substrate complex Structure of an enzyme: Every enzyme is characterized by the presence of one or more active site (s), regions to which the substrate binds , to form the enzyme substrate complex  An enzyme work by forming an enzyme-substrate complex Structure of an enzyme: The shape of the enzyme does not seem exactly complementary to that of the substrate. The binding of the substrate to the enzyme molecule causes a change, known as induced fit, in the shape of the enzyme. Induced fit helps break and form bonds  An enzyme work by forming an enzyme-substrate complex Structure of an enzyme: Scientists usually name enzymes by adding the suffix -ase to the name of the substrate. The enzyme sucrase acts on sucrose The enzyme maltase acts on maltose Properties of enzymes 1- Enzymes are specific : acts as a key and a lock, for each substrate , there is a specific enzyme. The enzyme urease, which decomposes urea to ammonia and carbon dioxide, attacks no other substrate. The enzyme sucrase splits only sucrose and so on Properties of enzymes 2- Many enzymes require co-factors: Some enzymes consist only of proteins :Pepsin Other enzymes has two components: protein called Apoenzyme + chemical component called cofactor,( can be organic or inorganic)(magnesium ,calcium ions) both of them should be present to ensure enzyme function. Neither the apoenzyme nor the cofactor alone has catalytic activity; only when the two are combined does the enzyme function. An organic, nonpolypeptide compund that binds to the apoenzyme and serves as a cofactor is called a coenzyme Most vitamins, which are organic compounds that an organism requires in small amounts but cannot synthesize itself, are coenzymes or components of coenzymes Properties of enzymes 3-Each enzyme has optimal temperature: (at which the rate of the reaction is fastest) As the temperature increases the enzymatic activity increase until it reaches an optimal temperature. Enzymatic reactions occur slowly or not at all at low temperatures (enzymes are inactive at 0’C). In human enzymes, the favorable temperature is between 35-40’C In human, when the temperature is above 50’c the enzyme is destroyed (denatured). In heat-tolerant bacteria and archeae (thermophiles), the condition is different, the optimal temperature is between 70-80’C  Properties of enzymes 4-Each enzyme has an optimal PH: Most enzymes active at a narrow pH range and have an optimal pH at which rate if reaction is fastest. The optimal pH for most human enzyme is between 6 and 8 pepsin acts in acidic medium PH=2,it is a protein digesting enzyme in the stomach. Trypsin (secreted by pancreas) acts in basic medium pH=8, it is an enzyme splitting proteins in the small intestine. salivary amylase in neutral medium pH=7, acting on starch. The activity of an enzyme is altered by any alteration in pH  Evaluation In the figure below, which of the following matches CORRECTLY a. A is the substrate, E is the product b. A is the substrate, C is the enzyme c. E is the product, C is the enzyme d. All of the above  Activation Energy All reactions have activation energy: The energy required to break the existing bonds and start reaction Enzymes lower activation energy necessary to start a reaction  Reaction proceeds more quickly  enzymes speed up reaction rates  Enzyme and Substrate Concentration Enzymes have a regulated activity: If pH and temperature are constant (as in the cell), the rate of a reaction is affected by the enzyme concentration and by substrate concentration  Rate of reaction Substrate concentration (b) In this example, the rate of the reaction is measured at different substrate concentrations, and enzyme concentration, temperature, and pH are constant. If the substrate concentration is relatively low, the reaction rate is directly proportional to substrate concentration. However, higher substrate concentrations do not increase the reaction rate, because the enzymes become saturated with substrate.  Enzyme inhibition Reversible inhibition occurs when an inhibitor forms weak chemical bonds with the enzyme Reversible inhibition can be: – competitive (inhibitor competes with substrate for binding to the active site of the enzyme) – noncompetitive (inhibitor binds at a site other than the active site). Such inhibitor inactivates the enzyme by changing its shape so that the active site cannot bind the substrate Competitive Inhibition Noncompetitive Inhibition  Enzyme inhibition Irreversible inhibition – inhibitor combines with enzyme and permanently inactivates it. ( Example: mercury and lead)