Transcript Ch.21Pt.5
Heavy metal poisoning is an example of noncompetitive inhibitor of an enzyme. What type of bonds are affected by heavy metal ions? Irreversible Inhibition Covalent bonds make a permanent change to enzyme’s active site shape. e.g. organophosphate insecticides & nerve gases. Enzyme is permanently inhibited. Sarin nerve gas Foreign vs.Normal Inhibition • Foreign: reversible & irreversible – Caused by the influence of agents foreign to the normal cells • Normal: allosteric enzymes – Regulation by “normal” cell components Allosteric regulation of Enzyme Activity Allosteric enzymes have 2 or more protein chains (o4 structure) And 2 kinds of binding sites: 1. Substrate binding site = active site 2. Regulator binding site - molecule binding here alters structure of active site Reversible and rapid in action Regulators bind to a specific site on the enzyme called the allosteric site positive regulator negative regulator Feedback control is a common mechanism for allosteric activity enzyme 1 enzyme 2 enzyme 3 A ----> B ----> C ----> D Product D is a negative regulator of enzyme 1, so... Feedback & Feedforward control: Activation or inhibition of the 1st reaction in a reaction sequence is controlled by a product of the reaction sequence. Feedback animation In positive feedforward, earlier reactants in a metabolic reaction sequence feed-forward positively on later steps If A is accumulating, speed up down stream products to use “A” up. The enzyme phosphofructokinase (PFK) is regulated by: • High concentrations of ATP, which inhibit PFK • High concentrations of ADP, which stimulate PFK • High concentrations of AMP, which stimulate PFK • High concentrations of citrate, which inhibit PFK ATP and citrate are allosteric negative regulators ADP and AMP are allosteric positive regulators. DNA Digestion lab animation Restriction Fragment Lengths Polymorphism Early genetic splicing experiment Polymerase Chain Reaction Another mechanism of regulating enzyme activity is to keep dangerous enzymes in compartments where access to their substrates is limited. e.g. proteolysis of cell proteins by enzymes is controlled by keeping these enzymes within lysosomes Or, take a proteolytic enzyme (dangerous because they break peptide bonds) and keep it in an inactive form until needed. The inactive form of a proteolytic enzyme is called a proenzyme or a zymogen Digestive and blood clotting enzymes are kept this way until needed, then they are activated! Covalent modification: enzyme activity controlled by other enzymes or chemicals Enzyme exists in 2 states: modified (e.g. phosphorylated) = “activated” unmodified (e.g. unphosphorylated) = “inactivated” Here the proteolytic enzyme zymogen form is activated by HCl acid in the stomach: e.g. pepsinogen is converted to pepsin when needed. Conversion of zymogen to proteolytic enzyme often involves removal of a polypeptide chain segment from the zymogen structure. Bacterial infection (pathogens) account for many severe diseases world-wide. Examples: Tuberculosis and Leprosy Staph and Strep Syphilis Bubonic plague Cholera and typhoid fever Gastroenteritis Urinary tract and Wound infections Many sexually transmitted diseases Much research on fighting bacterial infections revolves around inactivating bacterial enzymes. Earliest “antibiotics” were the Sulfa drugs Antibiotics kill or stop the growth of bacteria. Sulfanilamide is a structural mimic of PABA (p-aminobenzoic acid) which is needed by bacteria to make a coenzyme called Folic acid. Humans get Folic acid from diet, don’t use PABA. Structures of selected antibiotic sulfa drugs in use today PABA Penicillins Discovered accidentally by Fleming in 1928 Developed ten years later into penicillin products Inhibit enzyme: transpeptidase used to strengthen polysaccharide strands in gram + bacterial cell walls. Structures of selected penicillins in use today The selective binding of penicillin to the active site of transpeptidase. Disorders and diseases frequently “leak” organ enzymes into blood and urine. Doctors utilize this for diagnoses of various ailments.