Enzymes Part 3-4 (1) PDF - Ross University

Cellular Biology & Homeostasis ENZYMES - Part 3 VP Summer 2023 Clara Camargo, DVM LEARNING OBJECTIVES 1. Explain cofactors (coenzyme, cosubstrate, prosthetic group) and give some examples 2. Understand what is a zymogen and its physiological relevance 3. Define apoptosis and generally explain how...

Cellular Biology & Homeostasis ENZYMES - Part 3 VP Summer 2023 Clara Camargo, DVM LEARNING OBJECTIVES 1. Explain cofactors (coenzyme, cosubstrate, prosthetic group) and give some examples 2. Understand what is a zymogen and its physiological relevance 3. Define apoptosis and generally explain how it works 4. Describe the mechanisms of enzyme activity regulation Enzymes Part 3 COFACTORS The catalytic activity of many enzymes depends on the presence of small Cofactor Enzyme Organic molecules Thiamine pyrophosphate Pyruvate dehydrogenase Flavin adenine dinucleotide (FADH) Monoamine oxidase Nicotinamide adenine dinucleotide (NADH) Lactate dehydrogenase • small non-protein molecules Pyridoxal phosphate Glycogen phosphorylase • “helpers” Coenzyme A (CoA) Acetyl CoA carboxylase Biotin Pyruvate carboxylase 5′-Deoxyadenosyl cobalamin Methylmalonyl mutase Tetrahydrofolate (THF) Thymidylate synthase molecules: cofactors or coenzymes Metal Cofactors can be subdivided into two Zn 2+ Carbonic anhydrase Zn 2+ Carboxypeptidase 2+ EcoRV 2+ Hexokinase Mg groups: Mg Ni Urease 1. Inorganic metals Mo Nitrate reductase Se Glutathione peroxidase 2. Small organic molecules (coenzymes) Mn 2+ K + 2+ Superoxide dismutase Propionyl CoA carboxylase COFACTORS Enzymes Part 3 Cofactors Enzymes can exist in inactive forms (apoenzyme) and later be converted to active forms (holoenzyme) → with the help of a cofactor.  If the cofactor is a small organic molecule  coenzyme Apoenzyme → Coenzyme binding → Holoenzyme  Tightly bound (covalent bonds) → prosthetic groups (Heme group is an example)  Loosely bond → co-substrates bind to and are released from the enzyme just as substrates and products are. Many enzymes acquire full enzymatic activity after they acquire the right folding (recall the posttranslational modifications). COENZYMES • Coenzymes are often derived from vitamins. • Can be either tightly or loosely bound to the enzyme. • Are associated with the enzyme’s active site that assists with their catalytic function. • Vitamins cannot be synthesized by humans and most animals; and must be supplied from the diet. • Many vitamins are essential components of enzymes or provide those as coenzymes Enzymes Part 3 COENZYMES - Biotin • Biotin is a water-soluble vitamin Enzymes Part 3 • Attached to distinct lysine residues in and serves as a coenzyme for five histones, affecting chromatin structure carboxylases in humans. and mediating gene regulation (epigenetic modification) • Forms transient covalent bond to the carboxyl group (COO- group) transferring it from one molecule to another. (i.e, pyruvate carboxylase) ZYMOGENS • Some enzymes are synthesized as inactive precursors that are activated by proteolytic cleavage of one or a few specific peptide bonds. • The inactive precursor is called a zymogen (or a proenzyme). • The biochemical change usually occurs in the Golgi apparatus. Enzymes Part 3 ZYMOGENS Enzymes Part 3 Proteolytic cleavage = activation Unlike allosteric control and reversible For the cleavage, energy (ATP) is not covalent modification, proteolytic cleavage needed. occurs just once in the life of an enzyme molecule, the process is irreversible. Proteins (enzymes) located outside cells can be activated by proteolytic cleavage. The digestive enzymes that hydrolyze Site of synthesis Proenzyme/ Zymogen Active enzyme proteins are synthesized as zymOGEN or Stomach Pepsinogen Pepsin Pancreas Chymotrypsinogen Chymotrypsin Pancreas Trypsinogen Trypsin Pancreas Procarboxypeptidase Carboxypeptidase Pancreas Proelastase Elastase PROenzyme in the stomach and pancreas. Zymogens Cascade of events regulating digestive enzymes Proenteropeptidase Proteins Enteropeptidase Trypsinogen Chymotrypsinogen Proelastase Procarboxypeptidases A and B Pancreatic prolipase Pepsin Trypsin Large peptides Gastric and pancreatic Chymotrypsin Elastase zymogens Carboxypeptidases A and B Pancreatic lipase Small peptides Aminopeptidases Dipeptidases Tripeptidases Free amino acids + Triglycerides ENZYME ACTIVATION BY PROTEOLYSIS Zymogens Specific proteolysis is common in cellular physiology.  The digestive enzymes that hydrolyze  Many protein hormones are also proteins (proteases) are synthesized as synthesized as inactive precursors. zymogens in the stomach and pancreas. o i.e.; proinsulin → insulin (proteolytic o Pepsinogen → pepsin (stomach) o Trypsinogen → Trypsin (pancreas)  The pancreas secretes zymogens partly to prevent the enzymes from digesting removal of a peptide) • Accidental activation of zymogens can happen when the secretion duct in the pancreas is blocked by a proteins in the cells in which they are gallstone resulting in acute synthesized. pancreatitis. DIETARY PROTEIN DIGESTION BY PROTEASES Zymogens - Proteases  Pepsinogens are converted in the gastric lumen by gastric acid to pepsins. o Once this reaction begins, pepsins can autocatalyze the conversion of pepsinogens to pepsins. 2 steps to regulate enzyme activation Gastric gland SECRETION OF ZYMOGEN GRANULE BY CELLS OF THE PANCREAS • Darker-staining cells form clusters called acini, which are arranged in lobes separated by a thin fibrous barrier • The secretory cells of each acinus surround a small intercalated duct • Secretory function, these cells have many small granules of zymogens that are visible Zymogens ZYMOGEN AND COENZYME - COAGULATION CASCADE FYI Blood clotting is mediated by a cascade of proteolytic activations that ensures a rapid and amplified response to trauma. VITAMIN K → coenzyme participating in the synthesis of blood coagulation factors II, VII, IX and X WARFARIN (reduces blood clot formation) → Compete irreversibly with a liver enzyme (epoxy reductase complex) → Depletion of active Vit K ZYMOGEN - APOPTOSIS Programmed cell death, or apoptosis, is mediated by proteolytic enzymes called caspases, which are synthesized in precursor form as procaspases. Unlike necrosis (traumatic cell death), apoptosis is highly regulated. When activated, caspases function to cause cell death in most multicellular organisms. It produces special cell fragments (apoptotic bodies), which are cleared by macrophages. Zymogens ZYMOGEN - APOPTOSIS Zymogens Initiator caspases  executioner caspases kill the cell by degrading proteins indiscriminately Cannot stop once it has begun. FYI Can be initiated through: • intrinsic pathway • extrinsic pathway  Both pathways use caspases (proteases) APOPTOSIS - OTHER ROLES OF ZYMOGENS IN BIOLOGY Zymogens • Many developmental processes are controlled by the activation of zymogens. • For example, in the metamorphosis of a tadpole into a frog, large amounts of collagen are resorbed from the tail. • The conversion of procollagenase into collagenase (the active protease) is precisely timed in these remodeling processes.  Likewise, much collagen is broken down in a mammalian uterus after delivery. From: Alberts. Mol. Biol. of the Cell MECHANISMS FOR REGULATING ENZYME ACTIVITY Some enzymes with specialized regulatory functions can be regulated when physiologic conditions change, by: • Regulation of allosteric enzymes • Regulation of enzymes by covalent modification • Induction and repression of enzyme synthesis/degradation An organism must coordinate its different metabolic processes by:  Regulating the reaction velocity of enzymes depending on the substrate concentration (Km range) (increased substrate → increased reaction rate) Enzymes Part 3 MECHANISMS FOR REGULATING ENZYME ACTIVITY Enzymes Part 3 • Cofactors/cosubstrates (can be reversible) • Zymogens/proenzymes (irreversible activation) • Enzyme activation cascades (irreversible once started) REGULATION OF ENZYME ACTIVITY Enzymes Regulation ALLOSTERIC ENZYMES • Allosteric enzymes change shape upon Increase or decrease affinity for substrate binding of an effector. • Effectors (modifiers/regulators) bind noncovalently at a site other than the active site.  Altering the affinity of the enzyme for its substrate OR  Modifying the maximal catalytic activity of the enzyme https://www.youtube.com/watch?v=ApKM-IkSElY REGULATION OF ENZYME ACTIVITY ALLOSTERIC ENZYMES Positive effector Negative effector • Modify maximal catalytic velocity (Vmax) • Effectors can influence affinity of enzyme for its substrate (K0.5) • Both Enzymes Regulation Enzymes Regulation REGULATION OF ENZYME ACTIVITY ALLOSTERIC ENZYMES • Homotropic effectors: when the substrate itself serves as an effector • Most allosteric substrates function as positive homotropic effectors: the presence of the substrate molecule at one site of the enzyme enhances the catalytic properties of the other substrate-binding sites. • Hemoglobin is a homotropic allosteric protein From: Harvey. Biochemistry REGULATION OF ENZYME ACTIVITY Enzymes Regulation ALLOSTERIC ENZYMES • Heterotropic effectors: effector different from the substrate • Classical example is a feedback inhibition of a metabolic pathway (end-product inhibition) Enzymes Regulation REGULATION OF ENZYME ACTIVITY COVALENT MODIFICATION • Covalent modifications: usually addition or removal of phosphate groups from specific amino acids of the enzyme (Ser, Tyr, Thr). • Phosphorylation reactions are catalyzed by kinases using ATP as a phosphate donor. • The phosphorylated protein may be more or less active. i.e., glycogen metabolism REGULATION OF ENZYME ACTIVITY Enzymes Regulation ENZYME SYNTHESIS • Cells can alter the rates of enzymes degradation or synthesis. • Enzymes subject to synthesis regulation are often those that are needed at only one stage of development or under certain physiological conditions. • Induction or repression of protein synthesis are slow (hours to days), compared with allosteric or covalent regulation of enzyme activity. REGULATION OF ENZYME ACTIVITY - Summary • Regulation of allosteric enzymes • Covalent modification of enzymes • Induction or repression of enzymes synthesis Enzymes Regulation HAPPY STUDYING Clara Camargo, DVM [email protected] ©2021 Ross University School of Veterinary Medicine. 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