BBYCT-137 PLANT PHYSIOLOGY Indira Gandhi AND METABOLISM National Open University School of Sciences VOL 2 PLANT PHYSIOLOGY AND METABOLISM BLOCK 3 ENZYMES AND RESPIRATION 5 BLOCK 4 NITROGEN METABOLISM AND PLANT GROWTH REGULATORS 109 PLANT PHYSIOLOGY AND METABOLISM This volume contains two blocks: Block 3 and block4 .We begin the Block III with Unit 9 which gives a description of the biological cayalsts – the enzymes that catalyse the metabolic reactions in any organism. Enzymes enhance the rate of specific metabolic reactions. To maintain the steady state in an organism, metabolic activities are regulated. Metabolic regulation is brought about by regulating the enzyme activity. You will also understand the mechanism of enzyme catalysis and the concept of Km. Unit 10 describes one of the most fundamental characteristics of all living organisms – Respiration. You will learn the major characteristics of the respiratory process under aerobic and anaerobic conditions and the concept of RQ. In addition, we have also described the ultrastructure of the ‘powerhouses’ viz., the mitochondria. The mechanism of the respiratory process has been discussed in details in Unit 11. Respiratory pathways of glycolysis, Krebs Cycle, and different shuttles operative across the mitochondrial inner membrane have been described along with the alternative pathways, like PPP. The concepts of electron transfer chain and ATP synthesis have also been discussed in the light of recent findings. Block IV comprises of five units. Unit 12 of this block deals with the nitrogen metabolism. Plants require nitrogen in relatively large amounts as it is a constituent of amino acids, proteins and nucleic acids Although molecular nitrogen is readily available in the atmosphere, it cannot be utilized by higher plants directly as they lack the gene for coding of the enzyme dinitrogenase. Instead, they depend on the nitrogen-fixing organisms. The mechanism of biological nitrogen fixation has been discussed in Unit 13. We will explain in detail the biochemistry and mechanism of nitrogen fixation by some “gifted” prokaryotes along with the metabolism of nitrate and ammonia. Sources of nitrogen and mechanism of its assimilation have been discussed in this unit. Unit 14 deals with the discovery, role and mechanism of action of plant growth regulators, along with their role in agriculture. Plant hormones have multiple effects on growth and particularly developmental processes are regulated by more than one hormone. Unit 15 describes the recent discoveries made in the field of photoperiodism and vernalization. In the last unit of this course viz.,. Unit 16, we have discussed about various types of stress conditions that plants have to face, and their varied responses to cope up with them. The possibility of manipulating the plants by genetic engineering to survive under stress situations is also explored. Objectives After reading this volume you would be able to : discuss the characteristics of enzymes and their activation energy; describe the enzyme-substrate complex and the Koshland’s Induced Fit Model; discuss the different types of enzyme inhibitors; discuss respiratory pathways of glycolysis, Krebs Cycle, and different shuttles operative across the mitochondrial inner membrane ,along with the alternative pathways, like PPP and appreciate the concepts of electron transfer chain and ATP synthesis; 3 describe the sources of nitrogen, the biochemistry of assimilation of nitrate, nitrite and ammonium ions by plants and describe biological nitrogen fixation by the ‘gifted’ species along with the nodule formation; explain the regulation of plant growth, development and differentiation by the growth hormones, and recent developments in the field of photoperiodism and vernalization; and describe the varied responses of plants under different stress conditions and the ways to adapt to them. 4 BBYCT-137 PLANT PHYSIOLOGY Indira Gandhi AND METABOLISM National Open University School of Sciences Block 3 ENZYMES AND RESPIRATION UNIT 9 Enzymes 9 UNIT 10 Respiration 42 UNIT 11 Respiration Mechanism 60 Course Design Committee Prof. G.C. Srivastava (Retd.) School of Sciences, IGNOU Former Head, Prof. M.S. Nathawat, Director, (Ex.) Department of Physiology, IARI, Pusa, Prof. Amrita Nigam New Delhi-110012 Prof. Jaswant Sokhi (Retd.) Prof. Vijay Paul Prof. Bano Saidullah (Retd.) Principal Scientist, Prof. Neera Kapoor Division of Plant Physiology IARI, Pusa, Dr. Eklavya Chauhan (Sr. Consultant) New Delhi-110012 Block Preparation Team Prof. Amrita Nigam Editor SOS, IGNOU, New Delhi-110068 Prof. G.C. Srivastava (Retd.) Dr. Eklavya Chauhan Former Head, Sr. Consultant, Department of Physiology, SOS, IGNOU, New Delhi-110068 IARI, Pusa, New Delhi-110012 Course Coordinator: Prof. Amrita Nigam Production Mr. Hemant Kumar SO(P), MPDD, IGNOU Acknowledgement • Dr. Kumkum Chaturvedi for giving useful inputs. • Sh. Manoj Kumar, Assistant for word processing and CRC preparation. March, 2021 Indira Gandhi National Open University, 2021 ISBN : All rights reserved. No part of this work may be reproduced in any form, by mimeograph or any other means, without permission in writing from Indira Gandhi National Open University. Further information on Indira Gandhi National Open University courses may be obtained from the University’s office at Maidan Garhi, New Delhi-110 068 or IGNOU website www.ignou.ac.in. Printed and published on behalf of Indira Gandhi National Open University, New Delhi by the Registrar, MPDD, IGNOU. Printed at BLOCK 3 : ENZYMES AND RESPIRATION You have read about the processes and mechanisms of photosynthesis operative in plants for the manufacture of food in Units 5, 6 and 7 of Block II. Also, the various pathways for translocation of these photo assimilates have been described in Unit 8. Block III is devoted to the mechanisms of utilization of these photo assimilates by the complex machinery of cellular respiration. This block comprises three Units 9,10 and 11. Unit 9 introduces you to the fascinating world of biocatalysts that catalyse biochemical reactions needed for the life processes. This unit deals with the aspects of discovery, structure and modern classifications of enzymes. The concept of activation energy of enzymes has been discussed in detail, along with a description of isozymes, abzymes, ribozymes and deoxy ribozymes. Regulation of enzyme action is explained through allosteric enzymes highlighting negative feedback mechanisms. Modern aspects of active site, mechanism of enzyme action and various types of enzyme inhibitions also find a detailed treatment in this Unit. Units 10 and 11 are devoted exclusively to respiration and its mechanism. Unit 10 introduces you to the nature of the respiratory process comparing it with combustion, and deals with the nature of respiratory substrates, and the concept of RQ (respiratory quotient). In addition, the processes of aerobic and anaerobic respiration have also been compared for their energy outputs. The location of the respiratory reactions within the powerhouse of a cell – the mitochondria, have been explained with the help of its ultrastructural features. A detailed description of the respiration mechanism follows in Unit 11, which forms an in- depth study of glycolysis. The preparatory phase and the payoff phase of glycolysis have been dealt with in details along with a balance sheet of ATP yield. Various steps of Krebs cycle along with its regulation are also discussed. The alternative pathways like Pentose Phosphate Pathway (PPP) have also been explained. In addition, the shuttle-mechanisms involved in the utilization of cytosolic NADH have also been described. This will make you familiar with various options present in the cellular machinery to utilize NADH molecules in the electron transport chain. The chemiosmotic model and ATP synthesis has been explained in the light of recent discoveries in the field. Objectives After studying the block, you will be able to: describe the role of biocatalysts in cell metabolism; appreciate the classification and nomenclature of enzymes; describe the mechanism of action and regulation of enzyme activity; describe the main features of anaerobic and aerobic respiration; familiarize with the concept of respiratory quotient of various substrates; get in-depth information of glycolysis including its variations in plants; describe the mechanism of Krebs cycle, electron transport chain and current knowledge on ATP synthesis, along with net yield in aerobic respiration; and appreciate the alternative shuttle mechanisms operative across the mitochondrial membrane. 7 Unit 9 Enzymes UNIT 9 ENZYMESENZYMESENZYMES StructureStructureStructure 9.1 Introduction 9.6 Enzyme Inhibition Objectives Irreversible Inhibitors 9.2 Structure and Function Reversible Inhibitors Discovery Uncompetitive Inhibition Structure 9.7 Regulation of Enzyme Prosthetic Group and Activity Coenzymes Feedback Regulation 9.3 Classification and Allosteric Enzyme Regulation Nomenclature Covalent Modulation 9.4 Properties of Enzymes 9.8 Factors affecting the Rate Activation Energy of Enzyme Action Isoenzymes Effect of Temperature Ribozymes Effect of pH Deoxyribozymes Effect of Enzyme Concentration Abzymes Effect of Substrate Allosteric Enzymes Concentration-Km 9.5 Mechanism of Enzyme 9.9 Summary Action 9.10 Terminal Questions Concept of Active site Enzyme-Substrate Complex 9.11 Answers Lock and Key Hypothesis Induced Fit Model 9.1 INTRODUCTION You have studied about the structure of cell membranes and the process of transport