Chapter 8 - Metabolism Mechanisms for Obtaining and Using Energy from the Environment

Metabolism The Sum of all Energy Transformations Within a Cell

Metabolic Reactions are Made Possible by Enzymes

Enzymes Proteins Which Act as Catalysts to Allow Biochemical Reactions to take place under Conditions Where they Ordinarily Would Not

Enzymes are Specific for the Reactions they Catalyze and the Substrates they Act Upon

Enzymes Have Multiple Levels of Structure and May Require Cofactors and Coenzymes

Induced-Fit Model of Enzyme Function Substrate(s) fit into active site Bonds are strained Reactive portions of molecule(s) are brought together Bond transfers occur Products are released

Enzyme-Substrate Interactions

Enzyme Cofactors Mineral ions frequently required for correct enzyme shape Vitamins may be needed to carry electrons during the enzymatic reaction No cofactors = No enzyme activity

Enzyme Types Constitutive enzymes are always present, such as those involved in glucose metabolism Induced enzymes are made only when their substrates are available (β- galactosidase, alcohol dehydrogenase)

Two Important Types of Enzymatic Reactions Condensation and Hydrolysis Reactions

Transfer Reactions Oxidation and Reduction: enzymes move electrons from one substrate to another; this process salvages electrons and their energy Aminotransferases move NH 2 groups Phosphotransferases move PO 4 Methyltransferases move CH 3 Decarboxylases release CO 2

A Few Notable Enzymes from Pathogens Coagulase – S. aureus Streptokinase – S. pyogenes Elastase and Collagenase – P. aeruginosa Lipase – C. perfringens

Enzymes are Sensitive to their Environment Wrong temperature, pH, ionic strength = no activity

Metabolic Pathways Reflect Batteries of Enzymes Working Together

Cell Energy Flow Energy = ability to cause a change in the condition of matter

Cell Energetics Cells carry out both types of chemical reactions: Exergonic – liberates energy Endergonic – consumes energy

Most Biological Energy Reactions Involve Oxidation and Reduction Oxidation = loss of electrons (and H) Reduction = gain of electrons (and H) In cellular respiration, glucose is oxidized to CO 2, and Oxygen is reduced to water

Objective of Respiratory Pathways Extract the Energy Contained in the Chemical bonds of Glucose and Use it to Make ATP

ATP – Adenosine Triphosphate A nucleotide containing the base adenine, the sugar ribose and three phosphate groups A very high amount of energy is stored in the terminal two PO 4 groups The phosphate bond energy can be used to drive many types of anabolism

ATP

Three Ways ATP can be Made Substrate-level phosphorylation directly transfers energy from glucose breakdown products Oxidative phosphorylation uses transport of electrons to build an H + gradient that efficiently drives ATP synthesis Photophosphorylation uses light energy to drive ATP synthesis

Metabolic Strategies In many cases, nutrient utilization is based on three possible catabolic pathways that convert glucose to CO 2 and release energy Aerobic respiration – glycolysis, the TCA cycle, and electron transport to O 2 Anaerobic respiration – glycolysis, the TCA cycle, and electron transport to a receptor other than O 2 Fermentation – glycolysis, organic compounds are the final electron acceptors without transport chain

The Aerobic Respiratory Pathway – Glycolysis, the Krebs Cycle and Electron Transport

Glycolysis Breaks glucose into two 3-carbon fragments Requires no oxygen Occurs in cytoplasm Fast but inefficient Produces 2 ATP, 2 NADH and 2 pyruvate molecules

Krebs Cycle Oxidizes the pyruvates from glycolysis to CO 2 Transfers electrons to NAD + and FAD to drive chemiosmotic phosphorylation Requires a membrane and electron transport proteins Produces 2 ATP directly and 34 by driving electron transport

Electron Transport System

Fermentation Incomplete oxidation of glucose or other carbohydrates in the absence of oxygen Uses organic compounds as terminal electron acceptors Yields a small amount of ATP Production of ethyl alcohol by yeasts acting on glucose Formation of acid, gas and other products by the action of various bacteria on pyruvic acid

Fermentation

Products of Fermentation