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10/26/17 CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry THE CHEMISTRY IN BIOCHEMISTRY • Metabolism can be summarized in the overall scheme given below: Ø Matter is conserved & Energy is conserved • To understand cellular metabolism, we need to discuss how different functional groups chemically react with each other CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry FUNCTIONAL GROUPS IN BIOCHEMICAL REACTIONS • The field of biochemistry classifies reactions based on the chemical behavior of specific functional groups • Each organic functional group we studied is associated with one or more characteristic biochemical reactions • You should know these! CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry CLASSES OF BIOCHEMICAL REACTIONS • There are six basic chemical reactions that describe the bulk of biochemistry: 1. Oxidation-reduction reactions 2. Hydration-dehydration reactions Chapter 10 3. Acyl group transfer reactions Material 4. Phosphoryl group transfer reactions 5. Decarboxylation reactions Covered later 6. Reactions that form or break in metabolism carbon-carbon bonds lectures 1 10/26/17 CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry OUTLINE • 10.1 The Role of Functional Groups in Biochemical Reactions • 10.2 Oxidation-Reduction Reactions • 10.3 Hydration/Dehydration Reactions • 10.4 Acyl group transfer reactions • 10.5 Phosphoryl group transfer reactions CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry OXIDATION-REDUCTION REACTIONS • Oxidation-reduction (aka. “Redox”) reactions always involve a transfer of electrons. • There are four specific sub-classes of redox reactions we will consider: 1. Metal/nonmetal reactions 2. Combustion reactions 3. Hydrocarbon redox reactions involving C-C bonds 4. Redox reactions involving C-O bond containing functional groups CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry DEFINITION OF OXIDATION-REDUCTION REACTIONS • Redox reactions occur when an electron is transferred between two reactants: Ø The reactant that loses an electron is being oxidized Ø The reactant that gains the electron is being reduced • Remember it this way: The reactant that is being “reduced” is getting a reduction in charge • Note that both reactions must occur at the same time (one reactant is reduced, the other oxidized) 2 10/26/17 CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry AN EXAMPLE OF REDOX REACTIONS Mg(s) + 2HCl(aq) à H2 (g) + MgCl2(aq) • In this reaction, 2 electrons are donated (or lost) by the Mg, so it is oxidized: Mg à Mg2+ + 2e- • Two H+ receive (or gain) the electrons to form hydrogen gas, so they become reduced: + - 2H + 2e à H2 • The chloride (Cl-) ion remains unchanged; it is called a “spectator ion” CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry METAL-NONMETAL REDOX REACTIONS • In general, metals and non-metals exchange electrons in redox reactions. Ø Whenever a cation forms from a metal, it has undergone an oxidation (neutral à (+) positive) Ø Whenever an anion forms from a nonmetal, it has undergone a reduction (neutral à (-) negative) • Iron rusting in oxygen is a common redox reaction: 4 Fe + 3 O2 à 2 Fe2O3 Iron is Oxygen is Fe3+ O2- oxidized reduced CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry COMBUSTIONS ARE REDOX REACTIONS • Reaction of methane (natural gas) with oxygen is a typical combustion reaction: CH4(g) + 2O2 (g) à CO2(g) + 2H2O(g) • In this reaction, electrons are transferred from the methane to the oxygen Ø Methane is oxidized (nonpolar C-H bonds broken) Ø Oxygen is reduced (nonpolar O=O bonds broken) Ø New polar C=O and O-H bonds form • Organic compounds in the cell are oxidized in a similar way to generate energy during “cellular respiration” 3 10/26/17 CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry HOW TO TELL WHEN ORGANIC COMPOUNDS UNDERGO REDOX REACTIONS • The redox reactions of organic molecules involve changes to bond structure (“partial” e- transfer) 1. The number of polar bonds to carbons increases and the number of non-polar bonds to carbons decreases 2. The more electronegative atom is reduced, while the less electronegative atom is oxidized glucose CH4(g) + 2O2 (g) à CO2(g) + 2H2O(g) methane oxygen C6H12O6(s) + 6O2 (g) à 6CO2(g) + 6H2O(g) General, Organic, and Biochemistry Chapter 10: Figure 10.3 REDOX REACTIONS DRIVE CONVERSIONS BETWEEN FUNCTIONAL GROUPS • Oxygen containing Redox Conversion Reactions functional groups are at different levels of oxidation: alcohol aldehyde oxidation carboxylic acid • Oxidation of different alcohols results in distinct functional groups formation CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry HYDROCARBON REDOX REACTIONS • Changes to C-C bonds are also redox reactions • Less saturated carbons are more oxidized: oxidation reduction oxidation alkane à alkene à alkyne • The overall energy per bond changes in single vs. double vs. triple C-C bonds 4 10/26/17 CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry CATALYTIC HYDROGENATION • The food industry uses catalytic hydrogenation to reduce liquid plant oils (alkenes) into solids such as margarine (mostly alkanes): Ø High heat and a metal catalyst (Pt or Pd) is used Ø The reducing agent is hydrogen gas (H2) catalytic hydrogenation reactions Alkene Alkane CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry A PROBLEM WITH HYDROGENATIONS • Industrial hydrogenation of oils is not always a complete process….some C=C bonds remain: Ø The naturally occurring C=C bonds found in plant oils are all cis-double bonds Ø “Partial hydrogenation” of cis C=C bonds results in the production of trans C=C bonds • Trans-fats are associated with increased risk for obesity and cardiovascular disease. http://bio1151b.nicerweb.net/Locked/media/ch05/05_fatty-acids2.gif CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry PRACTICE PROBLEM Determine whether the following reactions represent an oxidation or a reduction of the organic substance shown. 5 10/26/17 CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry ENZYMATIC REDOX REACTIONS • All redox reactions require an oxidant AND reductant for the electron transfer to occur: Ø Reactants receiving the electrons from a molecule undergoing oxidation are called oxidizing agents Ø Reactants providing electrons to molecules undergoing reductions are called reducing agents • Biochemical redox reactions involving enzymes often use coenzymes to serve as electron carriers Ø Coenzymes can accept electrons during catabolism Ø Coenzymes can donate electrons during anabolism CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry VITAMINS AND HEALTH • Vitamin deficiency results in a number of diseases. • Two classes of vitamins: Ø Water soluble: B complex and C Ø Fat soluble: A, D, E, and K • Several of the water soluble vitamins are used to form important Coenzymes. General, Organic, and Biochemistry Chapter 10: Figure 10.9 B-VITAMINS USED AS COENZYMES vitamin B 3 NAD+ (oxidized form) Niacin vitamin B2 FAD (oxidized form) Riboflavin vitamin B5 Coenzyme A Pantothenic acid (CoA) 6 10/26/17 CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry THE COENZYME NAD+/NADH • In biochemical systems, the coenzyme NAD+ is commonly used (with an enzyme) to oxidize oxygen containing functional groups: NAD+ (oxidized form) NADH (reduced form) 1°Alcohol Aldehyde (reduced) (oxidized) CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry OXIDATION OF ALCOHOLS • The oxidation of alcohols is a two-step process that involves the NAD+ coenzyme: 1° alcohols à aldehydes à carboxylic acids 1. The coenzyme NAD+ is the oxidant that removes both electrons and protons from ethanol & acetaldehyde 2. The catalysts driving each step are enzymes 2e- 2e- + + NAD NADH NAD NADH + H+ + H+ CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry PRACTICE PROBLEM Predict what product forms in the following reactions: + NAD+ à + NADH à ß NADH + + NAD+ à 7 10/26/17 CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry THE COENZYME FAD/FADH2 • In biochemical systems, the coenzyme FAD is commonly used (with an enzyme) to oxidize C-C bonds in hydrocarbons: FADH FAD 2 (reduced form) (oxidized form) Alkane Alkene (reduced) (oxidized) CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry ANTIOXIDANTS ARE ELECTRON SCAVENGERS • An antioxidant is a compound that prevents harmful oxidation of important substances in the cell because it is an effective reducing agent. What sort of functional groups on antioxidants would you expect to mediate their “electron scavenging” function? glutathione ascorbic acid (vitamin C) a-tocopherol (vitamin E) CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry HYDRATION REACTIONS • In a hydration reaction, water is added to an alkene, replacing the double bond: Ø The water is split: –OH and –H are singly bonded to the two carbons from the original alkene C=C bond • In biochemical systems, this occurs mostly when the alkene is adjacent to a carbonyl group 8 10/26/17 CHAPTER 10: Reactions of Organic Functional Groups in Biochemistry DEHYDRATION REACTIONS • Dehydration reactions are simply the reverse of hydration reactions: Ø A water is removed from an alcohol to form an alkene Ø Removal of the alcohol is a reduction reaction • These also commonly occur adjacent to a carbonyl group in biochemical systems CHAPTER 10: Reactions of Organic Functional
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