Christopher G. Hamaker, Illinois State University, Normal IL © 2008, Prentice Hall Chapter 20 Biochemistry INTRODUCTORY CHEMISTRY INTRODUCTORY CHEMISTRY Concepts & Connections Fifth Edition by Charles H. Corwin

Chapter 20 2 There are 30 elements that are essential for life. The study of the chemistry of living things is biochemistry. Biological compounds are often large and complex with molar masses greater than 1,000,000 g/mol. These large molecules are polymers of smaller molecules. Introduction to Biochemistry

Chapter 20 3 Biological Compounds

Chapter 20 4 Proteins are naturally occurring polymers composed of many amino acids. An amino acid has both an amine and a carboxylic acid functional group. Amino acids are linked together by amide bonds, which are referred to as peptide linkages. Proteins

Chapter 20 5 Sources of Protein Meat, fish, poultry, cheese, milk products, and many beans are rich in protein.

Chapter 20 6 A carbohydrate is either a simple sugar or a polymer composed of many simple sugars. A carbohydrate usually contains either an aldehyde or ketone functional group and several alcohol groups. Starch is a carbohydrate polymer composed of glucose units linked together by glycoside linkages. Carbohydrates

Chapter 20 7 A lipid is a water-insoluble compound usually composed of an alcohol and one or more carboxylic acid molecules. Fats and oils are esters of glycerol, an alcohol that has three –OH groups. As a result, each molecule of a fat or an oil contains three ester groups from three carboxylic acid molecules joined to one glycerol molecule. Lipids

Chapter 20 8 A nucleic acid is a biochemical polymer composed of a very large number of individual units. Each unit in the nucleic acid contains a sugar molecule attached to an organic nitrogen-containing molecule and an attached phosphate group. The units are attached together by phosphate linkages. Nucleic Acids

Chapter 20 9 Proteins are composed of amino acids. An amino acid has an amine (-NH 2 ) and a carboxy group (-COOH) attached to a carbon atom with a side chain (R) attached to the  -carbon. There are 20 naturally occurring amino acids in human proteins. Each one has a different side chain (R). Amino Acids

10 Acidic Amino Acids Basic Amino Acids Neutral Amino Acids The 20 Natural Amino Acids

Chapter In proteins, amino acids are linked together with peptide linkages or a peptide bond. In a dipeptide, two amino acid units are joined together by a peptide bond. In a polypeptide, up to 50 amino acids are linked together. Human insulin is composed of 48 amino acids. Polypeptides

Chapter Proteins are polypeptides composed of hundreds or thousands of amino acid units. The primary structure of a protein is the sequence of amino acids. Individual amino acids are represented by the ovals in the figure below. The replacement of one amino acid by another in the primary structure of a protein can completely alter its biological activity. Primary Structure of Proteins

Chapter When the peptide chain twists and bends, proteins acquire a secondary structure. There are two primary types of secondary structure:  -helix and pleated sheet. An  -helix is analogous to that in a coiled telephone cord. Secondary Structure of Proteins

Chapter In a pleated sheet, the chains run antiparallel to each other. It looks like a sheet of paper folded in an accordion shape. Secondary structure in proteins is a result of hydrogen bonds. Pleated Sheet

Chapter The overall three-dimensional structure of a protein is referred to as its tertiary structure. The tertiary structure of a protein may be long and extended, or compact and folded. The tertiary structure of a protein is held together by intermolecular forces. Tertiary Structure of Proteins

Chapter An enzyme is a protein that acts as a biological catalyst. Enzymes are incredibly selective for specific molecules. An enzyme can speed up a biochemical reaction so that the rate is a million times faster than it would be in the absence of the enzyme. Many reactions catalyzed by enzymes would be too slow without the enzyme to sustain life. Enzymes

Chapter The location where the reaction occurs on the enzyme is the active site. The molecule that reacts is the substrate. We can use the lock-and-key model to describe enzyme mechanisms. In the model, the key is the enzyme and the lock is the substrate. Enzyme Mechanisms

Chapter An enzyme reaction takes place in two steps. First, the substrate (S) binds to the active site on the enzyme (E). Step 1: E + S → ES Second, the enzyme releases two or more products (P 1 and P 2 ). Step 2: ES → E + P 1 + P 2 Enzyme Reactions

Chapter The active site of an enzyme has a shape designed to specifically bind its substrate. Once the reaction has occurred, the active site is free to bind to another substrate molecule and repeat the reaction. Enzyme Reactions

Chapter The enzyme ptyalin is in human saliva and catalyzes the breaking down of starch molecules into sugar units. The reaction would take about two years to occur without the enzyme. The Enzyme Ptyalin

Chapter The word carbohydrate means “hydrates of carbon,” and many have the empirical formula CH 2 O. Carbohydrate names usually end in the suffix -ose. Carbohydrates have an aldehyde or ketone functional group and several hydroxyl groups. Carbohydrates

Chapter A simple sugar molecule typically has three to six carbons, an aldehyde or ketone group, and a few hydroxyl groups. A monosaccharide with an aldehyde group is an aldose, and one with a ketone group is a ketose. Simple Sugars

Chapter In aqueous solution, sugar molecules usually exist in ring structures. Glucose forms a cyclic structure by joining a hydroxyl group to an aldehyde group. The resulting ring structure has 5 carbon atoms and 1 oxygen atom. Aqueous Structures of Sugars

Chapter A disaccharide is 2 monosaccharide molecules joined together. In the formation of a disaccharide, 2 simple sugars split out water and are joined together by a special –O– bond called a glycoside linkage. Disaccharides

Chapter A polysaccharide is a polymer of many monosaccharide linkages joined by glycoside linkages. Starch and cellulose are both polysaccharides composed only of glucose units, but with slightly different 3-dimentional structures. Polysaccharides starch cellulose

Chapter Unlike most other biological compounds, lipids are not water soluble. Lipids include –fats and oils –waxes –steroids –the water insoluble vitamins A, D, E, and K Lipids

Chapter A fatty acid is a carboxylic acid with a long hydrocarbon chain. Fatty acids can be saturated or unsaturated. Fatty Acids

Chapter A triglyceride is a lipid formed from glycerol, a trihydroxy alcohol, and three fatty acids. When three fatty acids react with glycerol, a triglyceride is formed with three ether linkages. Triglycerides (Triglycerols)

Chapter In triglycerides obtained from animals, the fatty acids are mostly saturated and have few double bonds. A semisolid lipid obtained from an animal source is a fat. The following structure is an example of a saturated triglyceride from animal fat. Fats

Chapter A liquid lipid obtained from a plant source is an oil. Triglycerides obtained from plants have mostly unsaturated fatty acid chains. The following structure is an example of an unsaturated triglyceride from a vegetable oil. Oils

Chapter Saponification of Fats and Oils The ester linkage in a triglyceride can be broken by treatment with aqueous sodium hydroxide. The products of the reaction are glycerol and the sodium salt of the fatty acid. The sodium salts of fatty acids are soaps.

Chapter Soaps Soap molecules have a polar “head” (COO-Na + ) and a long nonpolar “tail” (R, R ’, R ” ). The soap molecules are able to dissolve oil with the tail, and the heads stick into the water.

Chapter Phospholipids A phospholipid is a glyceride found in living cells and membranes. Phospholipids have two fatty acids esterified to the glycerol. The third –OH is linked to a phosphoric acid.

Chapter Steroids A steroid belongs to a special class of lipids that has a structure composed of four rings of carbon atoms fused together.

Chapter Chemistry Connection: Vitamins Vitamins are biochemical compounds required for normal metabolism. Vitamins are not synthesized in the human body, and therefore must be included in the diet. Excess water-soluble vitamins are easily excreted from the body. Excess doses of fat-soluble vitamins (A, D, E, and K) accumulate in fatty tissue and can be toxic.

Chapter Nucleic acids are biochemical compounds found in every living cell. Nucleic acids contain the genetic information responsible for the reproduction of a species. There are two basic types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). A nucleic acid is a polymer composed of many repeating units, each of which is called a nucleotide. Nucleic Acids

Chapter Each nucleotide consists of three similar component molecules: a five-carbon sugar, a nitrogen-containing base, and a molecule of phosphoric acid. The five-carbon sugars are similar in RNA and DNA. The two sugars are identical except for a hydroxyl group. The structures are shown here: Nucleotides

Chapter There are five nitrogen bases. Three are found in both DNA and RNA, one is found only in DNA, and one is found only in RNA. Their structures are shown below: Nitrogen Bases

Chapter Nucleotides for DNA and RNA are the sugar with a phosphoric acid and the nitrogen base attached. Shown here are structures of a DNA and RNA nucleotide. Nucleotide Structures

Chapter DNA and RNA are polymers of individual nucleotides. A segment is shown here. A DNA molecule is actually two strands of DNA wound together in a spiral structure called a double helix. The double strand is held together by hydrogen bonds between nitrogen pairs. DNA and RNA

Chapter An adenine (A) on one strand always hydrogen bonds to a thymine (T) on the other strand. Also, a cytosine (C) on one strand always hydrogen bonds to a guanine (G) on the other strand. These interactions give DNA its characteristic structure. DNA Double Helix

Chapter There are 4 major types of biological molecules: –Proteins are composed of amino acids. –Carbohydrates are composed of simple sugar molecules. –A lipid is a water-insoluble biological molecule. –A nucleic acid is a polymer composed of a sugar molecule, a nitrogen base, and a phosphoric acid. Chapter Summary

Chapter An enzyme is a catalyst for biological reactions. Enzymes work by a lock-and-key mechanism where only a specific substrate fits into the enzyme to react. Reactions catalyzed by enzymes can be completed in a matter of seconds where the reaction would normally take many years. Chapter Summary, continued

Chapter A triglyceride is an ester of glycerol and three different fatty acids. A fat is a triglyceride from an animal source. An oil is a triglyceride from a plant source. A nucleic acid is a molecule that carries genetic information. DNA and RNA are the two basic types of nucleic acids. Chapter Summary, continued