Concept 3.1 Nucleic Acids Are Informational Macromolecules

• The nucleic acids—DNA and RNA—are used mainly for information storage and expression.
   
• Nucleic acids are polymers of nucleotides. A nucleotide consists of one to three phosphate groups, a pentose sugar (ribose in RNA and deoxyribose in DNA), and a nitrogen-containing base. Review Figure 3.1 and WEB ACTIVITY 3.1
   
• In DNA, the nucleotide bases are adenine (A), guanine (G), cytosine (C), and thymine (T). Uracil (U) replaces thymine in RNA. The nucleotides are joined by phosphodiester linkages between the sugar of one and the phosphate of the next. RNA is usually single-stranded, whereas DNA is double-stranded. Review Figure 3.2
   
• Complementary base pairing, based on hydrogen bonds between A and T, A and U, and G and C, occurs in RNA and DNA. In RNA the hydrogen bonds result in a folded molecule; in DNA the hydrogen bonds connect two strands into a double helix. Review Figures 3.3 and 3.4 and WEB ACTIVITY 3.2
   
• DNA is expressed as RNA in the process of transcription. RNA can then specify the amino acid sequence of a protein in the process of translation. See ANIMATED TUTORIAL 3.1
   

Concept 3.2 Proteins Are Polymers with Important Structural and Metabolic Roles

• The functions of proteins include support, protection, catalysis, transport, defense, regulation, storage, and movement.
   
• Amino acids are the monomers from which polymeric proteins are made by peptide linkages. There are 20 different amino acids in proteins, each distinguished by a side chain (R group) that confers specific properties. Review Table 3.2 and WEB ACTIVITY 3.3
   
• The primary structure of a protein is the sequence of amino acids in the polypeptide chain. This chain is folded into a secondary structure, which in different parts of the protein may take the form of an α helix or a β pleated sheet. Review Figure 3.7
   
• Disulfide bridges and noncovalent interactions between amino acids cause polypeptide chains to fold into three-dimensional tertiary structures. Multiple polypeptides can interact to form quaternary structures. A protein's unique shape and chemical structure allow it to bind specifically to other molecules.
   
• Heat and certain chemicals can result in a protein becoming denatured, which involves the loss of tertiary or secondary structure. Review Figure 3.10 and WORKING WITH DATA 3.1
   

Concept 3.3 Some Proteins Act as Enzymes to Speed up Biochemical Reactions

• A chemical reaction must overcome an energy barrier to get started. An enzyme is a protein catalyst that affects the rate of a biological reaction by lowering the activation energy needed to initiate the reaction. Review Figure 3.12 and WEB ACTIVITY 3.4
   
• A substrate binds to the enzyme's active site —the site of catalysis—forming an enzyme–substrate complex. Enzymes are highly specific for their substrates.
   
• At the active site, a substrate enters its transition state, and the reaction proceeds.
   
• Substrate binding causes many enzymes to change shape, exposing their active site(s) and allowing catalysis. Review Figure 3.14
   
• Some enzymes require nonprotein "partners" to carry out catalysis. Review Table 3.3
   
• Substrate concentration affects the rate of an enzyme-catalyzed reaction. At the maximum rate, the enzyme is saturated with substrate. Review Figure 3.15
   

Concept 3.4 Regulation of Metabolism Occurs by Regulation of Enzymes

• Metabolism is organized into pathways in which the product of one reaction is a substrate for the next reaction. Each reaction in the pathway is catalyzed by a specific enzyme.
   
• Enzyme activity is subject to regulation. Some inhibitors bind irreversibly to enzymes. Other inhibitors bind reversibly. Review Figures 3.16 and 3.17 and ANIMATED TUTORIAL 3.2
   
• In allosteric regulation, a molecule binds to a site on the enzyme other than the active site. This changes the overall structure of the enzyme (including that of its active site) and results in either activation or inhibition of the enzyme's catalytic activity. Review Figure 3.18 and ANIMATED TUTORIAL 3.3
   
• The end product of a metabolic pathway may inhibit an enzyme that catalyzes the "commitment step" of that pathway. This is called feedback inhibition. Review Figure 3.19
   
• Environmental pH and temperature affect enzyme activity. Review Figure 3.20