bio_ch_3_outline - Chapter 3: Protein Structure and...

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Chapter 3: Protein Structure and Function Key Concepts Proteins are made of amino acids. Amino acids vary in structure and function because their side chains vary in composition. Proteins vary widely in structure. The structure of a protein can be analyzed at four levels that form a hierarchy —the amino acid sequence, substructures called α-helices and β-pleated sheets, interactions between amino acids that dictate a protein’s overall shape, and combinations of individual proteins that make up larger, multiunit molecules. Proteins vary widely in function. In cells, most proteins are enzymes that function as catalysts. Chemical reactions occur much faster when they are catalyzed by enzymes. During enzyme catalysis, the reactants bind to an enzyme’s active site in a way that allows the reaction to proceed efficiently. Section 3.1 Outline: What Do Proteins Do? The diverse functions of proteins include: defense, movement, catalysis, signaling, structure, and transport (Table 3.1). Section 3.2 Outline: Early Origin-of-Life Experiments To test whether the first steps of chemical evolution could have occurred on ancient Earth, Stanley Miller combined methane (CH 4 ), ammonia (NH 3 ), and hydrogen (H 2 ) in a closed system with water, and applied heat and electricity as an energy source (Figure 3.1). The products that resulted from Miller’s experiment included hydrogen cyanide (HCN) and formaldehyde (H 2 CO), important precursors for more-complex organic molecules and amino acids. Section 3.3 Outline: Amino Acids and Polymerization More recent experiments, similar to Miller’s, have confirmed that the second step in chemical evolution occurred early in Earth’s history. Amino acids and other organic molecules form easily under conditions that accurately simulate those on ancient Earth. The Structure of Amino Acids All proteins are made from just 21 amino acids. All amino acids have a central carbon atom that bonds to NH 2 (an amino functional group), COOH (a carboxyl functional group), H (a hydrogen atom), and a variable side chain (Figure 3.2). In water ( pH7 ), the amino and carboxyl groups ionize to NH 3 + and COO , respectively, which helps amino acids stay in solution and makes them more reactive. The Nature of Side Chains The 21 amino acids differ only in the variable side chain or R-group attached to the central carbon (Figure 3.3). R-groups differ in their size, shape, reactivity, and interactions with water. Nonpolar hydrophobic R-groups cannot form hydrogen bonds with water and tend to coalesce in aqueous solution, whereas polar hydrophilic R-groups interact readily with water (Table 3.2). The nature of the side chain influences chemical reactivity. Amino acids with hydroxyl, amino, carboxyl, or
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This note was uploaded on 02/12/2010 for the course BIOL 202-004 taught by Professor Dr.johnson during the Fall '09 term at Butler CCC.

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bio_ch_3_outline - Chapter 3: Protein Structure and...

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