1 (744)

Prévia do material em texto

24 Organic and Biological Chemistry Solutions to Exercises (b) Six (assuming the tripeptide contains all three amino acids): Gly-Ser-Glu, GSE; Gly-Glu-Ser, GES; Ser-Gly-Glu, SGE; Ser-Glu-Gly, SEG; Glu- Ser-Gly, ESG; Glu-Gly-Ser, EGS 24.65 (a) The primary structure of a protein refers to the sequence of amino acids in the chain. Along any particular section of the protein chain the configuration may be helical, may be an open chain, or arranged in some other way. This is called the secondary structure. The overall shape of the protein molecule is determined by the way the segments of the protein chain fold together, or pack. The interactions which determine the overall shape are referred to as the tertiary structure. (b) X-ray crystallography is the primary and preferred technique for determining protein structure. 24.66 The α-helix and ß-sheet are examples of regular orientations in protein chains that are termed protein secondary structures. Both patterns are formed by hydrogen bonding. The main difference is that an α-helix is formed by hydrogen bonds between amino acids in the same chain, while a ß-sheet is formed by hydrogen bonds between two chains (or a chain with a flexible loop that has bent back on itself to form hydrogen bonds). An α-helix is a column, with hydrogen bonds between a particular amino acid and a different amino acid several groups away. A ß-sheet has a hydrogen-bonding pattern that zippers together two uncoiled protein strands to form a "pleated" sheet. These two secondary structures are enabled by different amino acid sequences. And, many sequences form neither an α-helix nor a Carbohydrates and Lipids (sections 24.8 and 24.9) 24.67 (a) Carbohydrates, or sugars, are composed of carbon, hydrogen, and oxygen. From a chemical viewpoint, they are polyhydroxyaldehydes or ketones. Carbohydrates are primarily derived from plants and are a major food source for animals. (b) A monosaccharide is a simple sugar molecule that cannot be decomposed into smaller sugar molecules by (acid) hydrolysis. (c) A disaccharide is a carbohydrate composed of two simple sugar units. Hydrolysis breaks the disaccharides into two monosaccharides. (d) A polysaccharide is a polymer composed of many simple sugar units. 24.68 Glucose exists in solution as a cyclic structure in which the aldehyde function on carbon 1 reacts with the OH group of carbon 5 to form what is called a hemiacetal, Figure 24.22. Carbon atom 1 carries an OH group in the hemiacetal form; in α-glucose this OH group is on the opposite side of the ring as the group on carbon atom 5. In the (beta) form the OH group on carbon 1 is on the same side of the ring as the group on carbon 5. The condensation product of two glucose units looks like this: 738