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SECTION I. THE BASIC MOLECULES OF LIFE

Several families were studied whose affected individuals have nephrogenic diabetes insipidus. This disease causes childhood symptoms of polyuria (frequent urination), polydipsia (constant thirst and frequent drinking), poor growth, and hypernatremia (increased serum sodium concentration). Administration of antidiuretic hormone was not curative, focusing attention on a renal water loss due to a transport defect. A gene named aquaporin-2 was cloned from renal tubular epithelium, its amino acid sequence derived, and structural domains hypothesized to facilitate separation of mutations from benign variants. The hypothesized structure contained several transmembrane domains demarcated by β-turns, and these potential water channels were found to be mutated in affected individuals. Which of the following amino acids is most suggestive of β-turns?

A. Arginine and lysine

B. Aspartic acid and glutamic acid

C. Glycine and proline

D. Leucine and valine

E. Tryptophan and tyrosine

The answer is C. A β-turn structure consists of four amino acids in which the first residue is hydrogen bonded to the fourth residue of the turn (see Figure 1-5C). Glycine residues are small and flexible, whereas proline residues assume a cis or flattened conformation, making these residues amenable to tight turns. Transport proteins often have several membrane-spanning domains demarcated by β-turns that allow them to exit and return back into the membrane. These transmembrane domains form channels that regulate transport of ions and water in organs such as lung, gut, and kidney. Nephrogenic diabetes insipidus results when the kidney is less responsive to antidiuretic hormone excreted by the posterior pituitary, causing abnormal water excretion, dehydration, and electrolyte disturbances.

A 7-year-old female presents with dehydration after 3 days of explosive diarrhea after consuming a large amount of ice cream. Analysis of her fecal matter reveals a large amount of lactose. Which of the following sugar pairs would be found in this disaccharide?

A. Fructose and glucose joined by a α-1, 2 bond

B. Galactose and fructose joined by a β-1, 2 bond

C. Galactose and glucose joined by a β-1, 4 bond

D. Two molecules of glucose joined by a α-1, 1 bond

E. Two molecules of glucose joined by a α-1, 4 bond

The answer is C. Disaccharides are carbohydrates composed of two sugar molecules. Lactose in this case is composed of galactose and glucose joined by a β-1,4 glycosidic bond. Sucrose is a disaccharide of a fructose and a glucose molecule joined by a α-1,2 glycosidic ...

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