Chapter 16

Question 1

Glycogen breakdown requires three enzymes. …converts the glucosyl units at the nonreducing ends of glycogen to glucose-1-phosphate (G1P).

Answer 1

Glycogen phosphorylase

Question 2

…transfers an α(1→4)-linked trisaccharide to a nonreducing end and hydrolyzes the α(1→6) linkage.

Answer 2

Debranching enzyme

Question 3

… converts G1P to glucose-6-phosphate (G6P). In liver, G6P is hydrolyzed by glucose-6-phosphatase to glucose for export to the tissues.

Answer 3

Phosphoglucomutase

Question 4

Glycogen synthesis requires a diff erent pathway in which G1P is activated by reaction with …to form ….

Answer 4

UTP; UDP–glucose

Question 5

… adds glucosyl units to the nonreducing ends of a growing glycogen molecule that has been primed by glycogenin.

Answer 5

Glycogen synthase

Question 6

Branching enzyme removes an α(1→4)-linked 7-residue segment and reattaches it through an … linkage to form a branched chain

Answer 6

α(1→6)

Question 7

Glycogen metabolism is controlled in part by allosteric eff ectors such as AMP, ATP, and G6P. Covalent modifi cation of glycogen phosphorylase and glycogen synthase shifts their … and therefore alters their sensitivity to …

Answer 7

T ⇌ R equilibria; allosteric effectors

Question 8

The ratio of phosphorylase a (more active) to phosphorylase b (less active) depends on the activity of ….which is regulated by the activity of protein kinase A (PKA), a cAMP-dependent enzyme, and on the activity of phosphoprotein phosphatase-1 (PP1).

Answer 8

phosphorylase kinase,

Question 9

Glycogen phosphorylase is activated by …, whereas glycogen synthase is activated by …

Answer 9

phosphorylation; dephosphorylation

Question 10

Hormones such as glucagon, epinephrine, and insulin control …Hormone signals that generate cAMP as a second messenger or that elevate intracellular Ca2+, which binds to the calmodulin subunit of phosphorylase kinase, promote … Insulin stimulates glycogen synthesis in part by activating …

Answer 10

glycogen metabolism.; glycogen breakdown; phosphoprotein phosphatase-1.

Question 11

Compounds that can be converted to oxaloacetate can subsequently be converted to …. The conversion of pyruvate to glucose by gluconeogenesis requires enzymes that bypass the three …steps of glycolysis: Pyruvate carboxylase and PEP carboxykinase (PEPCK) bypass pyruvate kinase, fructose-1,6-bisphosphatase (FBPase) bypasses phosphofructokinase, and glucose-6-phosphatase bypasses hexokinase

Answer 11

glucose; exergonic

Question 12

Gluconeogenesis is regulated by changes in enzyme synthesis and by allosteric eff ectors, including …which inhibits FBPase and activates phosphofructokinase (PFK) and whose synthesis depends on the phosphorylation state of the bifunctional enzyme …

Answer 12

fructose-2,6-bisphosphate (F2,6P); phosphofructokinase-2/fructose bisphosphatase-2 (PFK-2/FBPase-2).

Question 13

Formation of glycosidic bonds requires …

Answer 13

nucleotide sugars

Question 14

… are synthesized by adding monosaccharides to a protein.

Answer 14

O-Linked oligosaccharides

Question 15

…are assembled on a dolichol carrier and then transferred to a protein.

Answer 15

N-Linked oligosaccharides

Question 16

glucose is synthesized from noncarbohydrate precursors by ….

Answer 16

gluconeogenesis

Question 17

The noncarbohydrate precursors that can be converted to glucose include the glycolysis products …and …., …. intermediates, and the carbon skeletons of most …

Answer 17

lactate; pyruvate; citric acid cycle; amino acids

Question 18

there is no pathway in animals for the net conversion of acetyl-CoA to ….

Answer 18

oxaloacetate

Question 19

… catalyzes the ATP-driven formation of oxaloacetate from pyruvate and HCO3-

Answer 19

Pyruvate carboxylase

Question 20

…(PEPCK) converts oxaloacetate to PEP in a reaction that uses GTP as a phosphoryl-group donor.

Answer 20

PEP carboxykinase

Question 21

Pyruvate carboxylase is a tetrameric protein of identical ∼1160-residue subunits, each of which has a …prosthetic group. This functions as a CO2 carrier by forming a carboxyl substituent at its ureido group

Answer 21

biotin

Question 22

(pyruvate carboxylase rxn) Phase I The cleavage of ATP to ADP acts to dehydrate bicarbonate via the formation of a “high-energy”… intermediate. The reaction of the resulting CO2 with biotin is exergonic. The biotin-bound carboxyl group is therefore “activated” relative to bicarbonate and can be transferred to another molecule without further …

Answer 22

carboxyphosphate; free energy input.

Question 23

(pyruvate carboxylase rxn) Phase II The activated carboxyl group is transferred from carboxybiotin to …in a three-step reaction to form …

Answer 23

pyruvate; oxaloacetate

Question 24

Oxaloacetate can therefore be considered as …with CO2 and biotin facilitating the activation at the expense of ATP.

Answer 24

“activated” pyruvate,

Question 25

In species with cytosolic PEPCK, oxaloacetate must fi rst be converted either to … or to … The difference between the two routes involves the transport of …

Answer 25

aspartate; malate; NADH reducing equivalents

Question 26

The malate dehydrogenase route (Route 2) results in the transport of reducing equivalents from the …to the …, since it uses mitochondrial NADH and produces cytosolic NADH. The aspartate aminotransferase route (Route 1) ….

Answer 26

mitochondrion; cytosol; does not involve NADH

Question 27

Cytosolic NADH is required for gluconeogenesis, so, under most conditions, the route through ..is a necessity. However, when the gluconeogenic precursor is lactate, its oxidation to pyruvate generates cytosolic NADH, and either transport system can then be used

Answer 27

malate

Question 28

, the glycolytic reactions catalyzed by phosphofructokinase (PFK) and hexokinase are …in the gluconeogenesis direction and hence must be bypassed by diff erent gluconeogenic enzymes.

Answer 28

endergonic

Question 29

The net energetic cost of converting two pyruvate molecules to one glucose molecule by gluconeogenesis is …ATP equivalents: …each at the steps catalyzed by pyruvate carboxylase

Answer 29

six; 2

Question 30

. Since the energetic profi t of converting one glucose molecule to two pyruvate molecules via glycolysis is two ATP (Section 15-1), the energetic cost of the futile cycle in which glucose is converted to pyruvate and then resynthesized is …ATP equivalents. Such free energy losses are the thermodynamic price that must be paid to maintain the independent regulation of two opposing pathways.

Answer 30

four

Question 31

The net fl ux through the substrate cycle created by the opposing actions of PFK and FBPase (described in Section 15-4B) is determined by the concentration of …

Answer 31

fructose-2,6-bisphosphate (F2,6P).

Question 32

F2,6P, which is not a glycolytic intermediate, is an extremely potent allosteric activator of …and an inhibitor of ….

Answer 32

PFK; FBPase

Question 33

The concentration of F2,6P in the cell depends on the balance between its rates of synthesis and degradation by …(PFK-2) and …(FBPase-2), respectively

Answer 33

phosphofructokinase-2; fructose bisphosphatase-2

Question 34

…is converted to pyruvate by the transfer of its amino group to an α-keto acid to yield a new amino acid and the α-keto acid pyruvate,
a process termed …

Answer 34

Alanine; transamination

Question 35

The regulation of glucose metabolism occurs not only through allosteric effectors, but also through long-term changes in the amounts of …

Answer 35

enzymes synthesized.

Question 36

Formation of the glycosidic bonds that link sugars to each other and to other molecules requires free energy input under physiological conditions (ΔG°′ = 16 kJ · mol−1). This free energy, as we have seen in glycogen synthesis is acquired through the synthesis of a …from a nucleoside triphosphate and a monosaccharide, thereby releasing PPi, whose exergonic hydrolysis drives the reaction

Answer 36

nucleotide sugar

Question 37

…is a long-chain polyisoprenol containing 17 to 21 isoprene units in a nimals and 14 to 24 units in fungi and plants. It anchors the growing oligosaccharide to the endoplasmic reticulum membrane, where the initial glycosylation reactions take place.

Answer 37

Dolichol