Chapter 16 Flashcards
Glycogen breakdown requires three enzymes. …converts the glucosyl units at the nonreducing ends of glycogen to glucose-1-phosphate (G1P).
Glycogen phosphorylase
…transfers an α(1→4)-linked trisaccharide to a nonreducing end and hydrolyzes the α(1→6) linkage.
Debranching enzyme
… converts G1P to glucose-6-phosphate (G6P). In liver, G6P is hydrolyzed by glucose-6-phosphatase to glucose for export to the tissues.
Phosphoglucomutase
Glycogen synthesis requires a diff erent pathway in which G1P is activated by reaction with …to form ….
UTP; UDP–glucose
… adds glucosyl units to the nonreducing ends of a growing glycogen molecule that has been primed by glycogenin.
Glycogen synthase
Branching enzyme removes an α(1→4)-linked 7-residue segment and reattaches it through an … linkage to form a branched chain
α(1→6)
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 …
T ⇌ R equilibria; allosteric effectors
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).
phosphorylase kinase,
Glycogen phosphorylase is activated by …, whereas glycogen synthase is activated by …
phosphorylation; dephosphorylation
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 …
glycogen metabolism.; glycogen breakdown; phosphoprotein phosphatase-1.
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
glucose; exergonic
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 …
fructose-2,6-bisphosphate (F2,6P); phosphofructokinase-2/fructose bisphosphatase-2 (PFK-2/FBPase-2).
Formation of glycosidic bonds requires …
nucleotide sugars
… are synthesized by adding monosaccharides to a protein.
O-Linked oligosaccharides
…are assembled on a dolichol carrier and then transferred to a protein.
N-Linked oligosaccharides
glucose is synthesized from noncarbohydrate precursors by ….
gluconeogenesis
The noncarbohydrate precursors that can be converted to glucose include the glycolysis products …and …., …. intermediates, and the carbon skeletons of most …
lactate; pyruvate; citric acid cycle; amino acids
there is no pathway in animals for the net conversion of acetyl-CoA to ….
oxaloacetate
… catalyzes the ATP-driven formation of oxaloacetate from pyruvate and HCO3-
Pyruvate carboxylase
…(PEPCK) converts oxaloacetate to PEP in a reaction that uses GTP as a phosphoryl-group donor.
PEP carboxykinase
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
biotin
(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 …
carboxyphosphate; free energy input.
(pyruvate carboxylase rxn) Phase II The activated carboxyl group is transferred from carboxybiotin to …in a three-step reaction to form …
pyruvate; oxaloacetate
Oxaloacetate can therefore be considered as …with CO2 and biotin facilitating the activation at the expense of ATP.
“activated” pyruvate,
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 …
aspartate; malate; NADH reducing equivalents
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) ….
mitochondrion; cytosol; does not involve NADH
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
malate
, the glycolytic reactions catalyzed by phosphofructokinase (PFK) and hexokinase are …in the gluconeogenesis direction and hence must be bypassed by diff erent gluconeogenic enzymes.
endergonic
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
six; 2
. 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.
four
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 …
fructose-2,6-bisphosphate (F2,6P).
F2,6P, which is not a glycolytic intermediate, is an extremely potent allosteric activator of …and an inhibitor of ….
PFK; FBPase
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
phosphofructokinase-2; fructose bisphosphatase-2
…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 …
Alanine; transamination
The regulation of glucose metabolism occurs not only through allosteric effectors, but also through long-term changes in the amounts of …
enzymes synthesized.
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
nucleotide sugar
…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.
Dolichol
