S/A Metabolic Regulation Flashcards
Describe the process of glycogen breakdown in muscle. Include a description of the structure of glycogen, the nature of the breakdown reaction and the breakdown product, and the required enzyme(s).
Muscle glycogen consists of linear polymers of (α1 → 4)-linked D-glucose, with many branches formed by (α1 → 6) glycosidic linkages to D-glucose. Glycogen phosphorylase in muscle catalyzes phosphorolytic cleavage of the terminal residue at the nonreducing ends, producing glucose 1-phosphate. When phosphorylase approaches (α1 → 6) branch points, a second enzyme (the “debranching enzyme”) removes the four glucose residues nearest the branch point and reattaches them in (α1 → 4) linkage at a nonreducing end. Now phosphorylase can continue to degrade the molecule.
Glycogen synthesis and glycogen breakdown are catalyzed by separate enzymes. Contrast the reactions in terms of substrate, cofactors (if any), and regulation.
Glycogen synthesis is catalyzed by glycogen synthase and employs UDP-glucose as the activated precursor:
UDP-glucose + glycogen (glucose)n → UDP + glycogen (glucose)n+1
Glycogen synthase is inactivated by phosphorylation, catalyzed by cAMP-dependent protein kinase; it is activated by dephosphorylation, catalyzed by phosphoprotein phosphatase. Glycogen breakdown is catalyzed by glycogen phosphorylase, which employs pyridoxal phosphate (PLP) as a cofactor. The reaction is a phosphorolysis; the glycosidic bond is broken by the attack of Pi:
Glycogen (glucose)n + Pi → glycogen (glucose)n –1 + glucose 1-phosphate.
Glycogen phosphorylase is activated by phosphorylation, catalyzed by phosphorylase kinase, and it is
inactivated by dephosphorylation, catalyzed by phosphorylase a phosphatase.
In mammalian liver, glucose-1-phosphate, the product of glycogen phosphorylase, can enter glycolysis or replenish blood glucose. Describe the reactions by which these two processes are carried out.
To enter glycolysis, glucose-1-phosphate must undergo isomerization to glucose-6-phosphate by phosphoglucomutase. To replenish glucose in the bloodstream, glucose-1-phosphate must be hydrolyzed to free glucose by glucose-1-phosphatase.
Diagram the pathway from glucose to glycogen; show the participation of cofactors and name the enzymes involved.
(1) Glucose + ATP → glucose 6-phosphate + ADP hexokinase
(2) Glucose 6-phosphate → glucose 1-phosphate phosphoglucomutase
(3) Glucose 1-phosphate + UTP → UDP-glucose + PPi UDP-glucose
pyrophosphorylase
(4) UDP-glucose → glycogen + UDP glycogen synthase
Show the reaction catalyzed by glycogen synthase.
The reaction is the addition of a glucose moiety from UDP-glucose to the nonreducing end of a glycogen chain; the linkage formed is (α1 → 4). (See Fig. 15-8, p. 568.)
What is the biological advantage of synthesizing glycogen with many branches?
Highly branched glycogen is more soluble than unbranched glycogen. In addition, both glycogen synthase and glycogen phosphorylase act at the nonreducing ends of glycogen chains. Branched glycogen has far more ends for these enzymes to work on than would the equivalent amount of linear glycogen chains. Having more ends effectively increases the concentration of substrate for the enzymes, thereby increasing the rate of glycogen synthesis and breakdown.
Explain the role of glycogenin.
Glycogenin is a protein that acts as the “primer” for the initiation of new glycogen molecules. It catalyzes the transfer of a glucose residue from UDP-glucose to a tyrosine hydroxyl group in glycogenin, then forms a complex with glycogen synthase. As more glucose residues are added, this first glucose residue, still attached to glycogenin, becomes the reducing end of the growing glycogen chain.
Describe four major principles of metabolic regulation that have selectively evolved throughout evolution.
- Maximize the efficiency of fuel utilization by preventing the simultaneous operation of opposing pathways (i.e., futile cycles).
- Partition metabolites appropriately between alternative pathways.
- Draw on the fuel best suited for the immediate needs of the organism.
- Shut down biosynthetic pathways when their products accumulate.
In the glycolytic path from glucose to phosphoenolpyruvate, two steps are practically irreversible. What are these steps, and how is each bypassed in gluconeogenesis? What advantages does an organism gain from having separate pathways for anabolic and catabolic metabolism? What are the disadvantages?
The two irreversible steps in glycolysis are conversion of glucose to glucose 6-phosphate, catalyzed by hexokinase, and conversion of fructose 6-phosphate to fructose 1,6-bisphosphate, catalyzed by phosphofructokinase-1 (Table 15-2, p. 573). The first reaction is bypassed during gluconeogenesis by the reaction catalyzed by glucose 6-phosphatase, an enzyme unique to the liver. The second is bypassed by fructose 1,6-bisphosphatase-1 (FBPase-1). By having separate pathways that employ different enzymes, an organism is able to control anabolic and catabolic processes separately, thus avoiding futile cycles. A potential disadvantage is the need to produce separate sets of enzymes for catabolism and anabolism.
Why is citrate, in addition to being a metabolic intermediate in aerobic oxidation of fuels, an important control molecule for a variety of enzymes?
As the key biochemical intermediate in the citric acid cycle resulting from the condensation of oxaloacetate and acetyl-CoA, citrate is at a junction of amino acid, fatty acid, and pyruvate oxidation, serving as an intracellular signal that the cell’s current energy needs are being met. In particular, it is an allosteric regulator of PFK-1, increasing the inhibitory effect of ATP, and further reducing the flow of glucose through glycolysis.
Under what circumstances does the bifunctional protein phosphofructokinase-2/fructose 2,6- bisphosphatase (PFK-2/FBPase-2) become phosphorylated, and what are the consequences of its phosphorylation to the glycolytic and gluconeogenic pathways?
Glucagon, signaling low blood sugar, stimulates cAMP synthesis, which activates protein kinase A (PKA) to phosphorylate PFK-2/FBPase-2 (among other proteins). This phosphorylation enhances FBPase-2 activity and inhibits PFK-2 activity of the enzyme, resulting in lower levels of fructose 2,6-bisphosphate (F26BP). In the absence of F26BP as an allosteric effector, the activity of PFK-1 is reduced (inhibiting glycolysis) and the activity of FBPase-1 is enhanced (stimulating gluconeogenesis), thus enabling the liver to replenish blood glucose. See Figs. 15-22 and 15-23.
Order the steps leading to glycogen breakdown resulting from the stimulation of liver cells by glucagon.
1) Activation of protein kinase A (PKA)
2) cAMP levels rise
3) Phosphorylation of phosphorylase b
4) Phosphorylation of phosphorylase b kinase
5) Stimulation of adenyl cyclase
The correct temporal order is 5-2-1-4-3.
Explain the distinction between metabolic “regulation” and metabolic “control” in a multienzyme pathway.
Regulation refers to rebalancing the levels of metabolites along a pathway in response to a change in flux through the pathway, while control is what determines the total flux through the pathway.
