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carb metabolism Flashcards

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1
Q

What is the net ATP and NADH yield from glycolysis per molecule of glucose?

A

2 ATP (net) and 2 NADH.

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2
Q

Which glycolytic step is the first committed step and which enzyme catalyzes it?

A

The phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate by Phosphofructokinase-1 (PFK-1).

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3
Q

What are the products of aldolase acting on fructose-1,6-bisphosphate?

A

Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde-3-phosphate (G-3-P).

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4
Q

Which steps in glycolysis are irreversible?

A

Steps 1 (Hexokinase), 3 (PFK-1), and 10 (Pyruvate kinase).

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5
Q

How is ATP generated in glycolysis?

A

Through substrate-level phosphorylation in reactions catalyzed by phosphoglycerate kinase and pyruvate kinase.

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6
Q

What is the role of magnesium ion (Mg²⁺) in glycolytic kinase reactions like hexokinase and PFK-1?

A

Mg²⁺ acts as a cofactor that stabilizes the negative charges on the phosphate groups of ATP, facilitating nucleophilic attack by the hydroxyl group of the sugar.

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7
Q

Why is the phosphorylation of glucose to glucose-6-phosphate considered a “trapping” mechanism?

A

It prevents glucose from diffusing out of the cell due to the added negative charge and commits it to intracellular metabolism.

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8
Q

What structural change occurs during the conversion of glucose-6-phosphate to fructose-6-phosphate, and why is it necessary?

A

The ring structure is opened and rearranged to move the carbonyl group from C1 to C2, forming a ketose and preparing it for a second phosphorylation at C1.

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9
Q

What is the function of the enediol intermediate in glycolysis?

A

It is a reactive intermediate formed during isomerization steps, such as in the phosphoglucose isomerase and triose phosphate isomerase reactions.

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10
Q

Describe how fructose-1,6-bisphosphate “prepares” for aldol cleavage.

A

Its symmetrical phosphate groups at C1 and C6 allow even cleavage into two three-carbon molecules: DHAP and G-3-P.

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11
Q

What type of reaction is catalyzed by aldolase and what kind of bond is broken?

A

Aldolase performs a retro-aldol cleavage, breaking the C3–C4 bond of fructose-1,6-bisphosphate.

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12
Q

What specific amino acid side chain in aldolase forms a Schiff base with fructose-1,6-bisphosphate?

A

A lysine residue forms a Schiff base (imine) with the carbonyl carbon, stabilizing the intermediate.

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13
Q

Why is only G-3-P used in the next glycolytic step after the aldolase reaction?

A

Because only G-3-P can be oxidized by glyceraldehyde-3-phosphate dehydrogenase; DHAP must be isomerized to G-3-P.

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14
Q

Which amino acid residues in triose phosphate isomerase facilitate the isomerization reaction?

A

Glutamate 165 (acts as a base) and Histidine 95 (acts as an acid).

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15
Q

Why is glyceraldehyde-3-phosphate dehydrogenase considered a unique glycolytic enzyme?

A

It couples oxidation and phosphorylation, forming a high-energy acyl phosphate without ATP input.

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16
Q

What is the role of the enzyme’s sulfhydryl group in the G-3-P dehydrogenase mechanism?

A

It forms a thiohemiacetal intermediate with the aldehyde group of G-3-P, facilitating oxidation.

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17
Q

Why must NAD+ be continuously regenerated for glycolysis to proceed?

A

It is the electron acceptor in the G-3-P dehydrogenase reaction; without regeneration, glycolysis halts.

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18
Q

What reaction in glycolysis is an example of substrate-level phosphorylation besides pyruvate kinase?

A

The phosphoglycerate kinase reaction (1,3-BPG → 3-PG + ATP).

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19
Q

How is the phosphoryl group transferred in the phosphoglycerate kinase reaction?

A

ADP acts as a nucleophile and attacks the high-energy phosphate on 1,3-BPG.

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20
Q

Why is 2-phosphoglycerate converted to phosphoenolpyruvate (PEP)?

A

PEP has a much higher phosphoryl transfer potential, essential for efficient ATP generation in the final step

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21
Q

What is the role of the enolase enzyme?

A

It removes water from 2-PG, forming a C=C bond and creating PEP.

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22
Q

What structural change in pyruvate after PEP transfer makes the pyruvate kinase reaction irreversible?

A

Tautomerization from enol-pyruvate to the more stable keto-pyruvate causes a large free energy drop.

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23
Q

Which three glycolytic enzymes are targets of allosteric regulation?

A

Hexokinase, PFK-1, and Pyruvate kinase.

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24
Q

How does fructose-1,6-bisphosphate regulate pyruvate kinase?

A

It acts as a feed-forward activator, ensuring that glycolytic flux proceeds to pyruvate if upstream intermediates accumulate.

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25
Why is glycolysis particularly important in red blood cells?
RBCs lack mitochondria and depend entirely on glycolysis for ATP production.
26
How is hexokinase regulated?
Isozymes I-III are inhibited by glucose-6-phosphate; Hexokinase IV (glucokinase) is not and requires higher glucose levels.
27
What activates PFK-1 in the liver?
Fructose-2,6-bisphosphate.
28
How does glucagon inhibit glycolysis in the liver?
It decreases fructose-2,6-bisphosphate by activating the phosphatase function of PFK-2, reducing PFK-1 activity.
29
How does insulin promote glycolysis?
By increasing PFK-2 kinase activity, raising fructose-2,6-bisphosphate levels, and promoting transcription of glycolytic enzymes via SREBP1c.
30
What are the energy requirements to form one molecule of glucose in gluconeogenesis?
6 ATP/GTP and 2 NADH.
31
What enzymes bypass the irreversible steps of glycolysis in gluconeogenesis?
Pyruvate → PEP: Pyruvate carboxylase + PEP carboxykinase Fructose-1,6-bisphosphate → Fructose-6-phosphate: Fructose-1,6-bisphosphatase Glucose-6-phosphate → Glucose: Glucose-6-phosphatase
32
What activates fructose-1,6-bisphosphatase?
Citrate.
33
What inhibits fructose-1,6-bisphosphatase?
AMP and Fructose-2,6-bisphosphate.
34
What is the function of the malate shuttle in gluconeogenesis?
to transport OAA as malate from mitochondria to cytoplasm and regenerate NADH for glyceraldehyde-3-phosphate dehydrogenase.
35
What are the key products of the pentose phosphate pathway?
NADPH and ribose-5-phosphate.
36
What is the key enzyme in the oxidative phase of the PPP?
Glucose-6-phosphate dehydrogenase (G-6-PD).
37
How is G-6-PD regulated?
Inhibited by NADPH; stimulated by GSSG (oxidized glutathione) and glucose-6-phosphate.
38
What vitamin is required for transketolase function in the nonoxidative phase?
Thiamine (Vitamin B1) as thiamine pyrophosphate (TPP)
39
What is the activated glucose donor in glycogen synthesis?
UDP-glucose.
40
What enzyme initiates glycogen synthesis on a protein primer?
Glycogenin.
41
What is the function of the branching enzyme in glycogenesis?
Amylo-(1,4 → 1,6)-glucosyl transferase creates α(1,6) linkages to form branches.
42
What two enzymes are involved in glycogen breakdown?
lycogen phosphorylase (cleaves α(1,4) bonds) Amylo-α(1,6)-glucosidase (debranching enzyme for α(1,6) bonds)
43
How does glucagon regulate glycogen metabolism?
Increases cAMP → activates PKA → activates glycogen phosphorylase → promotes glycogenolysis
44
What is the effect of insulin on glycogen metabolism?
activates glycogen synthase and inhibits glycogen phosphorylase through a phosphorylation cascade.
45
Why can't the glycolytic reactions catalyzed by hexokinase, PFK-1, and pyruvate kinase be reversed in gluconeogenesis?
They are highly exergonic and irreversible under physiological conditions, requiring alternate enzymes with separate pathways.
46
What is the function of biotin in pyruvate carboxylase?
Biotin serves as a coenzyme that carries activated CO₂ from bicarbonate to pyruvate, forming oxaloacetate.
47
What are the two major compartments involved in PEP synthesis in gluconeogenesis?
The mitochondrial matrix (pyruvate carboxylase) and the cytoplasm (PEP carboxykinase).
48
Why is the malate shuttle necessary in gluconeogenesis?
It transports oxaloacetate as malate across the mitochondrial membrane and regenerates cytosolic NADH.
49
Why is GTP used instead of ATP in the PEP carboxykinase reaction?
Because GTP is more abundant in the cytosol and PEPCK has a higher specificity for GTP.
50
How does fructose-2,6-bisphosphate regulate gluconeogenesis?
It inhibits fructose-1,6-bisphosphatase, thereby decreasing gluconeogenic flux.
51
Why is the conversion of glucose-6-phosphate to glucose restricted to certain tissues?
Because glucose-6-phosphatase is only expressed in liver and kidney, allowing these organs to release free glucose into the blood.
52
What is the energetic cost of synthesizing glucose from lactate?
6 high-energy phosphate bonds (4 ATP + 2 GTP) and 2 NADH per glucose.
53
What are the substrates for gluconeogenesis besides pyruvate?
Lactate, glycerol, alanine, and other glucogenic amino acids.
54
How does the glucose-alanine cycle connect muscle to liver metabolism?
Muscle transaminates pyruvate to alanine, which is transported to the liver, converted back to pyruvate, and used for gluconeogenesis.
55
What is the role of NADPH produced in the oxidative phase of the pentose phosphate pathway?
NADPH provides reducing equivalents for biosynthesis and protection against oxidative stress
56
In which tissues is the oxidative phase of PPP most active, and why?
Liver, adipose, adrenal cortex, and mammary glands—due to high lipid synthesis needs.
57
Which enzyme catalyzes the first committed step of PPP and how is it regulated?
Glucose-6-phosphate dehydrogenase; inhibited by NADPH and activated by oxidized glutathione (GSSG).
58
59
What type of reaction is catalyzed by 6-phosphogluconate dehydrogenase?
An oxidative decarboxylation that produces ribulose-5-phosphate, NADPH, and CO₂.
60
What coenzyme is required by transketolase in the nonoxidative phase?
Thiamine pyrophosphate (TPP), derived from vitamin B1.
61
How do transketolase and transaldolase differ in the number of carbon atoms they transfer?
Transketolase transfers 2-carbon units; transaldolase transfers 3-carbon units.
62
What is the functional outcome of the nonoxidative phase of PPP?
Interconversion of 5C sugars into intermediates of glycolysis: G-3-P and F-6-P.
63
When does a cell favor the nonoxidative phase over the oxidative phase of PPP?
When the cell needs ribose-5-phosphate for nucleotide synthesis but not NADPH
64
What’s the significance of the hexose monophosphate shunt in red blood cells?
It produces NADPH to maintain reduced glutathione, which protects against oxidative damage.
65
What is the precursor molecule for glycogen synthesis and how is it activated?
Glucose-1-phosphate is converted to UDP-glucose using UTP by UDP-glucose pyrophosphorylase.
65
How does PPP support biosynthetic demands in actively proliferating cells?
By providing ribose-5-phosphate for nucleotide synthesis and NADPH for reductive biosynthesis.
66
What role does pyrophosphatase play in glycogenesis?
It hydrolyzes PPi to 2 Pi, driving the formation of UDP-glucose forward.
66
What initiates glycogen synthesis when no existing glycogen primer is present?
Glycogenin autocatalytically adds glucose residues to a tyrosine side chain to initiate the primer.
67
What bond does glycogen synthase form, and what are its limitations?
It forms α(1→4) glycosidic bonds but cannot initiate chains or create branches.
68
What does the branching enzyme do in glycogenesis?
Amylo-(1→4→1→6)-glucosyl transferase creates α(1→6) branches by transferring 6–7 glucose residues to a nearby chain.
69
Why is glycogen highly branched?
Branching increases solubility and provides multiple nonreducing ends for rapid synthesis and degradation.
70
How does glycogen phosphorylase catalyze glycogen breakdown?
It uses Pi to cleave α(1→4) bonds at nonreducing ends, releasing glucose-1-phosphate.
71
What happens to the glucose molecule removed from an α(1→6) branch point?
It is released as free glucose by amylo-α(1,6)-glucosidase (the debranching enzyme).
72
What enzyme converts glucose-1-phosphate to glucose-6-phosphate in both glycogenolysis and glycogenesis?
Phosphoglucomutase.
73
What is a “limit dextrin”?
A glycogen molecule that has been degraded to the point where only branch residues remain.
74
What are the key hormones regulating glycogen metabolism?
Insulin (promotes glycogenesis), glucagon and epinephrine (promote glycogenolysis).
75
How does glucagon trigger glycogen breakdown?
It activates a cAMP-PKA cascade, phosphorylating and activating glycogen phosphorylase
76
How does insulin promote glycogen synthesis?
Via a receptor tyrosine kinase cascade that activates protein phosphatases, leading to activation of glycogen synthase.
77
What is the active form of glycogen phosphorylase?
Phosphorylase a, formed when phosphorylase b is phosphorylated by phosphorylase kinase.
78
What is the active form of glycogen synthase?
The I form (unphosphorylated), while the D form (phosphorylated) is inactive.
79
What effect does cAMP have on glycogen metabolism?
it activates PKA, which phosphorylates enzymes to activate glycogenolysis and inhibit glycogenesis.
80
How does Ca²⁺ influence muscle glycogenolysis?
Ca²⁺ activates phosphorylase kinase, promoting conversion of phosphorylase b to a.
81
What is the role of AMP in muscle glycogenolysis?
It allosterically activates glycogen phosphorylase b even without phosphorylation
82
What allosteric regulator inhibits glycogen phosphorylase in liver but not muscle?
Free glucose.
83
How does glucose-6-phosphate regulate glycogen metabolism?
It activates glycogen synthase and inhibits glycogen phosphorylase.
84
How does glucagon inhibit glycolysis in the liver?
It reduces fructose-2,6-bisphosphate by activating PFK-2’s phosphatase activity.
85
What is PFK-2 and how is it bifunctional?
It has both kinase and phosphatase domains that regulate fructose-2,6-bisphosphate levels.
86
What second messenger mediates glucagon signaling?
cAMP, which activates protein kinase A (PKA).
87
How does PKA inhibit glycolysis?
It phosphorylates and inactivates pyruvate kinase.
88
What does cortisol do to gluconeogenesis?
Stimulates gene transcription of gluconeogenic enzymes like PEPCK, F-1,6-bisphosphatase, and G6Pase.
89
How does insulin promote de novo fatty acid synthesis from glucose?
It activates transcription of lipogenic enzymes via SREBP1c and increases NADPH production.
90
What metabolic pathways are upregulated by high insulin and high glucose?
Glycolysis, glycogenesis, lipogenesis, and the pentose phosphate pathway.
91
How does AMPK regulate metabolism under low energy conditions?
It inhibits anabolic processes and activates catabolic pathways to restore ATP.
92
How does AMPK affect glycolysis in muscle?
It increases glucose uptake and glycolytic flux by enhancing GLUT4 translocation and PFK activity.
93
How does fructose enter glycolysis in the liver?
It is converted to fructose-1-phosphate by fructokinase, then split by aldolase B into DHAP and glyceraldehyde.
94
Why is fructose metabolism considered unregulated in the liver?
it bypasses the PFK-1 checkpoint, entering glycolysis downstream.
95
What happens to glyceraldehyde in hepatic fructose metabolism?
It is phosphorylated to G-3-P by glyceraldehyde kinase.
96
What enzyme phosphorylates mannose to enter glycolysis?
Hexokinase, forming mannose-6-phosphate.
97
How is mannose-6-phosphate converted to a glycolytic intermediate?
Isomerized to fructose-6-phosphate.
98
How does galactose enter glycolysis?
Through the Leloir pathway, converting it to glucose-1-phosphate via galactose-1-phosphate uridyltransferase.
99
What coenzyme is required in galactose metabolism?
UDP-glucose, which donates glucose to form UDP-galactose.
100
Why can defects in fructokinase or aldolase B be problematic?
They lead to hereditary fructose intolerance or essential fructosuria, causing hypoglycemia or benign fructosuria.
101
What is the clinical consequence of galactose-1-phosphate uridyltransferase deficiency?
Classic galactosemia, leading to liver failure, cataracts, and developmental delay.
102
Why is hepatic fructose metabolism implicated in metabolic diseases?
It rapidly produces lipogenic substrates, increasing fat synthesis and potentially contributing to insulin resistance
103
What is the Cori cycle?
The conversion of muscle lactate to glucose in the liver, then re-use of glucose by muscle
104
Why is the Cori cycle important during anaerobic exercise?
It allows ATP production in muscle and regenerates NAD+ for glycolysis.
105
What is the cost of one round of the Cori cycle?
Muscle makes 2 ATP; liver uses 6 ATP equivalents to regenerate glucose—net ATP loss.
106
What is the glucose-alanine cycle’s function?
It transports amino nitrogen from muscle to liver and provides carbon for gluconeogenesis.
107
What enzyme converts pyruvate to alanine in muscle?
Alanine transaminase (ALT), using glutamate as the nitrogen donor.
108
What happens to alanine in the liver?
It is deaminated to pyruvate, which enters gluconeogenesis, and the amino group enters the urea cycle.
109
What is the role of TPP in transketolase?
It stabilizes the carbanion intermediate during the transfer of 2-carbon units.
110
What type of reaction does transaldolase catalyze?
A reversible aldol transfer of a 3-carbon unit between sugar phosphates.
111
Which enzyme has a Schiff base intermediate during glycolysis?
Aldolase.
112
What is tautomerization and where is it important in glycolysis?
It is the rearrangement between enol and keto forms, critical in the pyruvate kinase step.
113
What is substrate-level phosphorylation and which reactions perform it in glycolysis?
Direct transfer of a phosphate to ADP from a high-energy intermediate: phosphoglycerate kinase and pyruvate kinase.
114
How does the liver prioritize glucose use post-meal?
By using GK’s high Km to avoid sequestering glucose until other tissues have taken what they need.
115
Why can muscle not contribute directly to blood glucose levels?
It lacks glucose-6-phosphatase, so G6P cannot be dephosphorylated to free glucose.
116
What allows liver glycogen to maintain blood glucose during fasting?
Glucose-6-phosphatase in hepatocytes releases free glucose into the blood.
117
What molecule integrates signals for energy status in cells?
AMP, via AMPK activation.
118
What enzyme controls the rate-limiting step of gluconeogenesis?
Fructose-1,6-bisphosphatase.
119
What happens when both PFK-1 and FBPase-1 are active simultaneously?
A futile cycle occurs, wasting ATP without net metabolic output.
120
Which metabolite is common to glycolysis, gluconeogenesis, and PPP?
Glucose-6-phosphate
121
How are NADH needs met for gluconeogenesis in the cytosol?
Via the malate shuttle, which delivers NADH alongside OAA.
122
What triggers the release of epinephrine and what is its metabolic effect?
Stress; it promotes glycogenolysis and inhibits glycogenesis in muscle and liver.
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