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SFM > Carbohydrate Metabolism > Flashcards

Carbohydrate Metabolism Flashcards

1
Q

Purpose of glycolysis

A
  • metabolize 1 molecule of glucose to 2 molecules pyruvate
  • generate 2 molecules ATP
  • Anaerobic process
  • fructose, galactose, and other sugars can also be used
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2
Q

Glycolysis location

A

the cytoplasm of all cell types - usually a precursor rxn

  • RBC do not have mitochondria, so they rely solely on glycolysis
  • glucose is the only fuel the brain uses under non-starvation conditions
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3
Q

Carbohydrate sources in diet

A
  • monosaccharides (glucose, fructose, galactose)
  • disaccharides - sucrose (gluc+fruc) and lactose (gluc+galac)
  • polysaccharides - starches (from plants) and glycogen (animals)
  • De novo synthesis (gluconeogenesis in liver)
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4
Q

Glucose transporters

A

GLUT1-4
GLUT1: all cell types (esp brain and RBC, high affinity)
GLUT2: Liver (low affinity-all digested nutrients go to liver first so it doesn’t need high affinity)
GLUT3: Main transporter in neurons (high affinity)
GLUT4: Skeletal muscle, heart, and adipose tissue (INSULIN DEPENDENT)

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5
Q

GLUT4

A

Insulin dependent glucose transporter

  • GLUT4 is sequestered in vesicles
  • when insulin binds, it signals for vesicles to fuse with membrane, incorporating GLUT4 and allowing more glucose to enter cell for glycolysis
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6
Q

Km

A

inversely proportional to the affinity (high affinity = low Km and vice versa)

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7
Q

3 phases of glycolysis

A

Investment (uses 2 ATP at different steps)
Splitting (6-C molecule splits into 2x 3C molecules)
Recoup/Payoff (4 ATP molecules formed - NET: 2 ATP, 2 NADH, 2 pyruvate)

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8
Q

Investment phase

A
  1. phosphorylation of glucose to G6P
    - Regulatory step of glycolysis - traps glucose in the cell - uses ATP
    - Enzymes (hexokinase-all cells, glucokinase-liver and pancreatic B cells)
  2. Isomerization of G6P to F6P (by phosphoglucose isomerase)
  3. Phosphorylation of F6P to fructose 1,6-bisphosphate (F1,6BP) **Rate limiting step
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9
Q

Rate limiting step of glycolysis

A

phosphorylation of F6P to fructose 1,6-bisphosphate

Enzyme: PFK1 (phosphofructokinase 1)

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10
Q

Hexokinase

A

investement phase of glycolysis - phosphorylates glucose to glucose-6-phosphate
- uses 1 ATP
- inhibited by glucose-6-phosphate (neg. feedback mech)
ALL CELL TYPES

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11
Q

Glucokinase

A
  • phosphorylates glucose to G6P - in liver and pancreatic B cells
  • uses 1 ATP
  • propelled by: glucose, fructose 1-p, insulin
  • inhibited by: glucagon, Fructose 6-P (next step in pathway)
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12
Q

Phosphofructokinase-1

A
  • phosphorylates F6P to fructose 1,6-bisphosphate (F1,6BP)
  • uses 1 ATP
  • rate limiting step of glycolysis
  • propelled by: AMP, Fructose 2,6BP
  • inhibited by: ATP (don’t need more), Citrate
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13
Q

Aldose A

A

Cleaves F1,6BP to 2x 3C molecules

  • DHAP and G3P
  • then triose phosphate isomerase favors G3P –> 2 G3P
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14
Q

Glyceraldehyde 3P dehydrogenase (GAPDH)

A

phosphorylates G3P
- Yields 2 NADH
produces 1,3BPG

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15
Q

Phosphoglycerate Kinase

A

converts 1,3BPG to 3PG

Yields 2ATP

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16
Q

Pyruvate Kinase

A

forms pyruvate
Yields 2ATP
Irreversible step

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17
Q

3 irreversible steps glycolysis

A 
Catalysts: Hexokinase/glucokinase (gluc to G6P), phosphofructokinase 1 (makes bisphosphate, rate limiting step), pyruvate kinase (makes pyruvate)
Activity Influenced by:
-ATP/AMP (energy status)
-glucose (feeding status)
-insulin and glucagon (hormone status)
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18
Q

Daddy molecules

A

insulin and glucagon

  • insulin stimulates a phosphatase (removes P), fed
  • glucagon stimulates a kinase (phosphorylates), fasting
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19
Q

Tarui disease

A
  • deficiency in PFK-1

* PFK-1 catalyzes rate limiting step of Glycolysis, so deficiency means that glycolysis is decreased

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20
Q

Glucose-6 phosphate

A

precursor for pentose phosphate pathway

- also used in galactose metabolism, glycogen synthesis, uronic acid pathway

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21
Q

Regulation of glycolysis during exercise

A

during exercise - ATP is decreased, slightly more AMP, which stimulates PFK1 –> makes bisphosphate at a higher rate which stimulates pyruvate kinase –> makes pyruvate (either makes CO2 + H2O moderate exercise or lactate in a sprint)
- this would be during muscle fiber contraction, glycolysis activated to meet energy requirements

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22
Q

Disorders of glycolysis

A

most result with hemolytic anemia, because RBC need glycolysis for energy.

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23
Q

Type 1 Diabetes

A

hyperglycemia (glycolysis disorder)
loss of pancreatic B cells (likely from immune destruction) causes severe insulin deficiency (GLUT4 not signaled into PM and glucose does not get in cell, stays in blood)

  • some potential causes include mutations in GK, mitochondiral tRNA genes, averrant conversion of proinsulin to mature insulin, defective insulin receptor, pancreatitis, pancreatic carcinoma, trauma, infection, etc
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24
Q

Type 2 diabetes

A

hyperglycemia (glycolysis disorder)

  • insulin resistance that progresses to loss of B cell function
  • (GLUT4 not signaled into PM and glucose does not get in cell, stays in blood)
  • insulin not activating pathway/signaling cascade for glycogen synthesis
  • some potential causes include mutations in GK, mitochondiral tRNA genes, averrant conversion of proinsulin to mature insulin, defective insulin receptor, pancreatitis, pancreatic carcinoma, trauma, infection, etc
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25
Q

Hemolytic Anemia

A

premature destruction of RBCs (glycolysis disorder)

  • clinical markers - elevated lactate dehydrogenase, unconjugated bilirubin
  • accumulation of intracellular Na causes swelling and lysis cell death
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26
Q

Fanconi-Bickel syndrome

A

autosomal recessive (glycolysis disorder)

  • mutation in GLUT2 transporter (liver, pancreatic V cell)
  • unable to take up glucose, fructose, and galactose
  • hepatomegaly, tubular nephropathy, abdominal bloating, resistant rickets
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27
Q

Gluconeogenesis

A
  • synthesis of new glucose from different sources (can be carbohydrate sources or non-carbohydrate sources)
  • occurs in liver, kidneys, and small intestine (not every cell type)
  • not a reversal of glycolysis
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28
Q

Precursors of gluconeogenesis

A

Lactate, amino acids, and glycerol

29
Q

Gluconeogenesis bypasses irreversible steps of glycolysis by:

A
  1. pyruvate carboxylase (mitochondrial pyruvate –> oxaloacetate)
  2. phosphoenolpyruvate carboxykinase (PEPCK) (OAA –> PEP)
  3. Fructose 1,6-Bisphosphatase (breaks down F1,6BP to F6-P) Rate limiting step
  4. Glucose 6-phosphatase
30
Q

Rate limiting step of gluconeogenesis

A

Fructose 1,6Bisphosphase to Fructose 6-phosphase
Enzyme: Fructose 1,6 bisphosphatase
Activated by: Cortisol and citrate
inhibited by: AMP and F2,6-BP

31
Q

Phosphoenolpyruvate carboxykinase (PEPCK)

A
  • gluconeogenesis
  • concurrent decarboxylation and phosphorylation of oxaloacetate to PEP (uses GTP)
  • transcription activated by cortisol, glucagon, thyroxine
32
Q

Glucose 6-Phosphatase

A 
gluconeogenesis
- dephosphorylation to form glucose
- liver, kidneys, small intestine, pancreas
located in ER lumen
activated by cortisol
33
Q

Cori Cycle

A
  • links lactate produced from glycolysis in RBC and exercising muscle to gluconeogenesis in liver
  • glucose produced in liver transported back to RBC and muscle
  • Prevents lactate accumulation and regenerates glucose
34
Q

F1,6BP deficiency

A

gluconeogenesis disorder

  • deficiency in rate limiting steps means they cannot generate glucose when they are starving
  • similar to Tarui disease in glycolysis
  • presents in infancy or early childhood - causes hypoglycemia, lactic acidosis, ketosis
35
Q

Von Gierke Disease (GSD1a)

A

gluconeogenesis deficiency

  • deficiency in glucose 6-phosphatase
  • inefficient release of free glucose into the blood stream by liver in gluconeogenesis and glycogenolysis
  • Pt exhibits marked fasting hypoglycemia, lactid acidosis, hepatomegaly due to buildup of glycogen, HLD, and potentially retarded growth
36
Q

Polyol pathway

A

conversion of glucose to fructose

glucose –(Aldose reductatse)–> sorbitol –(sorbitol dehydrogenase)–> fructose

37
Q

Why does excess fructose lead to fat production?

A

fructose breakdown bypasses the regulatory steps of glycolysis (PFK1 and PFK2), so it results in over-production of pyruvate and acetyl CoA.

  • Fructose-derived G3P and DHAP are processed by glycolysis to pyruvate and acetyl CoA - unregulated
  • excess acetyl CoA –> fatty acids –> triacylglycerols
  • Liver also accumulates fatty acids, resulting in fatty liver
38
Q

Galactosemia

A

disruption of galactose metabolism

  • classic (most common form) is inherited deficiency in GALT activity
  • treatment: remove galactose and lactose from liver
  • deficiency in galactokinase (nonclassical variant) leads to accumulation of galactose and galactitol in blood and urine, leads to caracts in early infancy
39
Q

GALT

A
  • galactose 1-phosphate uridyl transferase (GALT) activity
  • rate limiting step of galactose metabolism
    galactose 1-P –> Glucose 1-P
40
Q

Lactose intolerance

A

inability to metabolize lactose

- caused by deficiency in enzyme lactase (cleaves lactose into glucose and galactose)

41
Q

Pentose Phosphate Pathway (PPP)

A
  • oxidative pathway for glucose metabolism
  • does not produce direct energy
  • produces NADPH (needed for reductive biosynthesis)
  • produces sugar for DNA and RNA formation
  • Produces 2 NAPDH and Ribulose 5-P
  • pathway dependent on cellular demands (high demand for Ribose 5P in rapidly dividing cells, the oxidative phase favored) vs (high demand for NADPH: non-oxidative products channeled into gluconeogenesis for re-entry into PPP)
42
Q

Rate limiting step of PPP

A

G6P dehydrogenase catalyzes rate limiting step

oxidation of glucose 6-p with NADP+ –> NADPH

43
Q

G6P Dehydrogenase deficiency

A

affects PPP (rate limiting step)

  • hemolytic anemia with increased need for NADPH (because it is not formed) such as infection, oxidizing medications. Affects a significant population, particularly those of African descent
  • NADPH regenerates glutathione, an important antioxidant, detoxifies H2O2 with glutathione reductase
44
Q

PPP non-oxidative step

A

series of reversible reactions, where the end products shunt into glycolosis, gluconeogenesis, or necleotide synthesis pathways

45
Q

Processes with high demand for NADPH

A
  • Lactating mammary glands and adipose tissue active in fatty acid synthesis, 14-16 NADPH needed for each fatty acid (half of these come from PPP)
  • lung and liver tissue also high PPP activity
  • very high PPP activity in phagocytic cells
46
Q

Glycogen

A
  • homopolymer of glucose molecules w/ branches
  • linked with a1,4 glycosidic bonds
  • branches bonded with a1,6 glycosidic bonds
  • reducing end connected to protein glycogenin
  • degraded AND extended from non-reducing end (free OH group at C4)
47
Q

Glycogen storage

A
  • stored in liver and muscle tissues
  • stored as granules, also contain all the enzymes needed for making and breaking glycogen metabolism
  • helps get glucose quickly because it’s all in one kit
48
Q

Liver glycogen

A

regulates blood glucose levels

* distributes glucose as needed to other tissues

49
Q

Muscle glycogen

A

provides reservoir of fuel (glucose) for physical activity

* selfish - keeps glucose to itself to power glycolysis

50
Q

Glycogenesis

A

3 Key steps

  • Trapping and activation of glucose (glucokinase/hexokinase in cytosol phosphorylates gluc to G6P, then phosphoglucomutase isomerizes to G1P. UDP gluc pyrophosphorylase transfers G1P to UTP –> UDP-glucose = active form)
  • Elongation of glycogen primer (pre-existing glycogen polymer with glycogenin acts as primer, glycogen synthase rate limiting, transfers the glucose from UDP glucose to non-reducing end of glycogen chain forming 1,4 bonds)
  • Branching of glycogen chains (when chain reaches 11 residues, a fragment is broken off at a 1,4 bond and reattached elsewhere with 1,6 bond by glucosyl 4:6 transferase. Branching increases solubility of glycogen and increases the number of terminal non-reducing ends.)
51
Q

UDP-glucose

A

active form of glucose

  • formed when UDP-glucose pyrophosphorylase transfers G1P to UTP –> UDP-glucose
  • breakdown of pyrophosphate to Pi generates energy
52
Q

Glycogen synthase

A

rate limiting step of glucogenesis

  • transfers glucose from UDP-gluc to non-reducing end of glycogen chain, with a1,4 glycosidic bond
  • regulated by phosphorylation (dephospho form active, phospho form inactive)
53
Q

Glucosyl 4:6 transferase

A
  • breaks chain of ~7 residues from glycogen chain (breaks 1,4 glycosidic bond) and reattaches with 1,6 glycosidic bond elsewhere on the chain
  • done when branch reaches 11 residues
54
Q

Glycogenolysis

A

Breakdown of glycogen, Two steps

  • Chain shortening (release of Glu-1P) - glycogen phosphorylase rate limiting catalyzes cleavage of glucose residues as G1P. Uses pyridoxal phosphase (vitamin B6 as a cofactor). phosphorolysis until GP gets within 4 residues of a1,6 branch point
  • Branch transfer (release free glucose) - debranching enzyme uses transferase (4:4) to transfer block of 3 remaining residues to main chain with 1,4 bond. Then cleaves a1,6 bond on last residue from branch, releasing free glucose.
  • generates Glu1-P and free Glu in ratio of 10:1
55
Q

glycogen phosphorylase

A

Rate limiting step of Glycogenolysis

  • catalyzes cleavage of Glu-1-P (breaks a1,4 glycosidic bonds)
  • uses vitamin B6 as a cofactor
  • regulated by phosphorylation (dephospho form inactive, phospho form active)
56
Q

Liver Glu-1-P

A

(breakdown from glycogen)
- converted to Glu-6-P, then to free Glu by glucose-6-phosphatase (same as glyconeogenesis). Free glucose released to bloodstream

57
Q

Muscle Glu-1-P

A

(breakdown from glycogen)

  • use Glu1-P to generate energy via glycolysis and TCA cycle
  • do not have Glucose-6-phosphatase- cannot hydrolyze glu-6-p to glucose
58
Q

Glycogen metabolism

A
  • regulated to a.) maintain blood sugar and b.) provide energy to muscle
  • synthesis and degradation are separately regulated
  • key enzymes: glycogen synthase (rate limiting step of synthesis) and glycogen phosphorylase (rate limiting step of degradation)
  • both enzymes regulated by phosphorylation
59
Q

Glycogen regulation by Insulin

A
  • high blood glucose (promotes glycogen synthesis)
  • B cells release insulin, binds to receptor tyrosine kinase and activates signaling cascade
  • 4 proteins involved: GLUT4, Protein Kinase B (PKB), Protein phosphatase 1 (PP1), Glycogen synthase kinase 3 (GSK3)
  • PKB phosphorylates PP1 (activate) and GSK3 (inactivate)
  • active PP1 dephosphorylates glycogen synthase (activate) and dephosphorylates glycogen phosphorylase (inactivate)
  • Net result: Glycogen synthesis
60
Q

Glycogen Regulation by Glucagon

A
  • when blood glucose low (promotes glycogen breakdown)
  • a cells of pancreas release glucogon, binds to GPCR on hepatocytes and triggers signal cascade
  • Key enzymes and second messengers: G protein, Adenylate cyclase (AC) and cAMP, Protein kinase A (PKA), protein phosphatase 1 (PP1), and phosphorylase kinase (PK)
  • GPCR activates G protein –> activates cAMP –> activates PKA, phosphorylates glycogen synthase (inactivates) and PKA phos PK (active), PKA phos inhibitor –> activates PP1, active PK phosphorylates glycogen phosphorylase (activates)
  • Net result: glycogen breakdown (inactive glycogen synthase and active glycogen phosphorylase)
61
Q

Glycogen regulation by epinephrine

A

Epinephrine releasesd by adrenal glands, promotes degradation of glycogen (pathway similar to glucagon)

  • Glu-6-phosphate activates glycogen synthase and inactivates glycogen phosphorylase
  • especially relevant during periods of exercise
62
Q

GSD 0 (glycogen storage disease)

A
  • deficiency in glycogen synthase (cannot synthesize and store glycogen), rely on glucose in diet
  • Vulnerable to hypoglycemia when fasting, and have muscle cramps due to lack of glycogen in muscle
63
Q

GSD1a/Von Gierke Disease

A

defiency in glucose6-phosphatase (gluc6-P –x–> glucose)
ineffient release of free glucose into bloodstream by liver after gluconeogenesis
- mutations at catalytic site of glucose 6-phosphatase

64
Q

GSD II/Pompe Disease

A

Deficiency in Acid Maltase aka acid a-glucosidase

  • impairs lysosomal glycogenolysis - accumulation of glycogen in lysosomes
  • kids die of heart failure in infancy
65
Q

GSD III / Cori Disease

A

deficiency in a1,6 glucosidase (debranching enzyme)
glycogen molecules with a large number of short branches
- light hypoglycemia and hepatomegaly

66
Q

GSD IV/Anderson Disease

A

Deficiency in glucosyl (4:6) transferase (branching enzyme)

  • long glycogen with fewer branches
  • enlargement of liver and spleen, scarring of liver tissue (cirrhosis)
  • death by 5 years old
67
Q

GSDV/McArdle Disease

A

Deficiency in muscle glycogen phosphorylase

  • rate limiting step of glycogen breakdown
  • unable to supply muscles with enough glucose, exercise intolerance
68
Q

GSD VI/Hers Disesase

A

Deficiency in liver glycogen phosphorylase

  • prevents glycogen breakdown in liver
  • glycogen accumulates in liver causing hepatomegaly and low blood glucose levels

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