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
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
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)
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)
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
6
Q
Km
A
inversely proportional to the affinity (high affinity = low Km and vice versa)
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)
8
Q
Investment phase
A
- 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) - Isomerization of G6P to F6P (by phosphoglucose isomerase)
- Phosphorylation of F6P to fructose 1,6-bisphosphate (F1,6BP) **Rate limiting step
9
Q
Rate limiting step of glycolysis
A
phosphorylation of F6P to fructose 1,6-bisphosphate
Enzyme: PFK1 (phosphofructokinase 1)
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
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)
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
13
Q
Aldose A
A
Cleaves F1,6BP to 2x 3C molecules
- DHAP and G3P
- then triose phosphate isomerase favors G3P –> 2 G3P
14
Q
Glyceraldehyde 3P dehydrogenase (GAPDH)
A
phosphorylates G3P
- Yields 2 NADH
produces 1,3BPG
15
Q
Phosphoglycerate Kinase
A
converts 1,3BPG to 3PG
Yields 2ATP
16
Q
Pyruvate Kinase
A
forms pyruvate
Yields 2ATP
Irreversible step
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)
18
Q
Daddy molecules
A
insulin and glucagon
- insulin stimulates a phosphatase (removes P), fed
- glucagon stimulates a kinase (phosphorylates), fasting
19
Q
Tarui disease
A
- deficiency in PFK-1
* PFK-1 catalyzes rate limiting step of Glycolysis, so deficiency means that glycolysis is decreased
20
Q
Glucose-6 phosphate
A
precursor for pentose phosphate pathway
- also used in galactose metabolism, glycogen synthesis, uronic acid pathway
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
22
Q
Disorders of glycolysis
A
most result with hemolytic anemia, because RBC need glycolysis for energy.
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
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
25
Hemolytic Anemia
premature destruction of RBCs (glycolysis disorder)
- clinical markers - elevated lactate dehydrogenase, unconjugated bilirubin
- accumulation of intracellular Na causes swelling and lysis cell death
26
Fanconi-Bickel syndrome
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
27
Gluconeogenesis
- 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
28
Precursors of gluconeogenesis
Lactate, amino acids, and glycerol
29
Gluconeogenesis bypasses irreversible steps of glycolysis by:
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
Rate limiting step of gluconeogenesis
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
Phosphoenolpyruvate carboxykinase (PEPCK)
- gluconeogenesis
- concurrent decarboxylation and phosphorylation of oxaloacetate to PEP (uses GTP)
- transcription activated by cortisol, glucagon, thyroxine
32
Glucose 6-Phosphatase
```
gluconeogenesis
- dephosphorylation to form glucose
- liver, kidneys, small intestine, pancreas
located in ER lumen
activated by cortisol
```
33
Cori Cycle
- 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
F1,6BP deficiency
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
Von Gierke Disease (GSD1a)
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
Polyol pathway
conversion of glucose to fructose
| glucose --(Aldose reductatse)--> sorbitol --(sorbitol dehydrogenase)--> fructose
37
Why does excess fructose lead to fat production?
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
Galactosemia
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
GALT
- galactose 1-phosphate uridyl transferase (GALT) activity
- rate limiting step of galactose metabolism
galactose 1-P --> Glucose 1-P
40
Lactose intolerance
inability to metabolize lactose
| - caused by deficiency in enzyme lactase (cleaves lactose into glucose and galactose)
41
Pentose Phosphate Pathway (PPP)
- 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
Rate limiting step of PPP
G6P dehydrogenase catalyzes rate limiting step
| oxidation of glucose 6-p with NADP+ --> NADPH
43
G6P Dehydrogenase deficiency
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
PPP non-oxidative step
series of reversible reactions, where the end products shunt into glycolosis, gluconeogenesis, or necleotide synthesis pathways
45
Processes with high demand for NADPH
- 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
Glycogen
- 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
Glycogen storage
- 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
Liver glycogen
regulates blood glucose levels
| * distributes glucose as needed to other tissues
49
Muscle glycogen
provides reservoir of fuel (glucose) for physical activity
| * selfish - keeps glucose to itself to power glycolysis
50
Glycogenesis
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
UDP-glucose
active form of glucose
* formed when UDP-glucose pyrophosphorylase transfers G1P to UTP --> UDP-glucose
- breakdown of pyrophosphate to Pi generates energy
52
Glycogen synthase
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
Glucosyl 4:6 transferase
- 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
Glycogenolysis
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
glycogen phosphorylase
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
Liver Glu-1-P
(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
Muscle Glu-1-P
(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
Glycogen metabolism
- 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
Glycogen regulation by Insulin
- 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
Glycogen Regulation by Glucagon
- 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
Glycogen regulation by epinephrine
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
GSD 0 (glycogen storage disease)
- 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
GSD1a/Von Gierke Disease
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
GSD II/Pompe Disease
Deficiency in Acid Maltase aka acid a-glucosidase
* impairs lysosomal glycogenolysis - accumulation of glycogen in lysosomes
- kids die of heart failure in infancy
65
GSD III / Cori Disease
deficiency in a1,6 glucosidase (debranching enzyme)
glycogen molecules with a large number of short branches
- light hypoglycemia and hepatomegaly
66
GSD IV/Anderson Disease
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
GSDV/McArdle Disease
Deficiency in muscle glycogen phosphorylase
* rate limiting step of glycogen breakdown
- unable to supply muscles with enough glucose, exercise intolerance
68
GSD VI/Hers Disesase
Deficiency in liver glycogen phosphorylase
- prevents glycogen breakdown in liver
- glycogen accumulates in liver causing hepatomegaly and low blood glucose levels
