Carbohydrate Metabolism Flashcards
What are the net products of glycolysis?
1 glucose = 2 pyruvate, 2 ATP, 2 NADH
Where does anaerobic glycolysis occur?
What cells depend entirely on glycolysis for energy and why?
- in red blood cells and overworked muscles (lack 02)
- red blood cells, because they do not have mitcochondria so no TCA cycle
What are the main cells/organs that rely on or prefer glucose to other fuel?
- red blood cells (only fuel they can use)
- brain (only fuel it can use under non-starvation conditions)
- liver (de novo synthesis of glucose through gluconeogenesis)
What are the 4 important glucose transporters, where are they present, and what are their affinities for glucose?
- GLUT1: ubiquitous but high expression in RBC’s and brain; high affinity
- GLUT2: liver and pancreas; low affinity (liver gets a lot of recycled glucose from diet, thus it is not in dior need and does not require high affinity)
- GLUT3: neurons; high affinity
- GLUT4: skeletal muscle, heart, adipose tissue; insulin dependent affinity
How is GLUT4 transporter regulated by insulin? (3 “steps”)
- GLUT4 is kept within vesicles
- insulin signaling causes fusion of vesicles w/ plasma membrane and insertion of GLUT4 into membrane
- insulin increases GLUT4 induced glucose uptake
*fusion faciliated by exercise
What are the regulatory checkpoints within glycolysis and the associated enzymes within those reactions?
- phosphorylation of glucose to G6P using ATP (hexokinase (all cells), glucokinase (liver, pancreas, β-cells))
- phosphorylation of F6P to F 1,6-BP using ATP (rate limiting, phosphofructokinase-1)
- formation of pyruvate where ADP phosphorylated to ATP (irreversible, pyruvate kinase)
What are the steps in glycolysis that utilize ATP? produce ATP? and produce NADH?
Utilize ATP:
- glucose > G6P (hexokinase/glucokinase)
- F6P > F 1,6-BP (phosphofructokinase-1)
Produce ATP:
- 1,3-bisphosphoglycerate > 3-phosphoglycerate (phosphoglycerate kinase)
- phosphoenolpyruvate > pyruvate (pyruvate kinase)
Produce NADH:
- glyceraldehyde 3-P > 1,3-bisphosphoglycerate (glyceraldehyde 3-P dehydrogenase)
What are the hormonal and energy regulations on phosphofructokinase-1 (PFK-1) within glycolysis?
- insulin stimulates, glucagon inhibits
- AMP , NH4+, Pi stimulates; ATP, PEP, H+, citrate (TCA cycle) inhibits
- PFK2’s role: high insulin activates phosphatases, dephos FBP-ase2 triggers kinases, produces F2,6BP which also activates PFK-1
- PFK2’s role: low insulin induces high cAMP, activates protein kinase A, phospho PFK-2, triggers phospho activity, reduces PFK-1 activity
- deficiency in PFK-1
- exercise-induced myalgia, weakness
- hemolytic anemia
- high bilirubin, jaundice
- sx can be mild
Tarui disease
What are the hormonal and energy regulations on pyruvate kinase within glycolysis?
- insulin, F1,6BP stimulates
- glucagon, ATP, alanine inhibit (if this occurs, PEP will enter gluconeogenesis)
(high insulin: stimulates protein phosphatase, desphos of PK, activated form)
(high glucagon: cAMP activates protein kinase A, phospho of PK, inhibited form)
What is the critical junction point in glycolysis?
glucose 6-phosphate
- precursor for pentose phosphate pathway
- converted to glucose 1-phosphate for: galactose metabolism, glycogen synthesis, uronic acid pathway
What is the main cause of hemolytic anemia?
pyruvate kinase deficiency
Why are red blood cells impacted the most by defective glycolytic enzymes (i.e. ineffective glycolysis)
Because they don’t have mitochondria, thus they rely soley on energy from glycolysis for function
- glycolytic disorder causing hyperglycemia
- type 1 and type 2
- potential causes: mutations in GK and mito tRNA genes, aberrant conversion of proinsulin to insulin, defective insulin receptor, pancreatitis, pancreatic carcinoma trauma, infection
What is the difference between type 1 and type 2?
Type 1 is usually autoimmune, severe insulin deficiency due to loss of pancreatic β cells (likely immune destruction)
Type 2 is usually later onset due to poor diet, insulin resistance that progresses to loss of β cell function
- glycolytic disorder that results from premature destruction of RBC’s
- many different causes: inherited defects in RBC’s, hemoglobinopathies (thalassemia, sickle cell), nutritional deficiencies, infections, defects in glycolytic enzymes, reduced ATP, aberrant function of ATP dependent ion pumps > increases intracellular Na+ (swelling, lysis, cell death)
- clinical markers: elevated lactate dehydrogenase, unconjugated bilirubin
hemolytic anemia
- glycolytic disorder, autosomal recessive disorder
- mutation in GLUT2 transporter (liver, pancreas)
- cells unable to take up glucose, fructose, galactose
- sx: FTT, hepatomegaly, tubular nephropathy, abd bloating, resistant rickets
- fasting hypoglycemia, postprandial hyperglycemia
- tx: vit D, phosphate, uncooked corn starch
Fanconi-Bickel syndrome
How long can a person’s body glucose reserves sustain their glucose needs for?
1 day
Where does gluconeogensis occur in the body?
Liver, kidney, small intestine
What is the purpose of gluconeogenesis?
To convert pyruvate (other carbs and non-carb precursors) to glucose, especially during times of starvation, exercise, ketogenic diet. Important for the brain and muscles because significant gluconeogenesis does not occur in these areas
What are the regulatory steps within gluconeogenesis?
- pyruvate > oxaloacetate (pyruvate carboxylase)
- activated: acetyl CoA and cortisol - oxaloacetate > phosphoenolpyruvate (PEP carboxykinase)
- activated: cortisol, glucagon, thyroxine - fructose 1,6-BP > fructose 6-P (fructose 1,6-biphosphatase) (rate limiting)
- activated: coritsol, citrate; inhibited: AMP and F 2,6-BP - glucose 6-phosphate > glucose (glucose 6-phosphatase)
- activated: cortisol
How does gluconeogenesis “by-pass” the 3 irreversible steps of glycolysis?
It uses 4 enzymes that are not present during those irreversible steps
- pyruvate carboxylase
- phosphoenolpyruvate caboxykinase
- fructose 1,6-bisphosphatase
- glucose 6-phosphatase
How is pyruvate carboxylase regulated within gluconeogenesis?
- activated by acetyl-CoA and cortisol (make more glucose!)
- it is located in the mitochondria (duh, because it is acting on pyruvate) and uses a biotin cofactor
What is the role of malate in gluconeogenesis?
Malate serves as a way to get oxaloacetate out of the mitochondria. Since OA cannot leave the mito itself, it is reduced to malate by malate dehydrogenase, then re-oxidized to OA by malate dehydrogenase once in the cytoplasm
What is the role of Cori cycle?
- links lactate produced from anaerobic glycolysis in RBC and exercising muscle to gluconeogenesis in liver
- lactate in RBC/muscle > blood stream > liver, converted to pyruvate, pyruvate through gluconeogenesis converted to glucose > glucose to blood > glucose to RBC/muscle
- prevents lactate accumulation and regenerates glucose
What are the possible precursors of gluconeogenesis? (8) (not pyruvate, talking about before pyruvate)
- fructose
- galactose
- glycogen
- glycerol
- propionate
- lactate
- alanine
- amino acids (not leucine or lysine)
- disorder of gluconeogenesis (blue box)
- similar to Tarui disease in glycolysis
- presents in infancy, early childhood
- sx: hypoglycemia, lactic acidosis, ketosis
fructose 1,6-bisphosphatase deficiency
- disorder of gluconeogenesis (blue box)
- deficiency in glucose 6-phosphatase
- inefficient release of free glucose into the bloodstream by liver in gluconeogensis and glycogenolysis
- sx: fasting hypoglycemia, lactic acidosis, hepatomegaly, hyperlipidemia, retarded growth
- tx: diet management
Von Gierke disease (GSD1a)
What are the transporters that uptake fructose, glucose, and galactose respectively?
- fructose: GLUT5
- glucose/galactose: SGLT1
- How is glucose converted to fructose?
- What is the “benefit” of converting glucose to fructose?
- What are the differences within glucose/fructose metaboliosms?
- Polyol pathway: glucose reduced to sorbitol (aldose reductase), sorbitol oxidized to fructose (sorbitol dehydrogenase)
(cells that lack sorbitol dehydrogenase (kidneys, retina, Schwann cells) can accumulate sorbitol, triggers swelling, retinopathy, cataracts, and peripheral neuropathy
- fructose metabolizes faster than glucose
- by-passes rate limiting step, no PFK-1 or PFK-2 regulation, makes triacylglycerols
Why does fructose consumption lead to several pathological conditions?
What are some of these conditions?
- enzymes (fructokinase and triose kinase) bypass most important regulatory step in glycolysis (phophofructokinase-1 reaction), they also deplete liver of ATP and phosphate (liver dysfunction)
- products (G3P and DHAP) are processed in glycolysis to pyruvate and acetyl CoA in unregulated fashion
- excess acetyl CoA converted to fatty acids (triacyglycerols and obesity)
- liver accumulates fatty acids (fatty liver)
How is galactose metabolized into a “useful” intermediate (glucose 6-phosphate)?
- galactose > galactose 1-phosphate (galactokinase) > glucose 1-phosphate (GALT - rate limiting) > glucose 6-phosphate (phosphoglucomutase)
- defect in galactose metabolism
- deficiency in either galactose 1-phosphate uridyl transferase (GALT) or galactokinase
- sx: failure to thrive, vomiting/diarrhea (dairy), hepatomegaly, cirrhosis, cataracts, retardation
- tx: remove galactose and lactose (b/c it gets broken down into glucose and galactose) from diet (patients still suffer CNS malfunction, delayed language skills)
galactosemia
type 1: GALT deficiency; failure to thrive, liver failure, sepsis, bleeding
type 2: accumulation of galactitol in lens of eye leads to cataracts in early infancy
What does pentose phosphate pathway (PPP) produce?
- sugar (DNA/RNA formation by oxidation of G6P to ribulose 5-P)
- NADPH (reduction of NADP+ to NADPH)
- no energy
What is the rate limiting step of PPP?
When glucose 6-phosphate (G6P) is oxidized to 6-phosphoglucono-δ-lactone by G6P dehydrogenase.
(NADP+ is also reduced to NADPH in this rxn, NADPH then goes to regerate glutathione, an important antioxidant of H2O2)
- deficiency within PPP that prevents conversion of glucose 6-phosphate
- particulary affects those of African decent
- causes hemolytic anemia when NADPH need is elevated
- infection, oxidizing meds
glucose 6-phosphate dehydrogenase deficiency
What are the differences between the oxidative and non-oxidative phases within PPP? When are they favored?
- oxidative: products: 2 NADPH, 1 CO2
irreversible, catabolic
favored in rapidly growing cells where ribulose 5-P is in high demand
- non-oxidative: products: ribulose converted to multiple products for glycolysis, gluconeogenesis, and nucleotide synthesis
reversible, anabolic
favored when NADPH is in high demand, products channeled into gluconeogenesis for re-entry into PPP (lactating mammary glands, adipose tissue, lung/liver tissue, phagocytic cells)
What is the structure of glycogen? Specifically, how is the chain bonded together and how are the branch points bonded together?
What end is glycogenin on and what does it do specifically?
- chains are linked together by α-1,4 glycosidic bonds
- branches are formed via α-1, 6 glycosidic bonds
- glycogenin is on the reducing end, it is a protein that creats a glycogen polymer on itself, essentially serves as a primer for glycogen synthesis; it is degraded/extended from non-reducing end
Where and how is glycogen stored?
- in liver (blood glucose regulator), muscle (fuel during exercise), and other tissues
- stored as granules: contain glycogen and enzymes for glycogen metabolism
What are the 3 key steps of glycogenesis?
-
Trapping / activation of glucose
- trapping same as glycolysis (glucose > G6P by glucokinase/hexokinase)
- G6P > G1P by phosphoglucomutase (reversible)
- G1P > UDP-glucose by UDP-glucose pyrophosphorylase (active form of glucose)
-
Elongation of glycogen primer
- UDP-glucose transfered to non-reducing end of glycogen (α-1,4 glycosidic) by glycogen synthase (rate limiting enzyme)
-
Branching of glycogen chains
- when glyc chain reaches 11 residues, part is broken off at α-1,4 and reattached elsewhere via α-1,6 by glucosyl (4:6) transferase
- increases solubility and # of term non-reducing ends
What are the two key steps of glycogenolysis?
-
Chain shortening
- glucose 1-phosphate cleaved from non-reducing ends of glycogen by glycogen phosphorylase (GP, rate limiting enzyme) and vitamin B6 cofactor
- continues until GP is within 4 glucose residues of the α-1,6 branch pt of main chain
-
Branch transfer, release of glucose
- 3 of remaining 4 glucose are transfered to non-reducing end of main chain by debranching enzyme
- the enzyme cleaves the α-1,6 bond and releases glucose
* glucose 1-P and glucose generated in ratio of 10:1
What happens to glucose 1-phosphate when it is brought to 1) liver and 2) skeletal/cardiac muscles after glycogenolysis?
- liver: G1P converted to G6P by epimerase and then to glucose by G6Pase, free glucose then released into blood
- myocytes (skeletal/cardiac): lack G6Pase, thus they use G1P to generate energy via glycolysis and TCA cycle
What are the two regulatory enzymes in glycogenesis and glycogenolysis respectively? How are these enzymes regulated?
- glycogenesis: glycogen synthase (dephospho form active)
- glycogenolysis: glycogen phosphorylase (phospho form active)
In general, how are glycogenesis and glycogenolysis regulated?
- Glycogenesis: fed state
- blood glucose high
- insulin high
- cell ATP high
- (dephospho states of enymes predominant)
- Glycogenolysis: fasting state
- blood glucose low
- glucagon high
- during exercise (calcium high, AMP high)
- (phospho states of of enzymes predominant)
What are the 4 key proteins involved in regulation of glycogen metabolism by insulin?
- GLUT4: translocated to membrane
- protein kinase B (PKB): phosphorylates PP1 (active) and GSK3 (inactive)
- protein phosphatase 1 (PP1): dephos glycogen synthase (active) and dephos glycogen phosphorylase (inactive)
- glycogen synthase kinase 3 (GSK3): like PP1 but in opposite scenario (low glucose, high glucagon)
What are the 5 key enzymes in the regulation of glycogen by glucagon?
- G protein: binding of glucagon to GPRC turns on G protein which activates AC and forms cAMP
- Adenylate cyclase (AC) and cAMP: activates PKA
- Protein kinase A (PKA): phosphorylates glycogen synthase (inactive) and PK (active), and an inhibitor which inactivates PP1
- Protein phosphatase 1 (PP1)
- Phosphorylase kinase (PK): phosphorylates glycogen phosphorylase (active)
What is the regulation of glycogen metabolism by epinephrine?
- similar to glucagon regulation
- free glucose inhibits glycogen phosphorylase in liver by not muscle
- Ca2+: activates glycogen phosphorylase kinase
- AMP: activates glycogen phosphorylase
(aka exercise induced)
- glycogen storage disease
- deficiency in glycogen synthase
- patients cannot store/synthesize glycogen, thus they rely on glucose
- vulnerable to hypoglycemia, must eat often
- sx: myalgia
GSD 0 :)
- glycogen storage disease
- deficiency in glucose 6-phosphatase
- inefficient release of glucose into BS by liver
- sx: fasting hypoglycemia, lactic acidosis, hepatomegaly, hyperlipidemia, retarded growth
- tx: diet management
GSD1a / Von Gierke disease
- glycogen storage disease
- deficiency in acid maltase (acid α-glucosidase)
- impaired lysosomal glycogenolysis = accumulation of glu in lysosomes
- disrupts muscle and liver cell functioning
- sx: muscle weakness
- infants die of heart failure
tx: enzyme replacement therapy using recombinant human α-glucosidase
GSD II / Pompe disease
- glycogen storage disease
- deficiency in α-1,6-glucosidase (debranching enzyme)
- light hypoglycemia and hepatomegaly
GSD III / Cori disease
- glycogen storage disease
- deficiency in glucosyl (4:6) transferase (branching enzyme)
- hepatomegaly, splenomegaly, cirrhosis
- death by age 5
GSD IV / Andersen disease
- glycogen storage disease
- deficiency in muscle glycogen phosphorylase (rate limiting step in glycogenolysis)
- patients unable to supply muscles w/ glucose
- sx: weakness, fatigue, muscle cramping, muscle breakdown, myoglobinuria
- exercise intolerance
- tx: reduce strenuous exercise
GSD V / McArdle disease
- glycogen storage disease
- deficiency in liver glycogen phosphorylase
- sx: hepatomegaly, hypoglycemia
GSD VI / Hers disease
