Integrated Metabolism Flashcards
1
Q
Functions of Metabolism
A
- Supplies energy and biosynthetic precursors
- Provides mechanisms of excretion of waste products
- Provides protection
- Supplies molecules that operate control mechanisms
2
Q
Metabolic control and integration
A
- Hormonal controls
- CNS controls the release of the hormones
- Secondary messengers (intracellular and intercellular signalling)
- Availability of circulating substrates
3
Q
Co-operation between the different organs and tissues
A
- to continue to perform these key functions as conditions change, and to avoid substrate cycles
4
Q
Control and integration at whole body level
A
- Control of cellular activities and pathways via extracellular signals produced by other cells of the body, under control of the CNS
- circulating hormones, with specific mechanisms of action
5
Q
Regulation of enzyme activities
A
- allosterically (binding of a compound to another site on the enzyme);
- via covalent modification (mainly phosphorylation/ dephosphorylation)
- via changes in enzyme concentration, eg. new synthesis
6
Q
Regulation of pathways by regulatory enzymes
A
- to maintain balance between pathways
- to control synthesis and breakdown pathways in a reciprocal manner – activation of one and inhibition of the other – to avoid futile cycles
7
Q
Metabolic pathways involving carbohydrates
A
- Glycolysis
- Gluconeogenesis
- PDH reaction, Shuttles
- Pentose phosphates pathway
- Glycogen synthesis
- Glycogen breakdown
8
Q
Metabolic pathways involving lipids
A
- Fatty acid ß-oxidation
- Ketone bodies synthesis
- Fatty acid synthesis
- Complex lipid synthesis
- TAG synthesis
- TAG hydrolysis
- Cholesterol synthesis
9
Q
Metabolic pathways involving proteins
A
- Amino acids catabolism
- Transamination
- Amino acid synthesis
- Urea cycle
- Nucleotide synthesis
10
Q
Metabolic pathways involving energy
A
- Citric acid cycle (CAC)
- Electron transport chain (ETC)
- Oxidative phosphorylation (OxP)
11
Q
Metabolic features of brain
A
- Fuel: glucose (KB)
- Fuel store: none
- Fuel exported: none
- Pathways: glycolysis, PDH, AA cat, CAC, ETC, OxP
12
Q
Metabolic features of liver
A
- Fuel: glucose, fatty acids, AA
- Fuel store: Glycogen, TAG
- Fuel exported: glucose, KB, FA, VLDLs
- Pathways: glycolysis, PDH, FA Ox, AA cat, CAC, ETC, OxP;
Gluconeogenesis; Glycogen synthesis, glycogen breakdown, FA synthesis, TAG synthesis, KB synthesis; Urea synthesis
13
Q
Metabolic features of RBCs
A
- Fuel: glucose
- Fuel store: none
- Fuel exported: lactate
- Pathways: glycolysis
14
Q
Metabolic features of heart muscle
A
- Fuel: fatty acids; glucose
- Fuel store: none
- Fuel exported: none
- Pathways: glycolysis, PDH, FA Ox, AA cat, CAC, ETC, OxP;
15
Q
Metabolic features of adipose tissue
A
- Fuel: glucose, fatty acids, AA
- Fuel store: TAG
- Fuel exported: FA, glycerol
- Pathways: glycolysis, PDH, FA Ox, AA cat, CAC, ETC, OxP; TAG synthesis, TAG hydrolysis
16
Q
Metabolic features of skeletal tissue
A
- Fuel: fatty acids, glucose, AA
- Fuel store: glycogen
- Fuel exported: lactate, alanine
- Pathways: glycolysis, PDH, FA Ox, AA cat, CAC, ETC, OxP; glycogen breakdown; glycogen synthesis
17
Q
Metabolic control: aims, pathways and regulators
A
- Main aims: control blood glucose levels (glucose homeostasis), supply glucose to tissues which depend on it for their energy requirements (ATP synthesis)
- Main pathways: glycolysis and gluconeogenesis
- Main regulators: hormones - insulin and glucagon; epinephrine (adrenaline)
18
Q
Biochemical actions of insulin
A
- Activates/Increases: uptake of glucose in muscle cells and adipocytes, glycolysis, glycogen synthesis, TAG synthesis, protein, DNA and RNA synthesis
- Inhibits / Decreases: gluconeogenesis, lipolysis, protein hydrolysis
19
Q
Physiological actions of insulin
A
- Signals fed state
- Activates: fuel storage, cell growth and differentiation
- Decreases: blood glucose level
20
Q
Biochemical actions of glucagon
A
- Activates/Increases: cAMP level in liver and adipose tissue, glycogenolysis, TAG hydrolysis, gluconeogenesis
- Inhibits/Decreases: glycolysis, glycogen synthesis
21
Q
Physiological actions of glucagon
A
- Activates/Increases: glucose release from liver, blood glucose level
22
Q
Biochemical actions of epinephrine (adrenaline)
A
- Activates/Increases: cAMP level in muscle, glycogenolysis, TAG hydrolysis
- Inhibits/Decreases: glycogen synthesis
23
Q
Physiological actions of epinephrine (adrenaline)
A
- Activates/Increases: glucose release from liver, blood glucose level
- Inhibits/Decreases: glucose use by muscle
24
Q
Digestion and Nutrient absorption in intestine in absorptive (fed) state (steps 1-3)
A
- Carbohydrate digestion - absorption of glucose –> blood
- Lipid (TAG) digestion - formation of chylomicrons –> lymph –> blood
- Protein digestion - absorption of AA –> blood
25
What happens to blood glucose levels 2-4 hours after a meal? (step 4)
- increase
- Insulin is released from the β-cells in the pancreas
- Insulin levels are high
- Glucagon levels are low
26
Metabolic integration in liver in absorptive (fed) state (steps 5-7)
5. Glucose uptake by liver cells – Glycolysis (Insulin activated), PDH, CAC, ETC, OxP --> ATP, CO2 to meet the energy requirements of liver cells
6. Glycogen synthesis in liver cells (Insulin activated) – excess glucose is stored
7. Fatty acid and TAG synthesis in liver cells (Insulin activated) – excess glucose converted to fatty acids, esterified with glycerol and exported into blood as VLDLs
27
Metabolic integration in the brain during fed state (step 8)
- Glucose uptake by CNS cells (dependent on glucose for energy; FA transport across the blood brain barrier too slow)
- Glycolysis, PDH, CAC, ETC, OxP --> ATP, CO2 to meet the energy requirements of brain cells
28
Metabolic integration in RBCs during fed state (step 9)
- Glucose uptake by RBC (dependent on glucose for energy; no mitochondria)
- Glycolysis, Pyruvate reduced to lactate --> ATP, NAD+ to meet the energy requirements of RBC, lactate released in the blood
29
Glucose uptake by the muscle cells and adipose tissue cells (step 10 of absorptive state)
- Insulin-stimulated transport - - Glycolysis (Insulin activated), PDH, CAC, ETC, OxP ATP, CO2 to meet the energy requirements
- also to convert glycolysis intermediates to glycerol-3-phosphate required for TAG synthesis in adipose tissue cells
30
Glycogen synthesis in muscle cells (step 11 of absorptive state)
- Insulin activated
| - excess glucose is stored to be used during muscle contraction
31
Step 12 of absorptive state (TAG and VLDLs)
- TAG from chylomicrons and VLDLs are hydrolysed by lipoprotein lipases in capillaries --> FA and glycerol; FA taken up by adipose tissue cells
32
Metabolic integration in adipose during absorptive state (step 13)
- TAG synthesis (from FA and glycerol-3-phosphate) and TAG storage in adipose tissue cells
33
Metabolic integration in tissues during absorptive state (step 14)
- AA uptake into muscle, liver and other tissues is also activated by insulin --> protein synthesis; synthesis of other N-containing compounds; some are used for energy generation
34
Glucose uptake by brain
- GLUT3 transporters – high affinity for glucose
- Independent of insulin
- Glucose is phosphorylated by hexokinase – low Km for glucose
35
Glucose uptake by RBCs
- GLUT1 transporters – high affinity for glucose
| - Independent of insulin
36
Glucose uptake by liver
- Occurs only when blood glucose levels are high
- GLUT2 transporters – low affinity for glucose
- Independent of insulin
- Glucose: phosphorylated by glucokinase (higher Km for glucose than hexokinase and is not inhibited by glucose-6-phosphate)
- Glycogen synthesis is activated: glycogen synthase is dephosphorylated and activated (glycogen phosphorylase is phosphorylated and inhibited)
- Glucose-6-P is used mainly for glycogen synthesis
- PFK and pyruvate kinase are active; gluconeogenesis regulatory enzymes are inhibited
- Acetyl-CoA carboxylase is activated (catalyses rate limiting step in FA synthesis, the conversion of acetyl-CoA to malonyl-CoA)
- Malonyl-CoA inhibits carnitine transferase (FA aren't transported into mitochondria for β-oxidation: no futile cycle)
37
Glucose uptake by muscle cells
- Occurs only when blood glucose and insulin are high
- Insulin binds to its receptors: GLUT4 transporters are delivered to cell membrane (insulin-dependent uptake)
- Glucose is phosphorylated by hexokinase (low Km for glucose)
- Glycogen synthesis is activated: glycogen synthase dephosphorylated and activated (glycogen phosphorylase is phosphorylated and inhibited, as in the liver cells)
- Glucose-6-P is used mainly for glycogen synthesis, also used for glycolysis -> Acetyl-CoA -> PDH -> CAC -> ETC -> OxP -> ATP
38
Glucose uptake by adipose cells
- Occurs only when blood glucose and insulin are high
- Insulin binds to its receptors: GLUT4 transporters are delivered to cell membrane (insulin-dependent uptake)
- Glucose phosphorylated by hexokinase and glucose-6-P is used for glycolysis to DHAP stage - DHAP is reduced to glycerol-3-P needed for TAG synthesis
- Insulin activates lipoprotein lipase in adipose tissue capillaries -> hydrolyses FA from dietary TAG (in chylomicrons) and VLDLs -> FA taken up by the adipose tissue cells (glycerol returns to the liver cells) and used for TAG synthesis
- Insulin inhibits the hormone-sensitive lipase – no futile cycles
39
Blood glucose levels during basal fasting state
- Glucagon is released from the α-cells in the pancreas - Glucagon levels are high, Insulin levels are low
40
Glycogen breakdown in liver cells during basal fasting state
- about 2-3 hours after a meal
| - glucose is released in the blood, for brain and RBCs
41
Glucose uptake by brain cells and RBCs (steps 3-4 of basal fasting state)
3. Glucose uptake by brain cells - Glycolysis, PDH, CAC, ETC, OxP --> ATP, CO2 to meet the energy requirements of brain cells
4. Glucose uptake by RBC - Glycolysis, Pyruvate reduced to Lactate --> ATP, NAD+ to meet energy requirements of RBC
42
TAG hydrolysis in adipose tissue (step 5 of basal fading state)
- hormone-sensitive lipase --> FA and glycerol released in the blood for other tissues
43
FA uptake by muscle and liver cells during basal fasting state (steps 6-7)
6. FA uptake by muscle cells – FA Ox, CAC, ETC, OxP --> ATP, CO2 to meet energy requirements of muscle cells
7. FA uptake by liver cells – FA Ox, CAC, ETC, OxP --> ATP, CO2 to meet energy requirements of liver cells; KB synthesis --> KB released in the blood
44
KB uptake in muscle cells in basal fasting state (step 8)
- KB uptake by muscle cells --> Acetyl CoA - CAC, ETC, OxP --> ATP, CO2 to meet energy requirements of muscle cells
- Skeletal muscle cells also use their own glycogen stores --> ATP --> activity
45
Protein degradation in muscle cells in basal fasting state (step 9)
- AA released in the blood (mainly alanine and glutamine)
- alanine is taken up by liver cells to be used as substrates for gluconeogenesis; glutamine is metabolised mainly by intestine and kidney cells
46
Urea synthesis in liver cells in basal fasting state (step 10)
- urea released in the blood
| - elimination of nitrogenous waste products in urine, via the kidney
47
Substrates for gluconeogenesis (step 11-12 in basal fasting state)
11. lactate released in blood by RBCs taken up by liver cells, to be used as substrate for gluconeogenesis
12. glycerol released in blood from hydrolysis of TAG in the adipose tissue cells is taken up by liver cells, to be used as substrate for gluconeogenesis
48
Changes during fasting - transition from basal to prolonged fasting
- Prolonged starvation: increase in circulating FAs and ketone bodies
- Mobilisation of fat reserves for energy generation
- Muscle proteolysis decreases: reduced requirement of glucose
- Glucose and ketone bodies released are used as energy source by the brain
- Utilisation of FAs by other tissues spares glucose for those tissues that rely on glucose as only energy source
- 3 days – 30% energy, 40 days – 70% energy from ketone bodies.
49
Metabolic changes during prolonged fasting
| 3-5 days of fasting
- Use of KB by muscle tissue decreases; FA become the main energy substrate for skeletal muscle cells, as well as for heart muscle cells
- Blood KB levels rise
- uptake of KB by brain cells increases; the use of glucose decreases, but glucose remains major fuel for brain
- RBCs keep using glucose
- Liver gluconeogenesis decreases
- Less muscle protein is degraded to provide AA for gluconeogenesis, less AA catabolism, less urea produced
- KB act on β-cells of pancreas to release a small amount of insulin, reduces the rate of proteolysis and lipolysis
- body uses its fat stores, to conserve functional proteins
- Gluconeogenesis also occurs in kidney cells
50
Two ways to reversibly modulate the rate of an enzyme catalysed reaction in a cell
- Change the amount of enzyme present in the cell (change enzyme concentration)
- Change the rate of catalysis by a given amount of enzyme (change enzyme activity)
51
Change the amount of enzyme present in the cell (change enzyme concentration)
- slow changes (hours, days) at the gene activation level
- Change the rate of enzyme synthesis
- Change the rate of enzyme degradation
52
Change the rate of catalysis by a given amount of enzyme (change enzyme activity)
- rapid changes
- Allosteric control
- (instantaneous): activators or inhibitors (ligands) can bind to one or more allosteric sites (other than the active site for the substrates) – Km ↑ or ↓
- Covalent modification (rapid, but not instantaneous) usually triggered by chemical signals from other cells (hormones) - eg phosphorylation by kinases, or dephosphorylation by phosphoprotein phosphatases
53
Mechanism of hormonal control
- Hormone (glucagon, adrenaline (epinephrine)
- Binds to receptor of target cell
- Increases cAMP level inside cell
- cAMP activates a protein kinase (PKA)
- PKA phosphorylates
specific proteins
- Phosphorylation leads to
changes in enzyme activities and metabolic responses
54
Glucokinase (GK)
- bound to a regulatory protein (GKRP) in nucleus – inactive
- Released when blood glucose levels increase
- When fructose-6-P increases, GK is rebound to GKRP and ‘switched-off’
55
Phosphofructokinase (PFK1)
- Allosteric control
- controlled by activity of PFK2: bifunctional enzyme, with two catalytic sites: one synthesizes F-2,6-BP (kinase activity), other hydrolyses it back to F-6-P (bisphosphatase activity)
- kinase site is active when PFK2 is dephosphorylated -activated by insulin and inactivated by glucagon – levels of F-2,6-BP are high and Fructose 1,6-bisphosphatase is inactive
56
Fructose 1,6-bisphosphatase
- Allosteric control
- Transcriptional control
- Also controlled by the activity of PFK2
- F-2,6-BP decreases when the bisphosphatase site is active (phosphorylated state) - activated by glucagon and inactivated by insulin
- PFK2 is phosphorylated, Fructose 1,6-bisphosphatase is active
57
Regulation of PFK bi-functional protein
- Insulin aims to produce acetyl-CoA for FA synthesis – storage (fed - hyperglycemia)
- Glucagon aims to promote release of glucose (fasted - hypoglycemia)
- May be considered a modification mechanism - indirect
- Function is to control levels of fructose 2, 6-bisP – allosteric regulator
58
Pyruvate kinase
- Allosteric control
- regulated via phosphorylation
- glucagon released when the blood glucose levels are low activates the adenylate cyclase, and cAMP is produced
- cAMP activates protein kinase A
- Protein kinase A phosphorylates pyruvate kinase and inhibits its activity
- Phosphoenolpyruvate is used for gluconeogenesis
59
Glucose-6-phosphatase
- Activated by transcriptional changes, controlled by insulin and glucagon
60
PEP carboxykinase
- Allosteric control
| - Activated by transcriptional changes, controlled by insulin and glucagon
61
Pyruvate carboxylase
- Allosteric control
- Activated by high acetyl-CoA (which inhibits PDH)
- Gluconeogenesis is promoted
62
Control of glycolysis in muscle cells
- Muscle glycolysis influenced by ATP requirements
- primary regulatory control is the energy charge of the cell
ATP:AMP ratio
- ATP (allosteric inhibitor)
- AMP (allosteric activator)
- ADP not involved in allosteric control due to action of adenylate kinase, which salvages ATP from ADP in myocytes
63
Muscle hexokinase
- regulated by levels of glucose-6-P
- High [glucose-6-P] = maximal glycogen synthesis – Hexokinase is inhibited
- Not a major site of regulation as the product is precursor of other pathways
- PPP minimal in muscle: main exit routes for glucose-6-P are glycolysis and glycogen synthesis
64
Muscle phosphofructokinase PFK-1
- most important regulatory enzyme
- Tetrameric enzyme: L and M subunits e.g. M4, M3L, M2L2, L4, Myocytes M4, liver L4
- enzyme is inhibited by a decrease in pH (anaerobic production of lactate) to protect against acid damage
- Inhibited allosterically by high [ATP]: More active at low [ATP], reduced affinity for fructose-6-P
- Activated allosterically by AMP
- Activated allosterically by F-2,6-B, F-2,6-B levels depend on activity of PFK-2
65
PFK-2 in muscle tissue
- M isoform of PFK-2 in heart and skeletal muscle, L isoform in liver
- Differences in regulation – phosphorylation of M isoform activates kinase site (M isoform not controlled by glucagon) – activates glycolysis in muscle cells
- Hormonal regulation - by epinephrine
66
Pyruvate kinase (glycolysis control in liver cells)
- controls pyruvate production
- Inhibited allosterically by increased alanine levels
- Inhibited allosterically by ATP
- Tetrameric protein: each subunit is allosterically regulated by ATP
- High [ATP] inhibits the enzyme
- Activated allosterically by high [fructose 1,6-bisP]
67
Leptin
- Expressed by adipocytes
- Decreases appetite – hypothalamus
- Increases FA oxidation via activation of AMP-activated protein kinase (activated when the AMP/ATP ratio increases – low energy levels)
- Prevents accumulation of FA in non-adipose tissue
- Obesity – correlated with leptin resistance
68
Adiponectin
- Released by adipocytes – regulator of AMP-activated protein kinase activity (↑ phosphorylation, ↑ activity)
- Receptors on both liver and muscle cells (similar effect as insulin)
- In liver: increases FA oxidation and decreases gluconeogenesis
- In muscle: increases glucose uptake and glucose/FA oxidation
69
Ghrelin / Peptide YY (PYY3-36)
- Involved in short term regulation of appetite
- Ghrelin: peptide released by stomach, increases during fasting, stimulates appetite.
- PYY3-36: peptide released by intestine, inhibits food intake.
70
Roles of hypothalamus
- Several neuronal cell activities in the arcuate nucleus region of the hypothalamus
- NPY/AgRP neurons increase appetite (NPY = neuropeptide Y; AgRP = Agouti related protein)
- POMC/CART neurons suppress appetite (POMC = proopiomelanocortin; CART = cocaine- and amphetamine- regulated transcript)
