Flashcards in Medical Physiology Block 6 Week 3 Deck (62):
Describe glycolysis. What are the regulated steps?
glycolysis is a cytoplasmic pathway used by all cells to generate energy from glucose. One glucose molecule is converted into 2 pyruvate molecules, generating a net of 2 ATPs by substrate-level phosphorylation, and 2 NADHs; regulated step 1: glucose to G-6-P (enzyme is hexokinase in most tissues (low Km and negative feedback inhibition) and glucokinase (induced by insulin in the liver (high Km and no negative feedback); regulated step 2: F-6-P to F-1,6-BP (enzyme is phosphofructokinase-1 (rate limiting step upregulated by AMP, F-2,6-BP and downregulated by ATP and citrate); regulated step 3: conversion of phosphoenolpyruvate into pyruvate (enzyme pyruvate kinase (positive feedback from F-1,6-BP and negative feedback from ATP and alanine)
Which enzyme (not part of glycolysis cycle) upregulates glycolysis and decreases gluconeogensis? What is the product? What regulates the enzyme?
phosphofructokinase-2; fructose-2,6-bisphosphate; upregulated by insulin and downregulated by glucagon
What is the role of insulin on striated muscle?
glut 4 transporter insertion into the plasma membrane
Describe the role of thiamine, FAD, NAD, MG + 2 in the glycolytic pathway and explain which reactions require these cofactors
Dehydrogenase: oxidation of the substrate via transfer of (one or more) hydride ions (H") to an electron acceptor, often NAD+ or FAD; ATP must be bound to a magnesium ion to biologically active; thiamine is a crucial component in the conversion of pyruvate to acetyl coA (aerobic)
Compare and contrast the hormonal regulation of glycolysis in the liver for a person who has fasted overnight and for a person who has just eaten a piece of cheesecake. Specifically focus on the regulated steps in the glycolytic pathway.
in the fed state, insulin and glucose activate glycolysis glycogenesis; in the fasted state, glucagon increases intracellular cAMP which activates PKA; decreases hexokinase and pyruvate kinase; stimulates glycogenolysis; also decreases activity of phosphofructokinase-2
Compare and contrast the regulation of the glycolytic pathway in the muscle and liver.
in the fed state, regulation of muscle and liver is by insulin; during exercise, the muscle does not respond to glucagon signaling and uses glucose from the blood and from its own glycogen stores for energy; PFK-2 and pyruvate kinase have no site for PKA phosphorylation (promoting glycolysis); PFK-2 is activated by AMP kinase (through increases in AMP)
Can exercise recruit GLUT4 to the plasma membrane?
Do epinephrine and glucagon have the same effects on the liver?
Discuss the role of the 2 different mitochondrial shuttles during glycolysis.
glycerol-phosphate shuttle: hydrogens and electrons are transferred from NADH to glycerol phosphate which can diffuse through the membrane into the mitochondria. Inside the mitochondria, glycerol phosphate reacts with FAD coenzyme in enzyme complex 2 in the electron transport chain to make dihydroxyacetone phosphate which in turn diffuses back to the cytoplasm to complete the cycle.
malate-aspartate shuttle: NADH is used to reduce oxaloacetate (addition of hydrogens) to malate; malate is transported across the mitochondrial membrane (in exchange for alpha ketoglutarate); in the matrix, malate is oxidized to oxaloacetate (NAD is reduced to NADH again); oxaloacetate is converted in aspartate and transported out of the mitochondria (in exchange for glutamate); aspartate regenerates oxaloacetate
Why are shuttles needed? which shuttle is used in which cell types?
mitochondrial membrane is impermeable to NADH; glycerol phosphate shuttle in brain and muscle; malate-aspartate shuttle in heart and liver
Describe the role of amino acids in the regulation of blood glucose during fasting.
During fasting, amino acids from the muscle are converted to alanine and glutamine. Alanine is converted to glucose in the liver, while glutamine is converted to ammonia and glucose in the kidney.
Describe the role of fatty acids in the regulation of blood glucose during fasting.
The decline in [insulin] and the ensuing lipolysis are sufficient to supply FAs to extracerebral tissues (e.g., muscle, heart, liver) for fuel and glycerol to the liver for gluconeogenesis.
Describe how diabetes and starvation alter ketone body urinary urea nitrogen levels.
The kidney uses the carbon skeleton of glutamine for renal gluconeogenesis and converts the amino group to ammonia, which it excretes. This ammonia excretion is particularly important in maintaining body acid-base balance during fasting. The dependence of gluconeogenesis on proteolysis is reflected by an increase in urinary nitrogen excretion in the early phase of starvation.
Compare and contrast inter-organ relationships between muscle, liver and brain in the adaption to prolonged fasting.
CNS cannot make a complete transition to ketone utilization (still requires some glucose); Cardiac myocytes preferentially catabolize fatty acids in all metabolic contexts except starvation (switch to ketone bodies); liver uses fatty acids; red blood cells and renal medulla uses glucose
Describe the phases of glucose homeostasis in humans.
Fed state: An increase in glucose and insulin activates glucokinase in the liver (is not inhibited by its product G-6-P
Fasted state (glucagon signaling): increased glycogenolysis and beta oxidation
Early starved state: glycogen depleted; protein breakdown; TAG hydrolysis; some production of ketone bodies
Late starved state: protein breakdown slows down; TAG hydrolysis increases; production of ketone bodies
Describe the structure of fatty acids and triglycerides.
Hydrophobic tail (hydrocarbon chain) and hydrophilic head (carboxyl group); three fatty acid esterified to glycerol
Describe the fluxes of TGs and fatty acids between organs in the fed, fasted, and starved dietary states and explain the physical form in which transport between organs occurs.
Fed state: fatty acids from GI tract and fatty acids produced in the liver are esterified to glycerol-6-phosphate and transported as either chylomicrons or VLDL to adipose tissue for storage
fasted and starved state: trigylcerides are broken down and unesterified fatty acids are transported through albumin to the liver for beta oxidation or formation of ketone bodies
Name the different lipases. What is their location and function?
lipoprotein lipase: found in endothelium (free fatty acids are transported into muscle and adipose tissue); induced by insulin
hormone-sensitive lipase: found in adipose tissue (responds to phosphorylation via glucagon signaling); hydrolyzes the ester bonds at C1 or C3, liberating one FA, leaving a di-acyl glycerol (DAG) comprising the glycerol backbone with two remaining FA attached.
di-acylglycerol lipase removes a second FA, leaving mono-acylglycerol (MAG). Mono-acyl glycerol lipase then cleaves off the remaining FA, freeing glycerol
Explain how glycerol from TG breakdown is utilized.
Can be used in glycolysis or transported into the blood to the liver, where it can be used for either glycolysis or gluconeogenesis
List the major lipoproteins that transport TG through the blood. Describe the sites of synthesis and secretion of these lipoproteins and their sites of utilization.
chylomicrons: transport dietary TAGs and cholesterol from intestine to tissues (apoB-48, apoC-II, apoE); uptake by liver
VLDL: transports TAGs from liver to tissues (apoB-100, apoC-II, apoE)
LDL: deliver cholesterol into cells (apoB-100)
IDL (VLDL remnants): picks up cholesterol from HDL (apoE); taken up by liver
HDL: picks up cholesterol accumulating in blood vessels and delivers cholesterol to liver through scavenger receptor (also shuttles apoC-II and apoE in blood) (apoA-1)
What is a cofactor for lipoprotein lipase?
Describe the reaction of fatty acid activation in the cytosol.
FA must be linked to CoA in order to undergo lipogenesis. Fatty acyl CoA synthetase performs this reaction; The three activated FA are esterified to glycerol-3-phosphate in stages.
Describe the carnitine shuttle. Why is it important?
Acyl group is transferred from CoA to carnitine by carnitine palmitoyltransferase (CPT I) in the mitochondrial outer membrane; Acylcarnitine is transported across
the membrane by acylcarnitine/carnitine translocase (localized in the mitochondrial inner membrane) into the mitochondrial matrix; The acyl group is transferred back to CoA by carnitine palmitoyltransferase II in the matrix; Carnitine is returned to cytosolic side in exchange for another acylcarnitine
Mitochondrial inner membrane is impermeable to acyl-CoAs (activated fatty acids)
Describe beta oxidation.
takes place in the mitochondria and involves oxidative removal of 2 carbons per cycle to yield 1 NADH, 1 FADH2, and 1 acetyl-coA (oxidation, hydration, formation of ketone, thiolase)
Describe the breakdown of fatty acids with odd numbers of carbons and the oxidation of unsaturated fatty acids
Requires enoyl coA isomerase to convert cis double bonds to trans configuration; slower carnitine shuttling
Explain how ketone bodies are synthesized and utilized.
during fasting, the liver converts excess acetyl coA from beta oxidation of fatty acids into ketone bodies (can be used by muscle and brain tissues); regulated by mitochondrial HMG coA
What tissues synthesize fatty acids?
It occurs mainly in specialised fat cells; adipocytes, but also in liver, kidney and of course lactating mammary
Explain the pathway of fatty acid synthesis
occurs in the cytosol and involves the transport of acetyl coA from the mitochondria via the citrate shuttle, carboxylation to malonyl coA, and linking together 2 carbons per cycle to form long fatty chain acids (synthesis stops a C16 palmitoyl coA)
one cycle: condensation reaction, reduction, dehydration, and reduction (Beta-ketoacyl-ACP synthase transfers two chain carbon molecule from previous cycle to the next acyl chain
Describe the citrate shuttle.
Acetyl CoA condenses with oxaloacetate (in the
mitochondrial matrix) forming citrate, as in the
TCA cycle. This is catalysed by citrate synthase; Citrate is then exported from the matrix into the cell cytoplasm in exchange for malate; citrate lyase reforms oxaloacetate and acetyl coA; Oxaloacetate is reduced to malate by malate dehydrogenase (using NADH) and is transported and decarboxylated into pyruvate, which is transported back into the mitochondrial matrix to regenerate oxaloacetate (generating NADPH for fatty acid synthesis)
Describe the use of NADPH in fatty acid synthesis and explain why NADPH is used instead of NADH.
NADPH inhibits glucose-6-phosphate
dehydrogenase, and in doing so slows flux through
the pentose phosphate pathway (PPP); produced through pentose phosphate pathway
List the major sites of regulation of the fatty acid biosynthesis pathway
conversion of acetyl coA to malonyl coA (enzyme: acetyl coA carboxylase; upregulated by citrate and insulin and downregulated by glucagon and palmitoyl coA)
carnitine acyltransferase I (rate limiting for fatty acid oxidation; inhibited by malonyl coA)
What are the major functions of biliary secretions?
Elimination of many waste products (such as bilirubin and cholesterol); Promotion of digestion and absorption of lipids from the intestine
Define the polarities of a hepatocyte.
Hepatocytes form an epithelium, one cell thick, that constitutes a function barrier between fluid compartments with differing ionic compositions: the tiny canalicular lumen containing bile and the much larger sinusoid containing blood
What are stellate cells?
Stellate cells are in the space of Disse and are characterized morphologically by the presence of large fat droplets in their cytoplasm. These cells play a central role in the storage of vitamin A
What is the space of Disse?
perisinusoidal space (large proportion of microvilli project into this space)
What gives hepatocytes polarity?
Tight junctions and desmosomes separate apical membrane from basolateral membrane
What is the importance of having two blood supplies and one venous drainage in the hepatic circulation?
The peribilary plexus may provide the means for modifying biliary secretions through the bidirectional exchange of compounds such as proteins, inorganic ions, and bile acids between the bile and blood within the portal tract
Describe the organization of the liver.
a classic lobule is a hexagon in cross section, with a branch of the hepatic vein at its center and, at each of the six corners, triads composed of branches of the hepatic artery, portal vein, and bile duct.
Describe the different properties between zone 1 and zone 3 of the liver.
In zone 1, oxidative energy metabolism with beta oxidation, amino acid metabolism, ureagenesis, gluconeogenesis, cholesterol synthesis, and bile formation is particularly important (first zone to receive blood, oxygen, and nutrients)
Localized in zone III are glycogen synthesis from glucose, glycolysis, liponeogenesis, ketogenesis, xenobiotic metabolism, and glutamine formation
How are bile acids brought into the hepatocyte? What happens to these acids in the liver?
Basolateral uptake of bile acids into the hepatocyte involves a sodium-dependent transporter, a sodium independent (chloride or glutathione-dependent), as well as nonionic diffusion of unconjugated bile acids
Most of the bile acids that the liver secretes into the bile are conjugated (glycine or taurine); negatively charged (bases); increased water solubility; REMEMBER: the liver uses cholesterol to produce primary bile acids
Describe the urea cycle.
The breakdown of α-amino acids occurs by deamination to α-keto acids and NH + 4; The α-keto acids (“carbon skeleton”), depending on the structure of the parent amino acid, are metabolized to pyruvate, various intermediates of the citric acid cycle, acetyl coenzyme A (acetyl CoA), or acetoacetyl CoA.
Urea cycle: IN - NH4+ + HCO3- + Aspartate; OUT: Urea and fumurate
amino acids transported to the liver are transaminated to glutamate, which undergoes deamination to produce ammonium or transamination to make aspartate (produces alpha-ketoglutarate)
What are the major fates of cholesterol after entering the hepatocyte?
The major fates of cholesterol are secretion into bile, excretion in feces when intestinal cells are sloughed, and synthesis of steroid hormones
Describe metabolism of vitamin A
In the hepatocyte, retinyl esters may be hydrolyzed to release free retinol, which can be transported into the sinusoids bound to retinol-binding protein and prealbumin; Alternatively, retinyl esters may be stored in the hepatocyte or transported as RBP-bound retinol to stellate (Ito) cells; Retinol may also undergo oxidation to retinal and conversion to retinoic acid (conjugated and secreted into bile)
Describe metabolism of vitamin D
The first step in activation of vitamin D is the 25-hydroxylation of vitamin D, catalyzed by a hepatic cytochrome P-450 enzyme
Termination of the activity of 1,25-dihydroxyvitamin D also occurs in the liver by hydroxylation at carbon 24, mediated by another cytochrome P-450 enzyme.
Describe metabolism of vitamin E
The α-tocopherol is secreted again as a component of hepatically derived VLDL and perhaps HDL. The γ-tocopherol appears to be metabolized or excreted by the liver.
Describe the liver's storage of copper and iron
copper: is essential for the function of cytochrome c oxidase and superoxide dismutase; most secreted into bile; copper in blood is bound to albumin or ceruloplasmin
iron: can be toxic and therefore is bound to ferritin; used in ETC reactions
What are Kuppfer cells?
population of fixed macrophages removes particulate matter from the circulation
Describe how the liver detoxifies molecules.
phase I, mediated by cytochrome P450, transfer an oxygen to the molecule (increases the molecules polarity); phase II reactions conjugates the molecule (increases its solubility)
What is the location of sulfotransferases? UGT? glutathione-S-transferase?
cytosol; SER (glucourante); SER
How is bile secreted in the canalicular membrane? What about other molecules (specifically phospholipids)?
ATP-dependent transporter; MRP2 (anions; ATP-dependent); phospholipids: MDR3 (ABCB4) is a “flippase” that promotes the active translocation of phosphatidylcholine (PC) from the inner to the outer leaflet of the canalicular membrane. Bile salts then extract the PC from the outer leaflet so that the PC becomes a component of bile, where it participates in micelle formation
Describe the entero-hepatic circulation.
bile is secreted by the liver, reabsorbed in the intestine (active absorption in the ileum and passive absorption throughout), transported to the liver for another round of secretion
some bile acids/bases may be modified by colonic bacteria (may be reabsorbed or excreted in feces)
How is bilirubin absorbed? excreted?
absorbed through electroneutral, electrogenic, and chloride-mediated mechanisms; conjugated to glucuronic acid, secreted in the canalicular membrane through MRP2
Excretion of bilirubin from the body requires making the hydrophobic molecule hydrophilic (one of the last steps that occurs in either the kidney or colon)
What do cholangiocytes secrete?
Secrete bicarbonate rich fluid (similar to pancreatic duct cells)
What type of fluid is secreted following excretion of bile acids/bases in the canalicular membrane?
bicarbonate rich fluid
How is liver detoxification regulated?
Steroid and xenobiotic receptor (SXR) serves as a master regulator of xenobiotic metabolism: upregulates P450 family enzymes, and activates glutathione-S-transferase
Another nuclear receptor (constitutive androstane receptor or CAR) regulates all the components of bilirubin metabolism, including uptake, conjugation, and excretion: modulate OAPT (uptake), UGT (conjugation), and MRP2 (excretion)
How are molecules transported within the liver?
bound to fatty acid binding protein OR dihyrdiol dehydrogenase OR glutathione-S-transferase B
What signaling molecules determine insertion into the cell membrane of GLUT4 transporter?
AMP kinase and insulin
Hyperammonemia results in hepatic encephalopathy. Why?
ammonia is highly toxic for astrocytes
Is salivary amylase a critical component of carbohydrate digestion and absorption?
Does the gall bladder contract in the cephalic phase?
No; not stimulated by acetylcholine
Are fatty acid transporters present on the apical membrane of enterocytes?