Glucose Metabolism

first three enzymes in glycogenesis

Enzymes 1) Hexokinase/glucokinase (glucose -> G6P) 2) Phosphoglucomutase (convert G6P into G1P)- switch position of phosphoryl group UDP-Glucose Pyrophosphorylase - Combine G1P with UTP nucleotide (obtains nucleotide from UDP glucose)- transfer UDP moiety to G1P- UDP glucose is high energy molecule that drives reaction to completion- as enzyme catalyzes phosphoryl groups in UTP, reaction driven to completion)

review of hormonal control of glucose metabolism

muscle cells dont do gluconeogenesis glucagon is not a major regulator in skeletal muscle because it doesnt significantly express the glucagon receptor glucagon/epinephrine have opposite effect of insulin in the liver

Glycogen vs. fat

1) why glycogen for energy storage when fat is more abundant and energy rich? (Can extract more energy from oxidation of fat than glucose) a) muscles cannot mobilize fat as rapidly as glycogen b) fatty acids cannot be metabolized anaerobically (If sprinting, fat wont be energy source b/c cant be done anaerobically ) c) fatty acids cannot be converted into glucose (in animals); fat metabolism cannot maintain essential blood [glucose] (Can go glucose to fatty acids, but not fatty acids to glucose, so fatty acids cant be used to regulate glucose levels (brain and RBCs don't use fatty acids to make energy)

last two enzymes in glycogenesis

1) Glycogen synthase- adds UDP glucose to non reduing end of glycogen- as glucose is added, UDP nucleotide is liberated; forms alpha 1 -> 4 glycosidic bond; RATE LIMITING STEP 2) UDP must be restored to UTP (b/c UTP hydrolysis needed to drive glyocogenesis)- done with Nucleoside diphosphate kinase- convert UDP back to UTP (take phosphoryl group off of ATP and placed on UDP Glycogen synthase can only build on pre-existing glycogen molecules (cant start synthesis of glycogen molecule)

Gluconeogenesis

1) fasting/starvation -> inadequacy of glycogenolysis -> necessity for gluconeogenesis (Especially true for brain, can only store so much glycogen in body- eventually glycogen stores depleted ) 2) noncarbohydrate precursors -> glucose 3) occurs in the liver (primary) and kidneys (Takes place in kidneys under really prolonged fasting and starvation ) 4) utilizes all glycolytic enzymes, except for rate-limiting steps (bypass steps/reactions) (Share many enzymes with glycolysis, except for rate limiting steps ) 5) occurs primarily in the cytosol

Blood glucose levels

1) glucose metabolism is the heart of metabolism -> primary fuel source a) average body consumption ~ 160-200 g/day 2) brain is major consumer of glucose (~120 g/day) (Brain cant synthesize glucose- depends on constant supply of glucose in the blood; brain can store glucose but can only store in tiny amounts) 3) RBCs have absolute requirement for glucose (do not have mitochondria, thus cannot use fatty acids for energy ) 4) body glucose reserves a) extracellular glucose ~ 10g b) liver glycogen ~ 75 g c) muscle glycogen ~ 250 g

Glycogen metabolism as example of metabolic theme

1) glycogenolysis -∆G, so is glycogenesis + ∆G? (Breakdown of glycogen is thermodynamically favorable ; Glycogenesis must also be thermodynamically favorable if it is taking place in the body- has different pathway from glycogenolysis ) 2) Thermodynamic realities demand different pathways (Biosynthetic and degradative pathways for same molecule often use different pathways to achieve goal because both pathways must be favorable and operable at same metabolite concentration; and can be controlled independently) 3) biosynthetic vs degradative pathways a) requirement for same [metabolite] b) independent regulation and fine tune control of metabolism

Enzymes of glycogenolysis

3 enzymes: 1) Glycogen phosphorylase- cofactor PLP and Pi; in presence of cofactors, cleaves alpha 1,4 glycosidic bond- release from glycogen a glucose 1 phosphate molecule - RATE LIMITING STEP/CONTROL POINT 2) Glycogen debranching enzyme (2 functions) a. Goes along branch and cleaves last glycosidic bond before branch point bond, then adds branch to end of main chain so that glycogen phosphorylase can digest it b. Cleaves alpha 1,6 glycosidic bond that joins remaining branch point monomer to chain 3) Phosphoglucomutase a. Convert glucose 1 phosphate to glucose 6 phosphate (in liver, is moved from cytosol into lumen of ER by T1 G6P translocase (transport protein); in ER lumen, glucose 6 phosphatase dephosphorylates G6P into glucose, which is then transported out of ER lumen into cytosol by T3 transporter, transported from cytosol into blood by GLUT2

Glucose Alanine Cycle

Alanine- used to make glucose through this cycle In skeletal muscle, glucose converted to pyruvate through glycolysis, pyruvate is receiver of amino groups (nitrogen) As proteins and amino acids degraded, amino groups transferred to glutamate, which transfers amino group to pyruvate through alanine transaminase, converted into alanine Alanine serves as nontoxic way of delivering nitrogen from muscle that was acquired through breakdown of proteins Alanine transferred from skeletal muscle to liver- opposite reaction takes place, alanine transfers amino group to alpha ketoglutarate (makes glutamate), and alanine becomes pyruvate which fed into gluconeogenesis (glucose transferred back into muscle, but in fasting/starvation, glucose released into blood to supply other tissues- not directed to muscle) Muscle protein serves as source of glucose production through this cycle (in fasting conditions) Nitrogen from Glutamate eliminated through urea cycle

Reactions of gluconeogenesis

Different from glycolysis in rate limiting steps 1) catalyzed by pyruvate carboxylase- uses biotin cofactor to hydrolyze ATP, addss CO2 to pyruvate and converted into oxaloacetate (FIRST BYPASS STEP- for pyruvate kinase reaction- rate limiting step in glycolysis) 2) PEP carboxykinase- hydrolyze GTP and use P group to phosphrylate oxaloacetate to produce PEP and release CO2 (bypass for pyruvate kinase reaction- rate limiting step in glycolysis) a. If make PEP in mitochondria, pass through transport protein to pass in b. If made in cytosol, oxaloacetate must be present in cytosol (gets out of mitochondria through shuttle system- us malate or aspartate transporting system of malate aspartate shuttle, then reconverted into oxaloacetate in cytosol) 3) Enolase- reverse glycolysis reaction 4) Phosphoglycerate mutase -reverse glycolysis reaction 5) Phosphoglycerate kinase reverse glycolysis reaction 6) G3P dehydrogenase reverse glycolysis reaction 7) Triose phosphate isomerase- reverse glycolysis reaction 8) Aldolase- reverse glycolysis reaction 9) SECOND BYPASS STEP (no PFK)- fructose 1,6 bipshophatase- dephosrylate fructose bisphoshate to form fructose 6 phosphate 10) Phosphoglucose isomerase- reverse glycolysis reaction 11) FINAL BYPASS STEP (NOT hexokinase)- glucose 6 phosphatase dephosprhylase G6P to make glucose

Glycogenolysis

Breakdown of glycogen into individual glucose molecules by sequential removal (one monomer at a time) of glucose from nonreducing end of glycogen Glycogen is highly branched, every end of branch is nonreducing, so have many points to liberate glucose from (is why glycogen is more highly packed than amylopectin- animals have greater energy requirements for activity than plants) Occurs in muscle for immediate release of glucose to feed into glycolysis to generate ATP (muscle utilizes immediately) and in liver, glucose is released into blood to regulate blood glucose levels

Regulation of gluconeogenesis

Bypass steps are regulated 1) Glucose 6 phosphatase a. Insulin inhibits expression of enzyme (b/c produced when lots of glucose in blood, don't need to make more) 2) Fructose 1,6 bisphosphatase a. Allosteric i. Inhibited by AMP and F2,6P (most important method of control) 3) PEPCK a. Transcriptional control (hormones regulate) i. Glucagon, thyroid hormone, glucocorticoids- stimulate ii. Insulin inhibits 4) Pyruvate carboxylase a. Activated by acetyl CoA

gluconeogenic building blocks

Can use Lactate and pyruvate, glycerol, TCA cycle intermediates, and carbon skeleton of most amino acids (glucogenic amino acids) Before used, all precursors must be made into oxaloacetate- is the starting point for gluconeogenesis Leucine and lysine cant be used to make glucose, but can be used to make ketone bodies (are ketogenic, not glucogenic) KNOW WHICH AMINO ACIDS ARE GLUCOGENIC, KETOGENIC, OR BOTH Almost all fatty acids cant be used to make glucose- breakdown leads to acetyl CoA- carbon atoms are eliminated as CO2 in TCA cycle, so cant be used Interorgan substrate cycles- different organs produce precursors and deliver to liver so liver can make glucose

Cori cycle

First interorgan cycle - occurs between liver and skeletal muscle When ATP demands exceed ability of oxygen to be delivered to tissues, switch to ATP production through glycolysis and anaerobic- building up of lactate in skeletal muscle, transported to liver and converted into pyruvate; pyruvate feeds into gluconeogenesis Glucose can be stores in liver as glycogen or brought back into muscle to be stored as glycogen or used immediately for ATP production ATP produced in liver by fatty acid oxidation (sparing glucose for use by other tissues, don't use glucose to make ATP) After stop exercising, takes some time to make lactate back into glucose (need ATP)- body's oxygen needs remain very high for several minutes after to pay oxygen debt and fuel ATP production so liver can turn lactate back into glucose

Glycerol in gluconeogenesis

Glycerol- backbone molecule used to make trigylcerides Lipoportein lipase and hormone sensitive lipase- capable of degrading trigylcerides into three fatty acids and glycerol Glyverol transported from adipose tissue to liver 2 step process: - Phosohorylated by glyverol kinase (glyverol 3 phosphate) - Oxidized by NAD through glycerol 3 phosphate dehydrogenase- dihydroxyacetone phosphate (and NAD converted into NADH) Liver takes dihydroxyacetone phosphate an taking into glycolysis or gluconeogenesis (released from liver into tissues once glucose) Free fatty acids pass into blood and bind to albumin, which delivers them to different tissues (liver for energy production, heart/kidneys/skeletal muscle where oxidized to produce energy)

what enzymes are required to start glycogenesis? why?

Glycogenin- responsible for formation of nascent glycogen- when have 10 monomer long glycogen molecule, serves as primer for glycogen synthesis that glycogen synthase can build onto Glycogen branching enzyme- creates branches in glycogen molecule- takes glycogen chain, cleaves part of chain and attaches it as branch point

liver metabolic zonation

Hepatocytes that surround portal vein (circulation from organs to liver) - good at releasing glucose, enriched in enzymes of gluconeogenesis (responsible for synthesizing and releasing glucose) - PERIPORTAL ZONE Surrounding central vein (circulation from liver to organs)- enriched in enzymes of glycolysis- glucose uptake- PERIVENOUS ZONE Allows for hepatocytes to be specialized - avoids fuel cycling and waste of energy in liver

Regulation of blood glucose levels

Hyperglycemia: Pancreas produces insulin, signals glucose uptake in cells, cell can break down glucose and use in glycolysis or can store it in form of glycogen (glycogenesis) Hypoglycemia- pancreas produces glucagon, stimulates release of glucose in liver - Glucose produced by glycogenolysis in skeletal muscle is not released into blood like liver, but is used directly by skeletal muscle - Gluconeogenesis- make glucose from non-carbohydrate precursors Fight/flight - epinephrine prepares tissues for fight/flight- stimulates release of glucose from liver and skeletal muscle uptake and utilization of glucose

Regulation of glycogen metabolism: insulin

Insulin binds to insulin receptor- a receptor tyrosine kinase (auto phosphorylation, recruitment of other factors) - Stimulates insulin stimulated protein kinase (activates)- has two targets ○ GSK3- phosphorylation inactivates it (inhibiting inhibitor of glycogenesis) ○ Gm subunit- phosphorylation makes active, binds and activates phosphoprotein phosphatase 1, which dephosphorylates 4 targets § Own inhibitor: phosphoprotein phosphatase inhibitor 1- makes inactive § Phosphorylase kinase- inactive § Glycogen phosphorylase- inactive (decrease in glycogenolysis) □ ATP, G6P, glucose are allosteric inhibitors § Glycogen synthase- active (increase in glycogenesis) Allosterically activated by G6P

Regulation of glycogen metabolism: glucagon/epinephrine

Rate limiting steps in above 2 processes: - Glycogen synthase - Glycogen phosphorylase Glucagon (low blood glucose)/epinephrine - Bind to glucagon receptor or beta adrenoreceptor- both couples Gs alpha, activate adenyl cyclase, which makes cAMP, activates protein kinase A (major regulator for glycogen metabolism- phosphorylates phosphorylase kinase which then targets its substrates ○ Phosphoprotein phosphatase inhibitor 1- phosphorylation makes protein active, can bind and inhibit phosphoprotein phosphatase 1 ○ Glycogen phosphorylase (RATE LIMITING ENYME FOR GLYCOGEN BREAKDOWN) - when phosphorylated, becomes active (activates breakdown of glycogen into glucose) § Allosterically controlled by AMP (high AMP signals low ATP, liberate glucose to produce ATP) ○ GM subunit - phosphorylation makes inactive ○ Glycogen synthase (RATE LIMITING ENZYME FOR GLYCOGEN SYNTHESIS)- phosphorylation makes inactive, inhibition of glycogen synthesis § DAG produces protein kinase C, which phosphorylates and inhibits glycogen synthase § AMPK energy sensor in cell, if high AMP, is active, phosphorylates and inhibits glycogen synthase GSK3- phosphorylates and inhibits glycogen synthase

Glycogenesis

Synthesis of glycogen from individual glucose monomers- sequential addition of glucose at non reducing end of glycogen (only add one at a time) Builds main chain and induces creation of the branches Occurs in muscle tissue and in liver (muscle for storage of glucose for later energy use; in liver stored so that future glucose released to regulate blood glucose levels)