Metabolism Flashcards
Outline the bypass steps for Gluconeogenesis
1: Pyruvate -> Phosphoenolpyruvate
2: Fructose 1,6-BP -> Fructose 6-phosphate
3: Glucose 6-phosphate -> Glucose
Glycolysis and gluconeogenesis reciprocally regulated at the level of which enzyme?
Phosphofructokinase
Regulation points in glycolysis
Hexokinase
= ALLOSTERIC - Feedback Inhibition by product G-6P
Phosphofructokinase = ALLOSTERIC 1. ATP High ATP -> Allosteric site INHIBITION Low ATP -> Catalytic site ACTIVATION
- Glucose
High glucose -> drives rxn forwards by coordinate regulation
High glucose -> isomer F-2,6-BP, as not a glycolytic intermediate cell alerted to high glucose concentration and F-2,6-BP activates PFK to produce F-1,6-BP
=> F-2,6-BP increases PFK reaction velocity
Pyruvate kinase
= COVALENT MODIFICATION by phosphorylation
stimulated by F-1,6-BP
inhibited by ATP
less active in phosphorylated form
Pyruvate Dehydrogenase (Pyruvate-> Acetyl CoA) = COVALENT MODIFICATION by phosphorylation
phosphorylation -> Inactivation
high energy charge -> inhibition
low energy charge -> activation
How lactate removed from body
CORI CYCLE
Lactate dehydrogenase: Lactate -> Pyruvate -> Gluconeogenic Pathway
occurs in liver
requires 6 ATPs = oxygen debt
Gluconeogenesis = manufacture of ‘new’ glucose form non-carbohydrate precursors.
What are some Gluconeogenic precursors?
- Glucogenic amino acids
- Glycerol
TAG -> FA + Glycerol
Glycerol -> Dihydroxyacetone phosphate G3P - Lactate
Lactate dehydrogenase: Lactate -> Pyruvate
** CAN NOT produce glucose from FA’s!!!
Diabetes Type 1/2
Type 1 - can’t produce insulin (autoimmune destruction of B cells)
Type 2 - tissues resistant to insulin
Anerobic respiration
Pyruvate -> Lactate
produces what useful by product?
NAD+
Regeneration of NAD essential - maintains glycolysis in absence of oxygen
FUNCTIONS OF LIPIDS
Major components of cell membranes.
Required to solubilise fat soluble vitamins
Biosynthetic precursors (e.g. steroid hormones from cholesterol)
Protection (e.g. kidneys are shielded with fat in fed state)
Insulation
chylomicron
Triacylglycerol + cholesterol + phospholipid + proteins form a lipoprotein complex called a chylomicron which transports the lipids in the circulation.
(Lipids are insoluble in plasma. In order to be transported they are combined with specific proteins to form lipoproteins)
The classes of lipoprotein
Source:
Function:
(all contain characteristic amounts TAG, cholesterol, cholesterol esters, phospholipids and apoproteins)
- Chylomicrons (CM)
Source: Intestine
Function: Transport dietary TAG to the adipose tissues where it can be stored as fat or to muscles where the constituent fatty acids can be used for energy. - Very Low Density Lipoproteins (VLDL)
Source: Liver
Function: Transport endogenously synthesised TAG to the extra hepatic tissues where it can be stored as fat or to muscles where the constituent fatty acids can be used for energy. The cholesterol is delivered to extra hepatic tissues once VLDL has been metabolised to LDL. - Low density lipoproteins (LDL)
Source: Formed in circulation by partial breakdown of IDL
Function: Delivers cholesterol to peripheral tissues - High density lipoprotein (HDL)
Source: Liver
Function: Removes “used” cholesterol from tissues and takes it to liver. Donates apolipoproteins to CM and VLDL
What are Triglycerides
= Highly concentrated energy store
Formed by esterification of FAs to glycerol at each of the hydroxyl groups
Reside in adipocytes (fat cells) = energy store
source = dietary lipids
How are triglycerides broken down
TG broken down by series of lipolytic reactions
Pancreatic lipases digest TGs (small intestine)
Triacylglycerol –Lipase–> Diacylglycerol –Lipase–> Monoacylgylcerol –MAG lipase–> FA + Glycerol
Bile salts
Made and stored in gall bladder e.g. glycocholate
Contain a hydrophobic structure and an ionic structure -> physiological detergents - act to dissolve and emulsify TGs in the small intestine & make them accessible to pancreatic lipase
Bile Salts emulsify dietary TGs & then lipase act on TG micelles
Chylomicrons
Triacylglycerols broken down by lipase into monoacylglycerols + FAs which cross gut wall
Inside mucosal cell reassembled into TGs
TGs combine with other lipids and proteins –> Chylomicrons –> Lymph system –> Adipocytes
Chylomicrons = protein-lipid complexes used to transport lipid in the blood stream and eventually into lymphatic system
Breakdown of TGs in adipocytes
Hormones Sensitive Lipase (HSL) hydrolyses TGs –> FAs (energy rich) + Glycerol (metabolised by glycolysis - converted into glycolytic intermediates DP & G3P + production of NADH)
HSL main regulators:
- Glucagon
- Adrenaline
=> Activation of 7TM membrane receptor –> elevation of cAMP (cyclic AMP) = secondary messenger - controls activity of protein kinases (=> +Pi)
Protein kinase A posphorylates: Perilipin + HSL=> activation of lipolysis
Adipocyte TG
- –> Glycerol -> Liver cell: Glycolysis ->Pyruvate / Gluconeogenesis -> Glucose
- –> FAs -> other tissues: FA oxidation -> Acetyl CoA -> TCA cycle -> CO2 + H2O
Fatty Acid B Oxidation
Location = Mitochondria (matrix)
Reaction sequence
- OXIDATION (FAD+)
- HYDRATION (H2O)
- OXIDATION (NAD+)
- CLEAVAGE /Thiolysis (HS-CoA)
FAs degraded by repetition of this reaction sequence
Activated FA (Activated Acyl CoA) –>–>–>–> Activated Acyl CoA (shortened by 2C atoms) + Acetyl CoA (2C)
Activated Acyl CoA renters cycle
Acetyl CoA -> TCA cycle => 2 NADH, 1 FADH, 1 GTP
NB: FAs activated as CoA derivatives via ATP
FA + ATP + HS-CoA => Acyl CoA + AMP
Acyl CoA needs to be transported to mitochondrial matrix for B-oxidation
Translocase transports FA-carnitine -> m.matrix
Acyl CoA + carnitine -> Acyl Carnitine (has translocase transporter) + HS-CoA
Carnitine
Combines with Acyl CoA (activated FAs) to transport them across membrane into mitochondrial matrix for B-oxidation as Acyl Carnitine.
Converted back into Acyl CoA once in matrix.
Oxidation of Polyunsaturated FAs
- Requires ISOMERASE and REDUCTASE
Isomerase => trans configuration = intermediate in B-oxidation
Reductase => reduces FA so less double bonds using NADPH
Unsaturated FA oxidation:
Odd numbered double bonds -> ISOMERASE
Even numbered double bonds -> ISOMERASE + REDUCTASE
Always require isomerase - unsaturated FAs must be in TRANS configuration for B-oxidation
Oxidation of odd numbered FAs
Final round of B-oxiation =>
C2 Acetyl CoA + C3 Propionyl CoA
Propionyl CoA converted to TCA cycle intermediate Succinyl CoA (vit B12 dependent, require ATP)
Ketone bodies
Can be used by CNS during starvation
= alternative fuel source during fasting or diabetes
- FA B-oxidation
FAs -> Acetyl CoA - Formation of ketone bodies
Acetyl CoA -> Ketone bodies e.g. Acetone, Acetoacetate - Ketone bodies -> Acetyl CoA
in Heart muscle, renal cortex and brain cells - Acetyl CoA -> Citric Acid Cycle
- Oxidative Phosphorylation
NB:
Ketone bodies formed by condensation of 3 x Acetyl CoA molecules
Ketone bodies converted back to Acetyl CoA as energy source:
Acetoacetate –CoA transferase–> Acetoacetyl CoA –Thiolase + CoA–> 2Acetyl CoA
Diabetic Ketosis
ketone bodies form
blood ph drops
coma and death result
Amino Acid Metabolism Overview
Proteins digested -> AAs in GI tract
Proteins tagged for degradation (normal protein turnover) with UBIQUITIN
Ubiquitin tagging = signal for proteosome to digest proteins into constituent amino acids
AA
- left intact for biosynthesis (AAs = precursors for other biomolecules)
or
-> Amino groups => Nitrogen disposal by UREA CYCLE => Oxidative degradation of AAs (Hepatic (liver): mitochondrial)
-> Carbon skeleton
=> FA synthesis
=> Glucose or glycogen synthesis
=> Cellular respiration => Catabolism of AA carbon skeleton in TCA cycle
Oxidative Degradation of AAs
GLUTAMATE = Initial NH4 acceptor
Each AA has it’s own aminotransferase
Amino Acid –Aminotransferase–> Glutamate (=universal nitrogen acceptor) –Glutamate dehydrogenase–> NH4+
Glutamate dehydrogenase releases NH4 from glutamate = OXIDATION (removal of H with associated electrons)
Overall:
- AA + α-ketoglutarate ketoacid + Glutamate
** Pyridoxine (vit B6) required as a cofactor for transamination
- Glutamate + NAD+ + H2O –Glutamate dehydrogenase–> α-ketoglutarate + NADH + NH4+
- NH4+ –UREA CYCLE–> Urea (excreted)
Urea Cycle
1st committed step requires energy investment => 2ATP (used to synthesise carbomyl phosphate)
.’. Regulation of urea cycle at first committed step!
1st committed step of urea cycle occurs in mitochondrion, L-Citrulline is then synthesised and transported out of mitochondrion.
CO2 + NH4+ => Carbamoyl Phosphate \+ Orthinine => Citrulline \+ Aspartate => Arginosuccinate => Fumarate + Arginine Arginine => UREA + Orthinine Orthinine + Carbomoyl-P => Citrulline
Point of integration between urea cycle and TCA cycle
Arginosuccinate
1* Formed from Citrulline + Aspartate
Aspartate derived from transamination of oxaloacetate
Aspartate Oxaloacetate
2* Arginosuccinate forms Arginine + Fumarate
- Arginine continues in urea cycle
- Fumarate enters TCA cycle = TCA cycle intermediate
Fatty Acid Biosynthesis
Location = Cytosol
- excess carbohydrate
- certain AA C skeletons
- Alcohol
==> Can be converted => FAs and stored as TAG (Triacylglyceride) - mainly occurs in Liver and Lactating mammary glands (small amount in kidney and adipose tissue)
Energy source = ATP
Reducing agent = NADPH
Starting point = Acetyl CoA (from carbohydrate breakdown)
- contains C=O carbon in oxidised form, aim to reduce all Cs - if molecule contains highly reduced form of C = good E source
**Acetyl CoA from mitochondrion –> Cytosol
ACA produced in mitochondria, excess needs to be transported to cytoplasm for FA synthesis – synthesis and breakdown of FAs occur in separate cellular compartments
FA oxidation - Mitochondria
FA reduction - Cytoplasm
ACP = Acyl Carrier Protein
- large protein, labels Malonyl CoA as destined for FA synthesis
Regulation of FA Biosynthesis
Occurs at level of:
- Acetyl CoA Carboxylase *
Inactivated by phosphorylation (action of kinase)
Activated by phosphatase (removes phosphate group)
Promoted by Citrate
Inhibited by Palmitoyl CoA
Important FA modification
Arachidonic Acid
Pentose Phosphate Pathway
Location = CYTOPLASM
Provides:
- NADPH
- Ribose 5-sugars
PPP = alternative pathway for glucose oxidation (linked to glycolysis&gluconeogenesis), but NOT for ATP synthesis
Glucose 6P -> IRREVESIBLE OXIDATIVE phase
Fructose 6P -> REVERSIBLE NON-OXIDATIVE phase
Glyceraldehyde 3P -> REVERSIBLE NON-OXIDATIVE phase
PPP used to meet the cells demands
- If cell needs lots of NADPH
G6P -> Oxidative phase and glycolytic intermediates renter PPP to generate more NADPH (No glycolysis/CAC occurs)
- If cell needs some NADPH and ATP
G6P -> Oxidative phase
Glycolytic intermediates renter glycolytic pathway - If cell needs lots of Ribose 5-Phosphate
F6P + G3P enter PPP - works backwards to produce Ribose 5-phosphate => Nucleotides
*Occurs when cell replicating - normal cell replication
- cancer cell
=> High robes 5-phosphate requirements for nucleotide synthesis
Pentose Phosphate Pathway
Oxidative Phase
=> Production of 2 NADPH
G6P NADPH + … H+ + 6-Phosphogluconate NADPH + Ribulose 5-phosphate (C5) + CO2
CO2 => exhaled
NADPH => reducing agent e.g. in FA biosynthesis
Ribulose 5P => Non-oxidative phase of PPP
Pentose Phosphate Pathway
Non-oxidative Phase
3 Ribulose Phosphates => 2 Fructose 6-Phosphates + 1 Glyceraldehyde 3-Phosphate => Feedback into glycolytic/gluconeogenic pathway
ETC and Oxidative Phosphorylation
Location = Mitochondria Cristae - highly folded => Increases SA for oxidative phosphorylation
Summary:
4 different complexes in ETC receiving either NADH or FADH2 generated in TCA cycle/glycolysis
E-‘s moved through complexes via a series of e- carriers to ultimately generate H2O from O2
Associated with the movement of e-‘s is the extrusion of H+ ions into intermembrane space => proton gradient
ADP regulates oxidative phosphorylation -> main regulator of TCA cycle
Electron Carriers in ETC
- Coenzyme Q
Q (oxidised) + e- + H+ ——-> QH2 (reduced)
e-s generally added to oxygen groups - Flavin mononucleotide (FMN)
FMN (oxidised) + 2e-s + 2H+ —> FMNH2 (reduced)
nitrogen groups accept e-/H+ - Fe-S clusters
Fe3+ + e- Fe2+
shuttling between oxidation states also a means of carrying e-‘s through ETC
Fe present in cytochromes (=haemproteins)
Chemostatic Hypothesis
Proton motive force = Chemical gradient (change in pH) + Charge gradient
ATP Synthase
ATP Synthase - Rotational Catalysis
Proton motive force causes a proton to enter cytoplasmic half channel from inter membrane space. Each proton follows a complete CLOCKWISE ROTATION of the C ring, and exits through the other half channel into the matrix. Energy of rotation used to synthesis ATP.
Glycerol-3-phosphate shuttle
(muscle)
pair of e-s transferred from NADH to dihydroxyacetone phosphate (glycolytic intermediate) -> glycerol 3-phosphate
G3P reoxidises to DP on the outer surface of inner mitochondrial membrane by a membrane bound isozyme of G3P dehydrogenase.
E- pair from G3P transferred to an FAD prosthetic group in this enzyme to form FADH2. Reduced flavin transfers it’s e-s to electron carrier Q, then enters respiratory chain as QH2.
NB: FADH2 less e. rich than NADH -> 1.5 ATP .’. NADH generated in glycolysis less E. rich than NADH generated in TCA cycle
Malate-aspartate shuttle
(liver and adipose tissue)
under some circumstance these shuttles can operate in energy neutral terms - not always energy loss associated with use of shuttles to transport reducing equivalents
ATP Translocase
ADP (cytoplasmic) binds to the translocase causing an allosteric change -> eversion .’. ADP released into the matrix.
ATP then binds -> eversion releasing ATP into the cytoplasm
Inner mitochondrial membrane Permeability
Inner mitochondrial membrane = IMPERMEABLE
to NAD+/NADH and ADP/ATP
NAD+/NADH -> transported by SHUTTLES into mitochondrial matrix
E.g: Glycerol-3-phosphate shuttle
Malate-aspartate shuttle
ADP/ATP -> transported by TRANSLOCASES into mitochondrial matrix
E.g: ATP translocase
Transporters used to transport substances across inner mitochondrial membrane - 40 transporters encoded in human genome E.g ATP-ADP translocate, Phosphate carrier (OH-Phosphate)
Glycogen Breakdown
Glycogen (n residues) + Pi Glucose 1-phosphate + Glycogen (n-1 residues)
Glucose 1-P = more E.rich than glucose
Glycogen Phosphorylase
Promoted by:
- Adrenaline (muscle &liver)
- Glucagon (liver only)
Stops cleaving when within 4 residues of a branch point .’. 2 additional enzymes required for Glycogen catabolism
1 - enzyme transfers 3 glucose units to end of glycogen molecule
2 - enzyme cleaves/hydrolyses off the single glucose unit remaining
=> Phosphorylase can start again
.’. Glycogen breakdown generates mainly G1P with some glucose (single hydrolysed residues)
ratio = 8:1
Fate of G1P & Glucose in LIVER
Glycogen => G1P -> G6P -> Glucose -> released into blood to maintain b.glucose levels
Fate of G1P & Glucose in MUSCLE
Glycogen => G1P-> G6P
Glycogen => Glucose -> G6P
G6P => Glycolysis in muscle to generate ATP
NB: Unlike liver cell, muscle doesn’t have glucose 6-phosphatase .’. can’t convert G6P -> glucose
.’. G6P trapped in muscle cell for use in glycolysis => ATP production
Regulation of Glycogen Metabolism
deficient glucose -> glycogen metabolism
excess glucose -> glycogen synthesis
2 processes = reciprocally controlled by hormones & allosteric control
AMP => ALLOSTERICALLY stimulates glycogen breakdown
ATP => inhibits glycogen breakdown
Regulation of Glycogen Metabolism:
Hormonal control by glucagon and adrenaline
=> glycogen breakdown stimulated, synthesis inhibited
Adrenaline & glucagon = Primary messenger
- bind membrane receptors => conformational change that is transmitted throughout cell => activation of cAMP
cAMP = Secondary messenger
=> AMPLIFICATION CASCADE
- catalysts (enzymes) activate other catalysts (enzymes) .’. only need one hormone molecule to activate thousands/millions of glycogen phosphorylase => large glycogen breakdown
Regulation of Glycogen Metabolism:
Hormonal control by insulin
Insulin released when blood glucose levels are elevate e.g. after a meal, to lower blood glucose to 5 mmol/L
=> glycogen breakdown inhibited, glycogen synthesis stimulated
Insulin receptor = Tyrosine Kinase
=> De-phosphorylation of glycogen synthase (activates enzyme) and glycogen phosphorylase (inactivates)
glycogen synthase inactivated by phosphorylation
glycogen phosphorylase activated by phosphorylation
Glycogen Synthesis
Surplus glucose -> glycogen = glycogenesis
Occurs in cytoplasm of liver & muscle cells
Glucose -> Glucose 6-phosphate Glucose 1-phosphate -> UDP-Glucose
UDP-Glucose + Glycogen (n residues) —Glycogen synthase—> UDP + Glycogen (n+1 residues)
Glycogen synthase promoted by:
- Insulin
- G6P (+ve regulation)
Formation of branches
- branching enzym transfers a block of glucose units from non-reducing terminal of a glycogen polymer to a more interior site i.e. an alpha 1-4 linkage is broken and an alpha 1-6 linkage is formed..
Deficiency of branching enzyme => liver failure and death in first year of life = Anderton’s disease
