Upload
nfshmi
View
5
Download
0
Tags:
Embed Size (px)
Citation preview
Learning objectives: Metabolism – definition Catabolism and anabolism – definition, example Identify/distinguish structure of coenzymes Identify structure of ATP
What is metabolism?• Definition: Metabolism is the sum total of the chemical
reactions of biomolecules in an organism• Metabolism consists of
1. catabolism: the breakdown of larger molecules into smaller ones; an oxidative process that releases energy
2. anabolism: the synthesis of larger molecules from smaller ones; a reductive process that requires energy
• Catabolism: the oxidative breakdown of nutrients
• Anabolism: the reductive synthesis of biomolecules
The study of the biochemical reactions in an organism, including the coordination, regulation and energy needs
Terminology of metabolism
Metabolic pathway: A sequence of reactions, where the product of one reaction becomes the substrate for the next reaction.- either linear pathway or cyclic pathway- metabolic pathways proceed in many stages, allowing for efficient use of energy
Metabolites: intermediates in metabolic pathway
Eg. 6 CO2(g) + 6 H2O(l) → C6H12O6(aq) + 6 O2(g)
Anabolism
C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O
Catabolism
light
photosynthesis
respiration
Comparison of anabolism and catabolismMetabolism is the sum total of the chemical reactions of
biomolecules in an organism
Metabolism Metabolism involves the energy flow in the cell Photoautotroph via photosynthesis transfers the energy
to heterotrophs Heterotrophs obtain the energy through
oxidation/reduction of organic compounds (carbohydrate, lipid and proteins)
Food supplies the energy Energy = ATP
The role of oxidation and reduction in metabolism
• Oxidation-Reduction (redox) reactions are those in which electrons are transferred from a donor to an acceptor– oxidation: the loss of electrons; the substance that loses
the electrons is called a reducing agent– reduction: the gain of electrons; the substance that gains
the electrons is called an oxidizing agent• Carbon in most reduced form- alkane• Carbon in most oxidized form- CO2 (final product of
catabolism)Reduced Oxidized
Oxidation and reduction in metabolism
CH3CH2OH + 2H+ + 2e-
Ethanol Acetaldehyde
NAD+ + H+ + 2e- NADH
CH3CH2OH + NAD+ + NADH +H+
CH3CHO
CH3CHO
CH3CHCOO-OH
+ 2H+ + 2e-
Pyruvate Lactate
NAD+NADH + H+ + 2e-
+ H++ NADH CH3CHCOO-OH
+ NAD+
CH3CCOO-O
CH3CCOO-O
Oxidation of ethanol by NAD+
Reduction of pyruvate by NADH
Oxidation – less e
Reduction – gain e
Oxidizing agent – e acceptor
reducing agent – e donor
Metabolism: featuresMetabolic pathway:
1. Enzymes – multienzymes 2. Coenzymes3. ATP – produced or used
Regulation of metabolic pathway: Feedback inhibition or Feed-forward activation
A group of noncovalently associated enzymes that catalyze 2 or more sequential steps in metabolic/biochemical pathway
Metabolism: Regulation• Regulation of metabolic pathway:
1. Feedback inhibition = product (usually ultimate product) of a pathway controls the rate of synthesis through inhibition of an early step (usually the first step)
A B C D E P
2. Feed-forward activation = metabolite produced early in pathway activates enzyme that catalyzes a reaction further down the pathway
A B C D E P
E1 E2 E3 E4 E5
E1 E2 E3 E4 E5
—
+
CoenzymesCoenzymes in metabolism: NAD+/NADH NADP+/NADPH FAD+/FADH2
Coenzyme A (CoASH) – activation of metabolites
Electron carriers
NAD+/NADH: An Important Coenzyme
Nicotinamide adenine dinucleotide (NAD+) is an important coenzyme
Acts as a biological oxidizing agent
The structure of NAD+/NADH is comprised of a nicotinamide portion.
It is a derivative of nicotinic acid
NAD+ is a two-electron oxidizing agent, and is reduced to NADH
Reduced form, NADH carries 2 electrons
NADP+/NADPH: Also comprised of nicotinamide portion
Nicotinamide adenine dinucleotide phosphate (NADP+) – oxidizing agent
NADPH involves in reductive biosynthesis
Differ with NAD+ at ribose (C2 contain a phosphoryl group, PO3
2- As electron carrier in
photosythesis and pentose phosphate pathway
Reduced form, NADPH carries 2 electrons Anabolism
The Structures Flavin Adenine Dinucleotide (FAD)
FAD is also a biological oxidizing agent
FAD – can accept one-electron or two-electron
FADH carries 1 electron, FADH2 carries 2 electrons
The terminal e acceptor (O2) can accept only unpaired e (e must be transferred to O2 one at a time)
FAD/FADH2• FADH (semiquinone form) carries 1 electron,
• FADH2 (fully reduced hydroquinone form) carries 2 electrons
Formation of fully reduced hydroquinone form bypass the semiquinone form
Coenzyme A in Activation of Metabolic Pathways
• A step frequently encountered in metabolism is activation– activation: the formation of a more reactive
substance – A metabolite is bonded to some other molecule
and the free-energy change for breaking the new bond is negative.
– Causes next reaction to be exergonic
Coenzyme A (CoASH) Coenzyme A – functions as a
carrier of acetyl and other acyl groups
Has sulfhydryl/thiol group
Acetyl-CoA: is a “high-energy” compound because of the presence of thioester bond – hydrolysis will release energy
Thioester bond
ATP- high energy compound ATP is essential high
energy bond-containing compound
Phosphorylation of ADP to ATP requires energy
Hydrolysis of ATP to ADP
releases energy
Phosphorylation: the addition of phosphoryl (PO32-) group/Pi
(inorganic phosphate)
nucleotide
The Phosphoric Anhydride Bonds in ATP are “High Energy” Bonds
“High Energy” bonds- bonds that require or release convenient amounts of energy, depending on the direction of the reaction
Couple reactions: the energy released by one reaction, such as ATP hydrolysis, provides energy for another reactions to completion – in metabolic pathway
Role of ATP as Energy Currency
Phosphorylation of ADP requires energy from breakdown of nutrients (catabolism)
The energy from hydrolysis of ATP will be used in the formation of products (anabolism)
Key words:Glycolysis, the fate of pyruvateSubstrate-level phosphorylation and
oxidative phosphorylation
GlycolysisGlycolysis is the first stage of glucose
metabolism
Glycolysis converts 1 molecule of glucose to 2 units of pyruvate (three C units) and the process involves the synthesis of ATP and reduction of NAD+
(to NADH)
The pathway has 10 steps/reactions
Glycolysis are divided into 2 stages/phases, Phase 1=1st 5 reactionsPhase 2=2nd 5 reactions
Linear pathway
Glycolysis Glycolysis are divided into 2 stages/phases,
1. Phase 1=1st 5 reactions Energy investment – A hexose sugar (glucose) is split into
2 molecules of three-C metabolite (glyceraldehyde-3-phosphate = GAP). The process consume 2 ATP
2. Phase 2=2nd 5 reactions Energy recovery – The two molecules of GAP are converted to
2 molecules of pyruvate with the generation of 4 ATP and 2 NADH.
Overall equation –Glucose + 2 NAD+ + 2 ADP + 2Pi
2 pyruvate + 2 NADH + 2 ATP + 2 H2O + 4H+
Glycolysis has a net “profit” of 2 ATP per glucose
The reaction of glycolysis
1. Phosphorylation of glucose to give glucose-6-phosphate
2. Isomerization of glucose-6-phosphate to give fructose-6-phosphate
3. Phosphorylation of fructose-6-phosphate to yield fructose-1,6-biphosphate
4. Cleavage of fructose-1,6,-bisphosphate to give glyceraldehyde-3-phosphate and dihydroxyacetone phosphate
5. Isomerization of dihydroxyacetone phosphate to give glyceraldehyde-3-phosphate – isomerase enzyme
1
2
3
4
5
Use ATP
Use ATP
glucokinase
phosphofructokinase
The reaction of glycolysis (cont’)6. Oxidation of glyceraldehyde-3-
phosphate to give 1,3-biphosphoglycerate
7. Transfer of a phosphate group from 1,3-biphosphoglycerate to ADP to give 3-phosphoglycerate
8. Isomerization of 3-phosphoglycerate to give 2-phosphoglycerate
9. Dehydration of 2-phosphoglycerate to give phosphoenolpyruvate
10. Transfer of a phosphate group from phosphoenolpyruvate to ADP to give pyruvate
transfer
isomerization
dehydration
transfer
Phosphorylation of ADP to ATP
Phosphorylation of ADP to ATP
6
7
8
9
10
Electron acceptor – NAD+
Glyceraldehyde-3-P dehydrogenase
GlycolysisDephosphorylation of ATPPhosphorylation of ADP
Oxidation of intermediates and reduction of NAD+ to NADH by dehydrogenase reactions- step 6 - glyceraldehyde-3-phosphate dehydrogenase
By kinase enzyme at step 1, 3, 7 and 10
ATP production• ATP is produced by phosphorylation of ADP - is through
substrate-level phosphorylation• Substrate-level phosphorylation – the process of
forming ATP by phosphoryl group transfer from reactive intermediates to ADP
• 1,3-bisphosphoglycerate and phosphoenolpyruvate – “high-energy” intermediates/compounds
• Oxidative phosphorylation – the process of forming ATP via the pH gradient as a result of the electron transport chain.
Glycolysis - Step 7 and 10
Fates of Pyruvate From GlycolysisOnce pyruvate is formed, it
has one of several fates
In aerobic metabolism-
pyruvate will enter the citric acid cycle, end product in aerobic metabolism CO2 and H2O
In anaerobic metabolism- the pyruvate loses CO2
produce ethanol = alcoholic fermentation
produce lactate = anaerobic glycolysis
Anaerobic Metabolism of PyruvateUnder anaerobic conditions, the most important pathway for the
regeneration of NAD+ is reduction of pyruvate to lactateLactate dehydrogenase (LDH) is a tetrameric isoenzyme consisting
of H and M subunits; H4 predominates in heart muscle, and M4 in skeletal muscle In muscle, during vigorous exercise –
demand of ATP but O2 is in short supply is largely synthesized via anaerobic glycolysis which rapidly generates ATP rather than through slower oxidative phosphorylation
Alcoholic Fermentation Two reactions lead to the production of ethanol:
Decarboxylation of pyruvate to acetaldehyde
Reduction of acetaldehyde to ethanol
In anaerobic bacteria
• Pyruvate decarboxylase is the enzyme that catalyzes the first reaction
• This enzyme require Mg2+ and the cofactor, thiamine pyrophosphate (TPP)
• Alcohol dehydrogenase catalyzes the conversion of acetaldehyde to ethanol
Key words Definition – citric acid cycle Explain the citric acid cycle Distinguish between glycolysis and citric acid cycle Understand -oxidation – catabolism of lipid
Citric acid cycle • Requires aerobic condition• Amphibolic (both catabolic & anabolic)
• Serves 2 purposes:1. Oxidize Acetyl-CoA to CO2 to produce energy (ATP &
reducing power of NADH & FADH2)-involved in the aerobic catabolism of carbohydrates, lipids and amino acids
2. Supply precursors for biosynthesis of carbohydrates, lipids, amino acids, nucleotides and porphyrins
TCA• Circular pathway• Two-carbon unit
needed at the start of the citric acid cycle
• The two-carbon unit is acetyl-CoA
• Involves 8 reactions• The overall reaction
from 1 acetyl-CoA produce 3 NADH, 1 FADH2, 2 CO2 and 1 GTP (equivalent to 1 ATP)
Pyruvate is converted to Acetyl-CoA – activation of pyruvate
Pyruvate dehydrogenase complex is responsible for the conversion of pyruvate to acetyl-CoA
Five enzymes in complexRequires the presence of cofactors TPP (thymine
pyrophosphate), FAD, NAD+, and lipoic acid and coenzyme A (CoA-SH)
The overall reaction of the pyruvate dehydrogenase complex is the conversion of pyruvate, NAD+, and CoA-SH to acetyl-CoA, NADH + H+, and CO2
Thioester, high energy compound
Oxidation of pyruvate and reduction of NAD+
3C
2CPyruvate = pyruvic acid
Coenzyme A (CoASH) Coenzyme A – functions as a
carrier of acetyl and other acyl groups
Has sulfhydryl/thiol group
Thioester bond
Acetyl-CoA: is a “high-energy” compound because of the presence of thioester bond – hydrolysis will release energy
CoASH
Features of TCA• Circular pathway• Two-carbon unit needed
at the start of the citric acid cycle
• The two-carbon unit is acetyl-CoA
• Involves 8 reactions• The overall reaction from
1 acetyl-CoA produce 3 NADH, 1 FADH2, 2 CO2
and 1 GTP (equivalent to 1 ATP)
How about 1 molecule of glucose?
X 2
Electron acceptor – NAD+ and FAD
Mitochondrial matrix
Citric acid cycle - features• Oxidation decarboxylation
- CO2 leaves at step 3 and 4
• Oxidation of intermediates and reduction of NAD+ to NADH by dehydrogenase reactions- step 3, 4 and 8- isocitrate dehydrogenase- α-ketoglutarate dehydrogenase- malate dehydrogenase
• Oxidation of intermediates and reduction of FAD+ to FADH2 by succinate dehydrogenase reaction- step 6
• Phosphorylation of GDP to GTP – step 5
Where does the Citric Acid Cycle Take Place?In eukaryotes, cycle takes place in the mitochondrial matrix
In prokaryotes? Cytoplasm
The Central Relationship of the Citric Acid Cycle to Catabolism
• TCA involves 8 series of reactions that oxidizes the acetyl group of acetyl-CoA to 2 molecules of CO2
and the energy is conserves in NADH, FADH2 and “high-energy” compound, GTP
• Acetyl-CoA – synthesize from pyruvate (glycolysis product)
Guanosine – Tri-Phosphate
Aerobic catabolism• NADH, FADH2 from glycolysis and
TCA will enter the Electron Transport Chain (ETC) to produce more ATP (oxidative phosphorylation)
• 1 NADH = 2.5 ATP,
• 1 FADH2 = 1.5 ATP
• ETC take place in mitochondria - inner membrane (eukaryotes) In prokaryotes?
Plasma membrane
In ETC
Oxidation of Pyruvate Forms CO2 and ATP
Aerobic metabolism is more efficient than anaerobic metabolism
Citric acid cycle - amphibolic Amphibolic (both catabolic
& anabolic) Serves 2 purposes:
1. Oxidize Acetyl-CoA to CO2 to produce energy (ATP & reducing power of NADH & FADH2)-involved in the aerobic catabolism of carbohydrates, lipids and amino acids
2. Supply precursors for biosynthesis (anabolism) of carbohydrates, lipids, amino acids, nucleotides and porphyrins
Replenish TCA- catabolism of amino a. and fatty a.
Anabolic pathway
Require aerobic condition
Differences between glycolysis & TCA cycle
Glycolysis is a linear pathway; TCA cycle is cyclic
Glycolysis occurs in the cytosol and TCA is in the mitochondrial matrix
Glycolysis does / does not require oxygen; TCA requires oxygen (aerobic)
Lipids are Involved in Generation and Storage of Energy
The oxidation of fatty acids (FA)in triacylglycerols are the principal storage form of energy for most organismsTheir carbon chains are in a highly reduced formThe energy yield per gram of fatty acid oxidized is greater
than that per gram of carbohydrate oxidized
C6H12O6 + 6O2
CH3(CH2)14COOH + 23O2
6CO2 + 6H2O
16CO2 +16H2O
-15.9
-38.9Palmitic acid
Glucose
Energy(kJ•mol-1)
Catabolism of Lipids - triacylglycerol• Lipases catalyze hydrolysis of bonds between fatty acid and
the rest of triacylglycerols• Phospholipases catalyze hydrolysis of bonds between fatty
acid and the rest of phosphoacylglycerols • May have multiple sites of action
Catabolism of fatty acid - -Oxidation• -Oxidation: a series
of reactions that cleaves carbon atoms two at a time from the carboxyl end of a fatty acid
• The complete cycle of one -oxidation requires four enzymes/steps
• Take place in mitochondria matrix
1 round of -oxidation = yield 1 NADH, 1 FADH2 and 1 acetyl-CoA
Spiral pathway
– Catabolism: the oxidative breakdown of nutrients
– Anabolism: the reductive synthesis of biomolecules
• Catabolism – features1. Release energy (ADP ATP)2. Oxidizing agent (NAD+, FAD)
• Anabolism – features1. Use energy (ATP ADP)2. Reducing agent (NADH ,FADH2)
Metabolism – the sum total of biochemical reaction carried out by organism
Metabolism Metabolism involves the energy flow in the cell Photoautotroph via photosynthesis transfers the energy to
heterotrophs Heterotrophs obtain the energy through
oxidation/reduction of organic compounds (carbohydrate, lipid and proteins)
Food supplies the energy Energy = ATP
GlycolysisGlycolysis is the first stage of glucose
metabolism
Glycolysis converts 1 molecule of glucose to 2 units of pyruvate (three C units) and the process involves the synthesis of ATP and reduction of NAD+ (to NADH)
The pathway has 10 steps/reactions
Glycolysis are divided into 2 stages/phases, Phase 1=1st 5 reactionsPhase 2=2nd 5 reactions
Linear pathway
Fates of Pyruvate From GlycolysisOnce pyruvate is formed, it
has one of several fates
In aerobic metabolism-
pyruvate will enter the citric acid cycle, end product in aerobic metabolism CO2 and H2O
In anaerobic metabolism- the pyruvate loses CO2
produce ethanol = alcoholic fermentation
produce lactate = anaerobic glycolysis
Glycolysis – in cytoplasm
ATP is essential high energy bond-containing compound
Phosphorylation of ADP to ATP requires energy
Hydrolysis of ATP to ADP
releases energy
Phosphorylation: the addition of phosphoryl (PO32-) group/Pi
(inorganic phosphate)
ATP- high energy compound ATP – energy carrier /
an energy transfer agent
nucleotide
Coenzyme A (CoASH) Coenzyme A – functions as a
carrier of acetyl and other acyl groups
Has sulfhydryl/thiol group
Thioester bond
Acetyl-CoA: is a “high-energy” compound because of the presence of thioester bond – hydrolysis will release energy
CoASH
TCA• Circular pathway• Two-carbon unit
needed at the start of the citric acid cycle
• The two-carbon unit is acetyl-CoA
• Involves 8 reactions• The overall reaction
from 1 acetyl-CoA produce 3 NADH, 1 FADH2, 2 CO2 and 1 GTP (equivalent to 1 ATP)
Citric acid cycle - amphibolic Amphibolic (both catabolic
& anabolic) Serves 2 purposes:
1. Oxidize Acetyl-CoA to CO2 to produce energy (ATP & reducing power of NADH & FADH2)-involved in the aerobic catabolism of carbohydrates, lipids and amino acids
2. Supply precursors for biosynthesis (anabolism) of carbohydrates, lipids, amino acids, nucleotides and porphyrins
Replenish TCA- catabolism of amino a. and fatty a.
Anabolic pathway
Require aerobic condition