Chp 19: Bioenergetics Flashcards
- Summarize the ATP-ADP cycle including the types of fuels used and the work accomplished.
Part 1 of the ATP-ADP cycle: The body transforms the chemical bond energy of fuels we eat into ATP
- We eat: carbohydrate (glucose), lipid (fat), and protein (amino acid)
- We use oxidative phosphorylation to synthesize ATP
- This process uses oxygen and food and releases heat, H2O, and CO2 as byproducts
- High energy bonds in ATP are synthesized from ADP and inorganic phosphate: ADP + Pi → ATP (Pi is inorganic phosphate, either HPO42+ or H2PO4-)
Part 2 of the ATP-ADP cycle: The body uses the chemical energy in ATP for:
- Muscle contraction
- Active transport (to maintain ion gradients across membranes)
- Biosynthesis of macromolecules
- Detoxification
- Thermogenesis
- The high energy bond of ATP is hydrolyzed to ADP and inorganic phosphate (ATP → ADP + Pi)
- In words, what is ΔG?
- ΔG is the change in free energy of a chemical reaction (change in the energy available to do work between products and substrates)
- Free energy of products minus free energy of substrates
- Delta= difference or change; G= Gibbs free energy
- If the substrates have more free energy than the products, ΔG is negative and the reaction is spontaneous/exergonic
- If the products have more free energy than the substrates, ΔG is positive and the reaction is non-spontaneous/endergonic
- Free energy will vary with temperature, pressure, and pH but since these are usually constant in humans we will ignore them
- What is a high energy bond? Given the structure of ATP, identify the high energy bonds.
- If the ΔG for hydrolysis of the bond is -7 kcal/mol or less (less being more negative), the bond is a high energy bond
- If ΔG > -4 kcal/mol or anything more (ex: -3, -2, -1, 0, +2, +9), the bond is a low energy bond
- The scope of this class doesn’t cover hydrolysis of bonds with a ΔG >-7 and/or < -4 so bonds with -6, -5, -4 kcal/mol aren’t covered
- A high energy compound is one containing a high energy bond
- To identify the high energy bonds of ATP, refer to Fig 19.2
- The phosphate anhydride bonds between the gamma and beta phosphates and between the beta and alpha phosphates are high energy bonds
- ATP → ADP + Pi, ΔG= -7.3 kcal/mol
- ATP → AMP + PPi, ΔG=-7.3 kcal/mol
- Hydrolysis of the phosphate-adenosine bond (a phosphoester bond) releases less energy (3.4 kcal/mol), and consequently this bond is not considered a high energy bond
- PPi is pyrophosphate, two phosphates joined by a phosphate anhydride bond (which is an acid anhydride bond)
- Understand the difference between ΔG and ΔG0!
- ΔG0! is the Standard Free Energy Change of a chemical reaction
- The substrates and products must all be at 1 molar concentrations – this never happens
- Why do we use ΔG0! if it is not as accurate as ΔG?
- Because it is easy to find in a laboratory
- Because when we do not know ΔG, the ΔG0! will often allow us to guess about the spontaneity of the reaction
- ΔG0! occurs at a pH=7 and 25°C – this also never happens in humans.
- ΔG is the Free Energy Change of a chemical reaction
- The concentrations of substrates and products are known. When the ΔG for a reaction is stated, you assume that the concentrations of reactants and products are known, even if they are not stated
- This is not a guess but an accurate statement about the spontaneity of a reaction
- The ΔG is calculated from the ΔG0! and the concentration of reactants and products. (Tab 19.2, bottom equation:)
- If you raise the concentration of the substrates or lower the concentration of the products, the ΔG becomes more negative (the reaction is more exergonic/less endergonic)
- If you lower the concentration of the substrates or raise the concentration of the products, the ΔG becomes more positive (the reaction is more endergonic/less exergonic)
- If ΔG is negative, the reaction is exergonic/spontaneous
- If ΔG=0, the reaction is at equilibrium
- Be able to explain how the free energy of ATP can be used to contract a muscle.
When the concentration of calcium ions rises in a muscle cell, the myosin, actin, and ATP are allowed to react. The contraction reaction between actin and myosin is an endergonic reaction (+ΔG). The hydrolysis of ATP is an exergonic reaction (-ΔG). Since the two reactions are linked (one cannot happen without the other) and the overall ΔG is negative, the reaction is spontaneous. Part of the energy in the ATP was used to drive the reaction and part is given off as heat
Mechanism (Fig 19.4): Calcium binds to the troponin complex, moving the tropomyosin from the actin. This allows contact of the myosin “head” with the actin fiber
- The ATP is bound to the myosin head. This allows themyosin head to separate from the actin fiber. As it separates, ATP is hydrolyzed and the myosin head changes conformation relative to the actin filament
- The hydrolyzed phosphate is released from the myosin head and binds to actin
- The above steps are repeated over and over until the concentration of calcium is lowered
- Be able to explain how the free energy in ATP can be used by the Na+, K+-ATPase to pump sodium and potassium ions through a cell membrane. (Fig 10.10)
- Potassium ions are constantly leaking from the cell and sodium ions are constantly leaking into the cell
- Keeping the intracellular concentration of these two ions at optimal concentrations is the job of Na+, K+-ATPase
- The removal of 3 Na+ and the uptake of 2 K+ is linked to the hydrolysis of ATP by Na+, K+-ATPase
- Transporting both of these ions against a gradient takes energy supplied by ATP
- Part of the energy in the ATP is used to the drive the reaction while part is given off as heat
- Mechanism: 3 Na+ are bound to transporter on the inner surface of the cell membrane, then ATP binds and is hydrolyzed
- The hydrolysis only takes place if the transporter changes its conformation so that Na+ is released to the extracellular space. ADP is released but a phosphoester bond remains
- If 2 K+ bind to the transporter on the outer surface of the cell membrane, the phosphoester bond can be hydrolyzed. This only happens if the enzyme changes its conformation and releases the K+ ions into the cytosol
- One can again see that the transport of the ions cannot take place without the hydrolysis of the high energy bond in ATP
- Be able to explain how the free energy in ATP can be used to synthesize glucose-6-phosphate from glucose and phosphate even though this reaction is endergonic. (Tab 19.3)
Reactions:
- Glucose + Pi → Glucose-6-P + H2O, ΔG0!= +3.3 kcal/mol, endergonic
- ATP + H2O → Pi + ADP, ΔG0!= -7.3 kcal/mol, exergonic
o Glucose + Pi + ATP → Glucose-6-P + Pi + ADP, ΔG0!= -4.0, exergonicE - xplanation: The reaction forming glucose-6-phosphate from glucose and phosphate is endergonic (ΔG0!= +3.3 kcal/mol) and would not proceed by itself
- The hydrolysis of ATP into ADP and Pi is exergonic (ΔG0!= -7.3 kcal/mol)
- The two reactions are linked by the enzyme hexokinase and the overall reaction is exergonic so it occurs spontaneously
- Given two equations with the ΔG0! for each reaction, be able to add or subtract
an equation and determine if the reaction is spontaneous. (See Other Help)
Glucose + ATP → Glucose-6-P + ADP (ΔG0!= -4.0 kcal/mol, exergonic)
Glucose-6-P → Glucose-1-P (ΔG0!= +1.65 kcal/mol, endergonic)
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Glucose + ATP → Glucose-1-P + ADP, ΔG0!= -2.35 kcal/mol, exergonic
- Given the equation at the bottom of Table 19.2, be able to explain why ΔG can be positive while ΔG0! is negative and visa versa
For the reaction aA + bB cC + dD
- ΔG = ΔG0! + RTln([CC][DD])/([AA][BB])
- The value for RT in this equation is a constant: 0.593 kcal/mol
- For both natural logarithms (ln) and log to the base 10 logarithms:
The log of any value greater than 1 is positive
The log of 1 is zero
The log of any value less than 1 is negative
- It follows, that any time the numerator is greater than the denominator, the value of RTln ([CC][DD]) will be positive
- If follows, that any time the numerator is less than the denominator, the value of RTln ([CC][DD]) will be negative
So no matter what the value of ΔG0!, the value of ΔG can be changed by raising or lowering the ratio of products to substrates
Ex: Given the ΔG0! for the above reaction is +3, how do we make the reaction spontaneous in a forward direction? – We adjust the concentrations of the substrates/products so that RTln([CC][DD]) has a value less than -3
Ex: Given the ΔG0! for the above reaction is -2, how do we make the reaction spontaneous in the reverse direction? – We adjust the concentration of the substrates/products so that RTln([CC][DD]) has a value greater than -2
- What is the function of the nucleoside monophosphate kinases and of nucleoside diphosphate kinase? Is it possible to have a high ratio of ATP to ADP and not have a high ratio of GTP to GDP?
The function of the nucleoside monophosphate kinases is to produce nucleoside diphosphates from a nucleoside monophosphate
The term kinase in a nucleoside monophosphate kinase suggests that the enzyme adds something. The enzyme is a phosphokinase but you can’t tell this from the name. It adds high energy phosphate to nucleoside monophosphates to form a nucleoside diphosphate. The high energy phosphate comes from ATP. The ATP used is synthesized from ADP by oxidative phosphorylation
Nucleoside monophosphate kinases catalyze the following types of reactions:
- UMP + ATP → UDP + ADP
- CMP + ATP → CDP + ADP
- GMP + ATP → GDP + ADP
- AMP + ATP → ADP + ADP (called adenylate kinase in muscle)
The function of nucleoside diphosphate kinases is to produce a nucleoside triphosphate from a nucleoside diphosphate
Nucleoside diphosphate kinases add high energy phosphates to nucleoside diphosphates to form a nucleoside triphosphate. As with the nucleoside monophosphate kinases, the high energy phosphate is donated by ATP that is made by oxidative phosphorylation in the mitochondria
Nucleoside diphosphate kinases catalyze the following types of reactions
- UDP + ATP → UTP + ADP
- CDP + ATP → CTP + ADP
- GDP + ATP → GTP + ADP
- All the above reactions are reversible
The ratio of all nucleoside triphosphates to nucleoside diphosphates rises and falls together. So if the ratio of ATP:ADP is high, then the ratio of GTP:GDP is also high
- Be able to write the reaction for adenylate kinase.
Adenylate kinase is the major nucleoside monophosphate kinase found in muscle and catalyzes the following reaction:
ADP + ADP → AMP + ATP or AMP + ATP → ADP + ADP
The reaction has an equilibrium constant of approximately 1. That means that the reaction is readily reversible and has a ΔG of approximately zero
During strenuous muscle activity, much ATP is converted to ADP. The concentration of ATP is decreased while the concentration of ADP is increased. Some of the ADP is converted into ATP by the adenylate kinase reaction and this ATP can be used to contract muscle
At rest, the muscle converts the AMP and ADP back to ATP
- In addition to the nucleoside phosphates, be able to identify 1,3- bisphosphoglycerate, Phosphoenolpyruvate, Creatine phosphate, and Acetyl CoA as high energy compounds. (Fig 19.7, Page 345)
Remember that in a multiple choice test, you will not be expected to write the chemical formula. You may be asked to recognize the structure or name as being a high energy compound
- Be able to describe how the hydrocarbons in our diet are converted to CO2, H20, and ATP. Include the reduction and oxidation of coenzymes and the creation and use of proton gradients in your explanation. (Fig 19.8)
The hydrocarbons of our diet contain much more free energy than the CO2, H2O, and ATP that they are eventually catabolized to. Much of this free energy is conserved in the high energy bonds of ATP and most of the rest is converted into heat.
The hydrocarbons are converted by various catabolic pathways into substrates for reactions that reduce NAD+ and FAD to NADH or FAD(2H). While doing so, their carbon atoms are, one by one, oxidized into CO2
NADH and FAD(2H) reduce the electron transport chain by feeding high energy electrons into the chain. That is, NADH and FAD(2H) are oxidized by the electron transport chain
As the electrons flow through the electron transport chain to eventually reduce oxygen and produce H2O, they force hydrogen ions to leave the mitochondria
The free energy in the high concentration of hydrogen ions outside the mitochondria as opposed to inside the mitochondria, the proton gradient, is used to synthesize ATP from ADP and Pi.
- What is oxidative phosphorylation?
- The formation of high-energy phosphate bonds by phosphorylation of ADP to ATP coupled to the transfer of electrons from reduced coenzymes to molecular oxygen via the electron transport chain
- Occurs in the mitochrondria
- Describe three general ways that oxidation or reduction occurs in the body.
Introduction: Oxidation-reduction reactions are ones in which electrons are transferred from a donor molecule (the reducing agent) to an acceptor molecule (the oxidizing agent)
At least one substrate is always oxidized while at least one substrate is always reduced
Three ways to spot an oxidation-reduction reaction:
- If a molecule gains hydrogen atoms during a chemical reaction, the molecule is reduced. If a molecule loses hydrogen atoms during a chemical reaction, the molecule is reduced.
- C6H12O6 + 6O2 → 6CO2 + 6H2O
- If a molecule gains an electron during a chemical reaction, the molecule is reduced. If a molecule loses an electron during a chemical reaction, the molecule is oxidized.
Ex: In the electron transport chain, iron atoms are continuously gaining and losing electrons. When the iron atom gains an electron, the iron atom is reduced. When an iron atom loses an atom, it is oxidized
- Fe3+ + e- → Fe2+, iron is being reduced when it gains an electron
- Fe2+ → Fe3+ + e-, iron is being oxidized when it loses an electron
