metabolism Flashcards
(32 cards)
What are metabolic pathways?
Metabolic pathways are a series of enzyme-catalyzed reactions that convert a starting molecule into an end product through a series of metabolic intermediates. Many pathways are shared between organisms and cell types but some differ between cells.
What are the two major purposes of metabolism?
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1. To obtain usable chemical energy from the environment, either by capturing solar energy (photosynthesis) or consuming and breaking down nutrients.
2. To make the specific molecules that cells need to live and grow.
What are the two main types of metabolic pathways?
○ Anabolic pathways: use energy to build larger molecules from smaller precursors and are generally reductive (electrons are gained to form new bonds).
○ Catabolic pathways: release energy (some of which is stored) by breaking down larger molecules and are generally oxidative (electrons are lost as bonds are broken).
What are amphibolic pathways?
Amphibolic pathways can function in both anabolic and catabolic processes depending on the conditions.
What are the most significant fuel sources for humans?
Polysaccharides (complex carbohydrates) that are broken down into monosaccharides (simple sugars) and triacylglycerol (fat), which is broken down into fatty acids.
How are carbohydrates and fatty acids stored in the body?
○ Carbohydrates are stored as glycogen, a polymer of glucose, in the liver and skeletal muscle.
○ Fatty acids are stored as fat (triacylglycerols) in adipocytes.
How is the oxidation state of carbon affected during catabolism?
Carbons in metabolites tend to become more oxidized. For example, a hydroxyl group might be converted to a carbonyl, or a carboxylate to carbon dioxide.
Oxidation is the loss of electrons during a reaction by a molecule
Are fats more or less reduced than carbohydrates?
Fats (and fatty acids) are more reduced than carbohydrates.
What is the typical end product of carbon catabolism?
Carbon dioxide
What is the standard free energy change of a reaction?
Standard free energy change (ΔG°′) is a thermodynamic term that describes the free energy change of a reaction under standard conditions:
* ○ ΔG°′ = ΔG°′f, products - ΔG°′f, reactants
○ ΔG°′ = ΔH°′ – TΔS°′
○ ΔG°′ = -RTln(K′eq)
(′) indicates the reaction is taking place under biochemical conditions
What are biochemical standard conditions?
○ pH = 7
○ [S] & [P] = 1M
○ Temperature = 25°C/298K
○ Pressure = 1 atm
○ [H2O] = 55M
How does the actual free energy change of a reaction relate to the standard free energy change?
The actual free energy change (ΔGreaction) depends on the concentrations of reactants and products present in the system:
○ ΔGreaction = ΔG°′ + RTln([products]/[reactants])
What does the sign of ΔG′ tell us about the spontaneity of a reaction?
○ ΔG′ > 0: The reaction will not occur spontaneously in the forward direction.
○ ΔG′ < 0: The reaction will occur spontaneously in the forward direction.
○ ΔG′ «_space;0: The reaction is considered irreversible (i.e., it will only proceed in the forward direction).
○ ΔG′ ~ 0: The reaction is considered reversible (i.e., it can proceed in both the forward and reverse directions).
How do the free energy changes of individual reactions in a pathway relate to the overall free energy change?
The free energy changes of individual reactions in a pathway are additive. For a pathway to proceed, the overall ΔG′ for the entire pathway must be negative (ΔG′ < 0). This is true for both catabolic and anabolic pathways.
What does it mean for a metabolic pathway to exist in a steady-state?
When a metabolic pathway is operating in a steady state, the concentrations of metabolic intermediates remain relatively constant, even though there is a constant flux of metabolites through the pathway.
Which steps in a metabolic pathway are typically regulated?
Irreversible steps (those with a large negative ΔG′) are usually regulated, while reversible steps are not. The rate-limiting step in a pathway is the irreversible, regulated reaction that determines the overall rate of the pathway.
What are some common mechanisms of enzyme inhibition in metabolic pathways?
○ Product inhibition: An enzyme is inhibited by the product of its reaction.
○ Feedback inhibition: An enzyme is inhibited by a metabolite further down the pathway.
What is feed-forward activation?
Feed-forward activation occurs when an enzyme is activated by a metabolite upstream in the pathway. This helps ensure that the pathway functions in concert and that intermediates do not accumulate.
What is reciprocal regulation?
Reciprocal regulation occurs when opposing pathways (which catalyze the reverse reactions of one another) are regulated to ensure that both do not operate simultaneously.
What are high-energy intermediates?
High-energy intermediates are compounds that contain “usable” chemical energy. This energy can be recovered for use in other reactions. These molecules have a simple reaction (hydrolysis) that is associated with a large, negative standard free-energy change (ΔG°′ < -25kJ/mol).
What are the three major types of high-energy intermediates?
○ Electron carriers (NADH, NADPH, FADH2, FMNH2)
○ Nucleoside triphosphates (NTPs: e.g. ATP, UTP, GTP)
○Thioesters
Describe the general roles of the electron carriers NAD+, NADP+, and FAD in metabolic reactions.
○ NAD+ and FAD are typically reduced during catabolic reactions (they act as oxidizing agents).
○ NADPH is typically oxidized during anabolic reactions (it acts as a reducing agent).
How are the cofactors NAD+, NADP+, and FAD reduced?
These cofactors are reduced by the addition of two electrons and two protons:
○ NAD+ + 2H+ + 2e- → NADH + H+
○ NADP+ + 2H+ + 2e- → NADPH + H+
○ FAD + 2H+ + 2e- → FADH2
What is the difference between a cosubstrate and a prosthetic group?
○ Cosubstrate: A molecule that binds transiently to an enzyme and is altered during the reaction. NAD+ and NADP+ are typically cosubstrates.
○ Prosthetic group: A molecule that is tightly (often covalently) bound to an enzyme. FAD is typically a prosthetic group.
