Exam II Flashcards
- There are five general principles governing metabolic pathways:
o A complex chemical transformation occurs in a series of separate, intermediate reactions that form a metabolic pathway
o Each reaction is catalyzed by a specific enzyme
o Most metabolic pathways are similar in all organisms, from bacteria to plants to humans
o In eukaryotes, many metabolic pathways are compartmentalized, with certain reactions occurring inside specific organelles
o Each metabolic pathway is controlled by key enzymes that can be inhibited or activated, thereby determining how fast the reactions will goo
- Free energy
– chemical energy available to do work
- Law of thermodynamics
A biochemical reaction may change the form of energy but not the net amount
- A biochemical reaction is a type of energy
– exergonic if it releases energy from the reactants or endergonic if energy must be added to the reactants
ATP Cycle
Cells use adenosine triphosphate as an energy currency. Some of the energy that is released in exergonic reactions is captured in chemical bonds when ATP is formed from adenosine diphosphate and inorganic phosphate (hydrogen phosphate; commonly abbreviated to Pi). The ATP can be hydrolyzed at other sites in the cell, releasing free energy to drive endergonic reactions.
- An ATP molecule - consists of the
of the nitrogenous base adenine bonded to ribose (a sugar), which is attached to a sequence of three phosphate groups. The Hydrolysis of a molecule of ATP yields free energy, ADP, and the inorganic phosphate ion
o ATP + H20 -> ADP + Pi+ Free Energy
- The change in free energy from the hydrolysis is
about -7.3 kcal/mol (-30 kJ/mol). Recall that a negative change in free energy means that the product molecules have less energy than the reactant, so the change is negative
- A molecule of ATP can also be hydrolyzed to – adenosine monophosphate and a pyrophosphate ion (P2O7^4-; commonly abbreviated as PPi).
- Two characteristics of ATP account for the free energy released by the loss of one or two of its phosphate groups:
o The free energy of the P-O bond between phosphate groups (called a phosphoanhydride bond and often denoted by wavy lines in chemical structures) is much higher than the energy of the O-H bond that forms after hydrolysis. So some usable energy is released by the following hydrolysis
o Because phosphate groups are negatively charged and so repel each other, it takes energy to get phosphate near enough to each other to make the covalent bond that links them together in the ATP molecule.
- In some reaction, ATP is formed by substrate level phosphorylation
because it involves the transfer of phosphate to ADP. This is the case for some reactions of glycolysis. But most of the ATP in living cells is formed by oxidative phosphorylation.
- Redox reactions
a reaction in which one substance transfers one or more electrons to another substance.
o Reduction – is the gain of one or more electrons by an atom, ion, or molecule
o Oxidation – the loss of one or more electrons
- Oxidation and reduction ALWAYS
occur together: as one chemical is oxidized, the electrons it loses are transferred to another chemical, reducing it. Thus some molecules are called oxidizing agents and others are reducing agents
- When a Molecule loses a hydrogen atom, it becomes oxidized
- The more reduced a molecule is, the more energy is stored in its covalent bonds.
- In a redox reaction,
some energy is transferred from the reducing agent to the reduced product. Some energy remains in the reducing agent (now oxidized), and some is lost to entropy
- Cells use the coenzyme
coenzyme nicotinamide adenine dinucleotide as an electron carrier in redox reactions This coenzyme exists in two chemically distinct forms, one oxidized (NAD+) and the other reduced (NADH). The reduction reaction:
o NAD+ + H^+ +2e^- ->NADH
o Involves the transfer of a proton and two electrons, which are released by the accompanying oxidization reaction. This reaction is highly endergonic, with a positive G about four times greater than the positive G for ATP formation.
- Within the cell, the electrons do not remain with NADH.
Oxygen is highly electronegative and readily accepts electrons from the reduced NADH molecule. The oxidation of NADH by O2 (which occurs in several steps:
o NADH + H^+ + 1/2O^2 -> NAD^+ + H2O
o Is highly exergonic, releasing energy with a lambda G of -52.4 kcal/mol (-219 kj/mol). Note that the oxidizing agent appears here a s1/2 O2 instead of O. This notation emphasizes that it is molecular oxygen, O2, that acts as the oxidizing agent
- Because the oxidation of NADH released more energy than the hydrolysis of ATP,
NADH can be thought of as a larger package of free energy than ATP.
- NAD+ is a common electron carrier in cells, but not the only one.
Others include FAD, which also transfers electrons during glucose metabolism, and NADP+ which is used in photosynthesis.
- We can summarize the two energy coupling coenzymes as follows
o ADP traps chemical energy to make ATP
o NAD+ traps the energy released in redox reactions to make NDH.
- Most chemical energy in cells is stored in the
C-H bonds of carbs and lipids. The release and reuse of this energy can be summarized as follows:
o Energy is released in catabolism by oxidation; this energy can be trapped by the reduction f coenzymes such as NADH
Energy for many anabolic and other energy requiring processes is supplied
by ATP. For example, active transport requires ATP
o Most of the energy releasing reactions in the cell produces
NADH, but most of the energy consuming reactions require ATP.
o Cells need a way to connect the two coenzymes;
; that is, to transfer energy from NADH to the phosphoanhydride bond of ATP. This transfer is accomplished in a process called oxidative phosphorylation-
the coupling of the oxidation of NADH
• NADH -> NAD+ + N+ + 2e- + energy
To the production of ATP
• Energy + ADP + Pi -> ATP
o This coupling is achieved via a mechanism called chemiosmosis- the diffusion of protons across a membrane, driving the synthesis of ATP.
o Chemiosmosis relies on
If the concentration of a substance is greater on one side of a membrane than the other, the substance will tend to diffuse across the membrane to its region of lower concentration
If a membrane blocks this diffusion, the substance at the higher c0oncentration has potential energy, which can be converted to other forms of energy
o Because the interior of a membrane is nonpolar, protons cannot readily diffuse across the membrane.
proton motive force
o Chemiosmosis convers the potential energy of a energy gradient across a membrane into the chemical energy in ATP.
o In prokaryotes, the gradient
is set up across the plasma membrane
o In eukaryotes, chemiosmosis occurs in the mitochondria and chloroplasts.
In the mitochondria, the H+ gradient is set up across the inner membrane, using energy released by the oxidation of NADH.
In chloroplasts, the H+ gradient is set up across the thylakoid membrane using energy from light.
o A membrane protein called ATP synthase uses
the potential energy of the H+ gradient to drive ATP synthesis.
o ATP synthase is a molecular motor composed of two parts
: the F0 unit, which is a trans membrane domain that functions as the H+ channel; and the F1 unit, which contains the active sites fir ATO synthesis.
o The f1 unit consists of six subunits (three each of two polypeptide chains), arranged like the segments of an orange around a central polypeptide. The potential energy set up by the proton gradient drives the passage of protons through the ring of polypeptides that make up the F- component. This ring rotates as the proton pass through the membrane, causing the F1 unit to rotate as well. ADP and Pi bind to active sites that become exposed on the F1 unit as it rotates, and ATP is made. The structure and function of ATP synthase are shared by living organisms as diverse as bacteria and humans. The molecular motors make ATP at rates of up to 100 molecules per second
- An oxidation reaction is always coupled with a reduction. When NADH is oxidized to
NAD+ in the mitochondria, the corresponding reduction reaction is the formation of water
o H+ + 1/2O2 -> H2O
- So the key role of O2 cells-
the reason we breath and have a blood system to deliver O2 to tissues- is to act as an electron acceptor and become reduced. In chloroplasts, the molecule ultimately reduce is NADP+, a relative of NAD+
- Chemiosmosis can be demonstrated experimentally.
