BCH 361 Lecture Notes - Lecture 11: Uncoupling Protein, Atp Synthase, Uch

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22 Nov 2020
Department
Course
Professor
BCH 361 Advanced Biochemistry I Fall 2020
1
Group Discussion Questions
Week 11 (November 23/24)
Case Study activities:
1. Prepare by reading the case study How to Make ATP Introduction, Parts I and IV (attached) and
attempting the questions included with Part I.
2. You will be given additional questions in class relating to Part IV.
Additional questions for study or discussion, time permitting.
1. Humans have only about 250 g of ATP at any one time, but require about 83 kg of ATP per day. How
is this discrepancy between requirements and resources reconciled?
2. What is the mechanistic basis for the observation that the inhibitors of ATP synthase also lead to an
inhibition of the electron transport chain?
3. The rate of oxygen consumption by mitochondria increases when ADP is added and then returns to
its initial value when ADP has been converted to ATP. Why does the rate of O2 consumption
increase?
4. In normal mitochondria the rate of electron transfer is tightly coupled to the demand for ATP. Thus
when the rate of utilization of ATP is relatively low, the rate of electron transfer is also low.
Conversely, when ATP is demanded at a high rate, electron transfer is rapid. Under such conditions
of tight coupling, the number of ATP molecules produced per atom of oxygen consumed when
NADH is the electron donor known as the P/O ratio is close to 3.
a. Predict the effect of a relatively low and a relatively high concentration of an uncoupling
agent on the rate of electron transfer and the P/O ratio.
b. The ingestion of uncouplers causes profuse sweating and an increase in body temperature.
Explain this phenomenon in molecular terms. What happens to the P/O ratio in the
presence of uncouplers?
c. The uncoupler 2,4-dinitrophenol was once prescribed as a weight-reducing drug. How can
this agent, in principle, serve as a reducing aid? Such uncoupling agents are no longer
prescribed because some deaths occurred following their use. Why can the ingestion of
uncouplers lead to death?
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Case copyright held by the National Center for Case Study Teaching in Science, University at Bualo, State University of New York.
Originally published March , . Please see our usage guidelines, which outline our policy concerning permissible reproduction of this
work. Licensed photo © Hennyvanroomen | Dreamstime.com,  .
Introduction
ere is an old joke about biochemists:
Q: How do biochemists gure out how a wristwatch works?
A: ey put it in a blender, grind it up, centrifuge out the solids and see what happens in the supernatant.
Why is this funny?
e joke goes back to the idea that in the early 20th century, many biochemists thought of cells as “bags of enzymes.
Biochemists elucidated many metabolic pathways during this time by studying the proteins, enzymes, and the reac-
tions they catalyzed. ey used an experimental approach where each component of the system (enzyme, substrate,
cofactors) was isolated and then reconstituted to demonstrate the reactions in vitro, or in a test tube. is was a very
powerful technique that was used to gure out most of the biochemical pathways we know today. Because this was the
main experimental approach, it was often called the conventional or orthodox approach.
Biochemists think about reaction pathways. If you have a starting substrate and wind up with a reactant, there must
be intermediate molecules with enzymes catalyzing each step of the pathway. For example, in glycolysis there are
enzymes that catalyze each reaction, forming intermediate molecules at each step between glucose and pyruvate. e
conventional experimental approach to guring out this pathway involved breaking cells open, isolating the compo-
nents essential to the reaction (enzymes, substrates, any cofactors) and then reconstituting the system.
Energy Within Cells
Energy ow is the prime characteristic of life. Living cells take energy from their environment and transform it and
store it so that the energy can then be used for all cellular processes such as growth, reproduction, communication,
and movement. Without ways to capture and store energy, cells could not function.
By the early 1940s, scientists knew that energy in
the cell was stored in adenosine triphosphate or
ATP. Making ATP is an endergonic or unfavor-
able reaction that requires the input of energy. In
biological organisms, that energy is supplied by
sunlight (for photosynthesis) or chemicals such as
sugars (for respiration). Once formed, ATP can
be hydrolyzed (or split) to adenosine diphosphate
(ADP) and inorganic phosphate (Pi ), releasing
a large amount of energy. is energy is used by
the cell to drive metabolism (Figure 1). Figure 1. ATP.
Relative Free Energy
40
0
10
20
30
PEP + ADP
pyruvate + ATP
substrate-level phosphorylation
ADP ATP
Three Classic Experiments in Biology
How to Make ATP:
by
Monica L. Tischler
Department of Biological Sciences
Benedictine University, Lisle, IL
NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE
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