CHEM-342 INTRODUCTION TO BIOCHEMISTRY
MIDTERM EXAMINATION - PART I (Individual work)
Thursday 21 March 1996 7 - 10 p.m.
H. B. White - Instructor
Read this before you turn the page.
1. You should have plenty of time to complete this examination. Therefore, take your time, organize your thoughts, and write "not that you may be understood but rather that you can not possibly be misunderstood." Stream of consciousness answers are rarely well organized. I will be looking for clear, well thought out answers that properly use examples from the course and personal experience.
2. PART I (75 points) You will have up to 90 minutes to answer Part I which contains four questions. You may use your course notes, handouts, and reader. However, texts or other reference books cannot be used on this section. If you finish Part I early, you may leave the room and relax until Part II begins around 8:30 p.m.
3. PART II (25 points) Problem 4 from Part I will be answered again in Part II but with the benefit of group discussion and with access to reference books. Each group will hand in a single answer to be graded and the group grade will be added to your score on Part I. If you do not agree with your group's answer, you may hand in a separate answer which will be graded and used in place of the group grade.
GOOD LUCK
1. Reread Section 10 of Stokes' article.
a. (15 points) In the space below, draw and label a graph that represents clearly what Stokes describes in Section 10. Annotate your graph to indicate what was done at different times to cause the observed changes.
b. (10 points) Create a model to explain qualitatively Stokes' observations of the reaction of cruorine and protochloride of tin with oxygen that you have drawn above.
2. The articles on hemoglobin by both Peters and Zinoffsky deal with stoichiometric relationships of iron to other elements. Peters showed that O2 binds to hemoglobin specifically and stoichiometrically with iron. The 1:1 ratio of O2 to Fe remains the same regardless of the animal source of the hemoglobin. Zinoffsky found a 2:1 stoichiometry of sulfur to iron in horse hemoglobin, a ratio that we can confirm from the now known complete three-dimensional, covalent structure of horse hemoglobin. In contrast to O2:Fe, however, the minimum S:Fe stoichiometry can vary among hemoglobins from different species as is revealed in the table below. In all cases the percent iron by weight is close to 0.34%.
Stoichiometry of sulfur and molecular oxygen to iron in different hemoglobins.
|
Animal Source of Hemoglobin |
|||||
|
Horse |
Cow |
Dog |
Human |
Chicken |
|
|
O2:Fe |
1:1 |
1:1 |
1:1 |
1:1 |
1:1 |
|
S:Fe |
2:1 |
5:2 |
5:2 |
3:1 |
9:2 |
a. (10 points) If Zinoffsky had studied hemoglobin from the cow, rather than the horse, how would it have changed his results and conclusions?
b. (15 points) Given what you have learned about hemoglobin from textbooks, how is it possible for the S:Fe stoichiometry to vary among hemoglobins from different species while the O2:Fe stoichiometry is constant?
c. (Answer this part only if you have not answered parts 2a and 2b; 15 points) What would you need to know, that you don't know, that would enable you to understand and answer parts 2a and 2b? Please phrase your answers as a series of learning issue questions.
Bonus (Answer only if you have finished the regular part of the examination or think you know the answer immediately; 10 points) Is it possible that the analysis of hemoglobin from someone in this class would give an accurate S:Fe stoichiometry of 11:4 or 13:4? No credit without an explanation that shows clear understanding.
3. a (10 points) Your professor draws the following chemical structure on the black board but does not identify the molecule. Describe in as much detail as possible, how you would go about identifying it without asking a person.
b. (5 points) Describe in words or an annotated picture, where in the Morris Library would one find books and periodicals dealing with biochemistry.
Keq
Hbn + nO2 = HbnO2n
Barcroft (1913) showed that the Hill Equation fit the data for oxygen binding to hemoglobin better than another model of the day. He argued for the generality of the model because it also accommodated CO binding, CO binding in the presence of O2, and the effect of CO2 on the binding of CO and O2. In points 1d and 1e of his conclusion, he noted that his results support several counter-intuitive aspects of the chemical model.
Answer one of the following questions. (Later you and your group will need to answer two.)
a. Derive the Hill Equation from the chemical model.
b. All of the chemical models produced by the class last week would have n = 1. Explain clearly why "n" cannot be 1.
c. Hill's chemical model does not recognize intermediates such as Hb4(O2)2,
yet solutions of hemoglobin that are 50% saturated are clearly possible.
Assume, as is now known, that hemoglobin occurs as aggregates of four oxygen-binding
subunits (n = 4). Draw a molecular representation (that would satisfy Hill
and Barcroft) of a solution in which hemoglobin is 50% O2-saturated.
(P.S. You can omit the water molecules.)
When you are done with Part I, you may hand it in and relax in the hall until Part II begins around 8:30 p.m.
MIDTERM EXAMINATION - PART II (Group work)
In Part II, your group will reanswer Question 4 from Part I. You may use any of the reference books available. Your group should come to consensus on a single group answer which constitutes 25% of this midterm examination. However, if after full discussion, your answer does not agree with that of the rest of your group, you may submit you answer for separate grading and separate credit.
The examination should end around 9:30 p.m. Any group or individual that needs more time can stay longer, within reason. Class will not meet tomorrow morning.
4. (25 points) The Hill Equation, y/100 = Kxn/(1 + Kxn) is a mathematical model derived from the following chemical model of oxygen binding to hemoglobin ("y" is the percent saturation of hemoglobin, "x" = pO2, "K" is the equilibrium constant, and "n" is the number of oxygen-binding units in the aggregate):
Keq
Hbn + nO2 = HbnO2n
Barcroft (1913) showed that the Hill Equation fit the data for oxygen binding
to hemoglobin better than another model of the day. He argued for the generality
of the model because it also accommodated CO binding, CO binding in the
presence of O2, and the effect of CO2 on the binding
of CO and O2. In points 1d and 1e of his conclusion,
he noted that his results support several counter-intuitive aspects of
the chemical model.
Answer two of the following three questions. (Continue on the back of the page as needed.)
a. Derive the Hill Equation from the chemical model.
b. All of the chemical models produced by the class last week would have n = 1. Explain clearly why "n" cannot be 1.
c. Hill's chemical model does not recognize intermediates such as Hb4(O2)2, yet solutions of hemoglobin that are 50% saturated are clearly possible. Assume, as is now known, that hemoglobin occurs as aggregates of four oxygen-binding subunits (n = 4). Draw a molecular representation (that would satisfy Hill and Barcroft) of a solution of hemoglobin that is 50% O2-saturated. (P.S. You can omit the water molecules.)