Note from Dr. White: My goal is your understanding. The best way to achieve that understanding is to first work on the problem alone to see if you can solve it. Don't give up without making a serious effort. Remember, your text book is a resource. If you can't solve a problem, please feel free to work and talk through it with other students in the class. However, write up your answer on your own to convince yourself and me that you really understand the problem and its resolution. If you do work with others, please acknowledge their participation. There is no penalty for such collaboration unless answers are copied and explanations paraphrased from another student's work. You may find it useful to printout pathway work sheets.
1. Varations on a Theme. (5 Points) The gram negative bacteria Pseudomonas fluorescens ferments glucose by a pathway different from glycolysis in its initial steps. A key intermediate in this pathway is 2-keto-3-deoxy-6-phosphogluconate (KDPG) which is cleaved by KDPG aldolase in a reaction analogous to that of fructose 1,6-diphosphate aldolase in glycolysis. The products of this reaction are metabolized by reactions familiar to you. The structure of KDPG is given below.
a. What are the products (D and E) of the KDPG-aldolase reaction? How are they formed mechanistically by analogy to fructose bisphosphate aldolase?
b. What amino acid is likely to be at the active site of KDPG-aldolase and react covalently with the substrate? How could this residue, among all the others of its type, be labeled specifically and irreversibly (chemically modified with a radioactive substituent)?
c. If l-14C-glucose forms carboxyl-labeled KDPG, where would the label appear in the end products of fermentation, ethanol and CO2? Compare this pattern to the glycolytic fermentation of l-14C -glucose to ethanol and CO2.
d. One ATP is used up in the formation of KDPG from glucose. How many net ATPs are generated in the conversion of glucose to ethanol and CO2 by this pathway? Compare this to the formation ethanol and CO2 by glycolysis (Embden-Meyerhoff Pathway). Which is more efficient?
e. Consider the conversion of glucose to KDPG and make
an intelligent guess at the intermediates A, B, and C between glucose and
KDPG. In so far as is possible, use well-known reactions. Indicate the
coenzymes, if any, involved in the four reactions.
2. Paying Attention to Details: (5 Points) The process of glucogenesis from pyruvate in liver does not involve the net incorporation of HCO3-; however, when liver tissue is incubated with pyruvate and H14CO3-, the glucose formed is radioactive.
a. Explain this result given that carbons 3 and 4 of glucose are labeled.
b. What carbons of glucose would be labeled starting with 3-14C-pyruvate?
This assignment has several purposes:
1. To review and consolidate
your understanding of Glycolysis, Gluconeogenesis, the Hexose Monophosphate/Pentose
Phosphate Pathway, and the TCA Cycle.
2. To illustrate the consequences
of enzyme specificity and stereochemistry on metabolism.
3. To show how useful radioisotopes
are in metabolic studies.
4. To apply chemical principles
to metabolism.
5. To exercise your biochemical
problem-solving skills.
This assignment will be evaluated on the following elements:
1. A clear analysis of the problems.
2. Answers that display an understanding
of the problems.
3. Handed in on time.