Exam 3

Answer the following. Physical constants and a table of standard reduction potentials are located on the last page of the test. Graph paper is attached. Assume all temperatures are 37^o C. There are 103 points on this test. Good Luck.

1) Ascorbic acid (Vitamin C, below) acts as an antioxidant in vivo, helping protect the body against oxidizing. mutation-inducing chemical agents as well as against radiation. Oxidized ascorbate (dehydroascorbate) isn’t much use, however. Plants can use NADPH to reduce the oxidized form of ascorbate (dehydroascorbate) back to ascorbic acid as shown in the reaction below:

Dehydroascorbate + NADPH + H⁺ ⇌ Ascorbate + NADP⁺

What is the equilibrium constant for this reaction? (Standard reduction potentials are on the last page) (6 pts)

Dehydroascorbate + 2e^- + 2 H^+	⇌	Ascorbate	+0.08 V
NADPH + H^+	⇌	NADP^+ + 2e^- + 2 H^+	+0.32 V
Dehydroascorbate + NADPH + H^+	⇌	Ascorbate + NADP^+	+0.40 V

2) The Krebs cycle produces both NADH and FADH₂. Both transfer their electrons to the electron transport chain and becomes reoxidized. Each NADH produced in the Krebs cycle results in the production of (optimally) about 2.5 - 3 ATPs, while each FADH₂ produced in glycolysis only results in (optimally) about 1.5 - 2 ATPs. Explain why this is true. (5 pts)

Electrons from FADH₂ enter ETC at Complex II, skipping complex I (NADH Dehydrogenase) and therefore pump fewer protons across the inner mitochondrial membrane.

3) You have the good fortune to be responsible for examining the biochemistry of an organism recovered from the subsurface ocean of Europa by a NASA probe. The organism is a small, sluglike creature living on the ocean floor. You measure the concentration of Potassium (K⁺) inside and outside the cells. Inside the cells, [K⁺] = 22 mM; outside the cells, [K⁺] = 660 mM. The cell membrane is leaky towards K⁺, by the way. Assuming the temperature of the animals in their native habitat is 4^o (it is a moon of Jupiter, after all), what is the electrical potential across the cell membrane? (10 pts)

DY = .081 V = 81 mV

4) We mentioned the fact that, under low-oxygen conditions, yeast and some other microorganisms convert the pyruvate produced in glycolysis into ethanol. Similarly, mammals under oxygen stress will convert the pyruvate to lactate. Why do they do that? (5 pts)

\NAD+ is a required reactant in the glyceraldehyde-3-phosphate dehydrogenase step of glycolysis. In the absence of oxygen, NAD+ cannot be regenerated from NADH by electron transport. Instead, NADH is used to reduce pyruvate.

5) Here it comes: diagram the reactions of glycolysis, including structures of the intermediates, enzymes, and products. (24 pts)

6) Differentiate among active transport, facilitated diffusion, and passive (i.e. plain old ordinary) diffusion. How do they differ? Why would I ever want to perform active transport in the first place? And how can I tell the difference between facilitated and passive diffusion? (8 pts)

Facilitated diffusion and passive diffusion both allow materials to cross the membrane in response to their concentration gradients. Passive diffusion involves molecule to which the membrane is permeable; facilitated diffusion involves the passage of molecules to which the membrane is impermeable through channels or transport molecules. As such, they require no additional input of energy. Active transport pumps materials against their concentration gradients and requires energy to proceed. Many materials need to be maintained in disequilibrium across the membrane – e.g. Na+ and K+ ions.

Facilitated diffusion can be distinguished from passive diffusion in that it is saturable.

7) Match the following compounds with their descriptions (3 pts each):

(a) OH _C__ wax

HC-CH=CH-(CH₂)₁₂-CH₃

| O _d__ saturated fatty acid

HC-NH-C-(CH₂)₈-CH=CH-(CH₂)₅-CH₃

| _a__ ceramide

CH₂OH

_b___ phosphatidylcholine

+ CH₃ O

(b) H₃C-N-CH₂-CH₂-O-P-O-CH₂ _e___ triacylglycerol

CH₃ O | O

HC-O-C-(CH₂)₁₆-CH₃

| O

HC-O-C-(CH₂)₁₆-CH₃

H

O

(c) H₃C-(CH₂)₇-CH=CH-(CH₂)₇ -C-O-(CH₂)₈-CH=CH-(CH₂)₇-CH₃

O

(d) HO-C-(CH₂)₁₈-CH₃

H O

(e) HC-O-C-(CH₂)₁₄-CH₃

| O

HC-O-C-(CH₂)₁₄-CH₃

| O

HC-O-C-(CH₂)₁₄-CH₃

H

8) Define the following terms in one or two sentences MAX!!! Provide enough info so that I know that you know what you are talking about; however, the “Great American Novel” answer will be penalized! (4 pts each)

a) Decoupler (or uncoupler) (Hint: proton gradient)

A decoupler breaks down the proton gradient across the inner mitochondrial membrane. Electron transport proceeds, bu the disrupted proton gradient interferes with ATP synthase activity.

b) Ubiquinone (also known as Coenzyme Q)

Electron carrier in the IMM which transfers electrons from complexes 1 and 2 to complex 3. Can carry 2 protons and 2 electrons.

c) ATP Synthase

Multienzyme complex anchored in the IMM. F0 portion is a membrane-bound rotor driven by the H+ gradient across the IMM. F1 portion is driven by the F0 rotor and synthesizes ATP fro ADP and Pi.

d) G-Protein

Multi-subunit protein associated with cell surface receptors. Binding of a hormone by the receptor triggers alpha subunit of the G protein to exchange ADP for ATP. ATP-bound form dissociates from the beta/gamma chains and activates membrane-bound enzymes like Adenylyl cyclase. The alpha subunit is also a slow ATPase; upon ATP hydrolysis, the alpha subunit returns to its original conformation and reassociates with the beta and gamma subunits.

e) Glycogen Phosphorylase

Enzyme responsible for releasing glucose-1-phosphate from the non-reducing ends of glycogen.

z

f) Futile cycle (hint – exploding patients)

Loss of reciprocal control between PFK-1 and fructose-1,6-bisphosphatase, resulting in the generation of massive amounts of heat with neither net oxidation of glucose nor net synthesis. One cause is exposure of susceptible individuals to halothane anaesthetics.

g) Cori cycle

Glucose in the muscles is converted to pyruvate by glycolysis. Lactate created from pyruvate in the muscles in order to regenerate NAD+ is transported in the bloodstream to the liver, where it is reoxidized to pyruvate. Pyruvate enters the gluconeogenic pathway, where it is used to make glucose. The glucose leaves the liver in the bloodstream and is delivered to the muscles.

Physical Constants and Parameters

Constant	Value
Avagadro’s Number	6.02 * 10^23
Boltzmann’s Constant	1.3807 * 10^-23 JK^-1
Charge on electron	-1.602 x 10^-19 coulomb
Gas Constant (R)	8.314 JK^-1mol^-1
Faraday’s Constant	96,485 J/Vmol
k = 1/(4pe)	8.99 Nm^2coulomb^-2
ln x	2.303 log x

Standard Reduction Potentials: