Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 438 - Introductory Biochemistry - Spring 2013

Lecture Notes:

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Mitochondrial Transport/Communication

Given the requirement for a very tight inner-mitochondrial membrane in order to maintain the proton electrochemical gradient, how do important charged molecules such as ATP and NADH get across the membrane?

ADP & ATP are obviously among the most important substances to transport into and out of the mitochondria. Adenine nucleotide translocase exchanges matrix ATP for cytosolic ADP in their magnesium-free forms. Note that in this exchange ATP4- leaves the matrix as ADP3- goes in, resulting in a net loss of (1-) for the matrix. This increases the charge gradient across the membrane, and thus must be driven by the mitochondrial proton gradient. Of course Pi must also be transported across the membrane with ADP to make ATP in the matrix. This is accomplished using another transporter which co-transports a dihydrogen phosphate and a single proton in an electroneutral process. Note the addition of these two processes is equivalent to moving one proton from the cytosol to the matrix, costing the gradient one proton (moving a negative charge out is equivalent to moving a positive charge in).

The net cost of providing an ATP to the cytosol is thus four protons: three to convert ADP + Pi to ATP and one to transport ATP out of, while bringing ADP and Pi into, the matrix. This accounts for the theoretical yield of ATP: (10 H+/NADH)/(4 H+/ATP) = 2.5 ATP/NADH. (Note that this means that bacteria may get more ATP (up to 38 ATP's instead of the 32 expected in mammals).

Reducing Equivalent Shuttles: In aerobic metabolism NADH from glycolysis must be regenerated to NAD+ in the mitochondria. Two shuttles are important in different tissues/organisms for this process:

Glycerol-3-P DH reaction

The glycerol-3-P is then reoxidized to dihydroxyacetone-P by Flavoprotein dehydrogenase. This enzyme is situated on the inner mitochondrial membranes outer surface. It uses FAD to oxidize the glycerol-3-P to dihydroxyacetone-P, passing the electrons to CoQ (note the similarity to succinate DH, except for the location on the outer instead of the inner surface of the membrane). The organism can thus get 1.5 ATP equivalents for this NADH.

Regulation of Mitochondrial Oxidative Phosphorylation

The ETS appears to be regulated largely by the availability of ADP and NADH. For most catabolic situations [ADP] will be the controlling factor. Note that the effects of [ADP] will integrate the regulation of TCA and Glycolysis with that of ETS.

P/O Ratios and Their Determination

A useful measure of metabolism is the number of ATP's captured for each oxygen atom reduced to water (ATP/H2O ) or the phosphate captured per oxygen atom = P/O.

Let's consider the P/O ratio for the conversion of citric acid to malate in the mitochondria.

 Reaction
Energy Product

factor
ATP Equivalents
(@2.5 ATP/NADH)
Oxygens consumed 
Isocitrate DH NADH

1

2.5
 1
2-oxoglutarate DH Complex NADH

1

2.5
 1
Succinyl-CoA Synthetase GTP

1

1
 0
Succinate DH FADH2

1

1.5
 1

  TOTAL=

7.5
3
P/O = ATP/O =
7.5/3 = 2.5

Lipids

Recall the lipid definition: The portion of an organism which will partition into a non-polar solvent.

Types of Lipids:

palmitic acid structure

stearic acid structure

oleic acid structure

glycerophospholipoid structure

membrane lipds and structures

public domain image via Wikipedia Creative Commons

 

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© R. A. Paselk 2010;

Last modified 15 April 2013