BCH/PPA 503 Assignment 5 February 25, 1999

 

Describe the "complete" b -oxidation of linolenic acid (18:3) from storage triglycerides in plants all the way to ATP, CO2 and H2O. Tabulate the numbers of ATP, CO2 and H2O molecules formed from each 18:3 molecule. Compare the complete oxidation of fatty acids to the utilization of fatty acids for the synthesis of glucose via gluconeogenesis. How does b -oxidation in plants differ from that in animals? Due by the start of class, March 4, 1999. (10 points)

 

 

Before b -oxidation can begin, fatty acids must 1st be released from triglycerides by a lipase and "activated" to an acyl-CoA e.g.:

18:3 + CoA + ATP à 18:3-CoA + AMP + PPi = consumption of 2 high energy bounds or 2 ATPs

b -oxidation of even numbered saturated fatty acid molecules involves four steps:

  1. Oxidation of the -CH2-CH2- between carbons 2 and 3 to -CH=CH- with reduction of FAD to FADH in animals and reduction of O2 to H2O2 à H2O in plants.
  2. Hydration of -CH=CH- to –HOCH-CH2- with consumption of 1 H2O
  3. Oxidation of –HOCH-CH2- to –O=C-CH2- with reduction of NAD+ to NADH
  4. Cleavage of a C2 unit producing acetyl-CoA and an acyl-CoA 2C shorter.

 

Complete b -oxidation of the saturated 18:3 precursor, 18:0, only involves these 4 steps repeated for 8 cycles:

18:0 + 2 ATP + 8 H2O à 8 FADH (8 H2O in plants) + 8 NADH + 9 acetyl-CoA

9 acetyl-CoA à 9 GTP + 9 FADH + 27 NADH

= 9 ATP + 17 FADH (à 34 ATP + 51 H2O) in mammals – 8 H2O

= 9 ATP + 9 FADH (à 18 ATP + 27 H2O) in plants – 8 H2O + 8 H2O

+ 35 NADPH (à 105 ATP + 140 H2O)

= 148 ATP – 2 = 146 ATP in animals

= 132 ATP – 2 = 130 ATP in plants

 

This is due to:

17 FADH + 8.5 O2 + 35 ADP + 34 Pi à 17 FAD + 51 H2O + 34 ATP in animals

  1. FADH + 4.5 O2 + 18 ADP + 18 Pi à 9 FAD + 35 H2O + 18 ATP in plants

(including the 8 H2O from the oxidase reaction in plants)

35 NADH + 17.5 O2 + 105 ADP + 105 Pi à 35 NAD+ + 140 H2O + 105 ATP

Thus b -oxidation of 18:0 in animals yields 146 ATP + 183 H2O

& b -oxidation of 18:0 in plants yields 130 ATP + 167 H2O

Some sources give 1.5 ATP/FADH and 2.5 ATP/NADH in mitochondria resulting in less ATP and H2O produced from b -oxidation of fatty acids. Nevertheless, large amounts of ATP and H2O would be formed.

b -oxidation in plants occurs in glyoxysomes or peroxisomes, but in animals it usually occurs in mitochondria. This results in the lack of reduction of FAD to FADH in the 1st oxidation of the b -oxidation cycle in plants.

 

The b -oxidation of 18:3-CoA has several differences compared with b -oxidation of 18:0:

The 1st 3 rounds of b -oxidation of 18:3-CoA is the same as for 18:0-CoA yielding

3 NADH + 3 FADH (or 3 H2O in plants) + 3 acetyl-CoA +

cis-3, cis-6, cis-9-dodecatrienoyl-CoA

The cis-3, cis-6, cis-9-dodecatrienoyl-CoA is isomerized to

trans-2, cis-6, cis-9-dodecatrienoyl-CoA which is processed by one more round of the b -oxidation cycle and the 1st step of another round. This results in loss of 1 FADH (1 H2O in plants) relative to the oxidation of 18:0 in the production of the trans-2, cis-4, cis 7-decatrienoyl-CoA. The trans-2, cis-4, cis 7-decatrienoyl-CoA must be reduced before it can be processed further by the b -oxidation cycle in animals. This reduction requires 1 NADH. The product, trans-2, cis 7-decadienoyl-CoA is processed by 2 additional rounds of b -oxidation yielding cis-3-hexenoyl-CoA. This must be isomerized to trans-2-hexenoyl-CoA before being fully processed by 3 more b -oxidation cycles. This circumvents production of another FADH (H2O in plants). Plants do not need to reduce the cis-4 double bond of trans-2, cis-4, cis 7-decatrienoyl-CoA since they have a hydratase that can hydrate this cis double bond and an epimerase that converts the resulting b -hydroxyacyl-CoA into the correct configuration for oxidation to the b -ketoacyl-CoA via the normal b -hydroxyacyl-CoA dehydrogenase operating in the 3rd step of b -oxidation. This saves an NADH.

Thus complete b -oxidation of 18:3

in mammals produces 2 less FADH and 1 less NADH resulting in 139 ATP + 173 H2O

& in plants produces 2 less H2O resulting in 130 ATP + 165 H2O.

Plants, but not animals, can convert fatty acids into glucose via b -oxidation and gluconeogenesis. In this process, acetyl-CoA resulting from b -oxidation is used to convert oxaloacetate to citrate, isocitrate and succinate in glyoxysomes:

acetyl-CoA + oxaloacetate à citrate à isocitrate à succinate

The succinate is transferred to mitochondria where it is converted to oxaloacetate:

succinate + FAD à fumarate + FADH

fumarate + H2O à malate

malate + NAD à oxaloacetate (OAA) + NADH + H+

In the cytosol:

OAA + ATP à phosphoenolpyruvate (PEP) + ADP + CO2

PEP à 2-P-glycerate à 3-P-glycerate

3-P-glycerate + ATP à 1,3-bisphosphoglycerate

1,3-bisphosphoglycerate + NADH à glyceraldehyde-3-P +NAD + Pi

glyceraldehyde-3-P ß à dihydroxyacetone-P

glyceraldehyde-3-P + dihydroxyacetone-P à fructose-1,6-bisphosphate

fructose-1,6-bisphosphate à fructose-6-P + Pi

fructose-6-P à glucose-6-P à glucose