BIOLOGY 2B03 Lecture Notes - Lecture 8: Dihydrofolate Reductase, Cross-Linked Polyethylene, Alpha Helix
6 Jun 2018
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Protein Targeting: Post-Translational Targetting
Structure and Function of Peroxisome
1) bound by a single membrane •
2) oxidative and synthetic reactions •
3) do not have a genome •
4) reproduction by fission •
TEM Image
in picture, each peroxisome seen to have single bilipid membrane •
inside there are dense regions that contain protein aggregates ◦
protein aggregates composed of exclusively catalase ◦
in animal cells, peroxisomes responsible for coolesterol synthesis •
some will end up in membrane ◦
in nerve cells - synthesize plasmalogen for cell membranes •
this is a component of the membrane that increases the insulating capacity of the membrane ◦
in liver, peroxisome is site of oxidation of toxins such as alcohol •
they are also required for conversion of fatty acids to carbs in plant cells - important in seeds •
in all cells - responsible for catalysis of fatty acids •
Fatty Acid Metabolism
major function of peroxisome - breakdown of long chain fatty acids via beta-oxidation •
animal cells - first oxidized to medium chain FA's which are shuttled to mitochondria •
FA's required for metabolic energy bc produce highest ATP yield •
byproduct = hydrogen peroxide •
catalase enzyme converts toxic hydrogen peroxide to oxygen and water molecules ◦
catalase also useful marker to visualize peroxisomes in cell - because abundant ‣
Peroxisome Biogenesis
formation of new peroxisomes by division and incorporation of new proteins •
peroxi proteins synthesized in cytosol and transported to peroxi •
Ex. PMP70 - targeted to precursor membranes first to create peroxisomal ghost ◦
membrane proteins transport peroxi matrix proteins (catalase) to interior of organelle •
more proteins = growing peroxi via fission •
Luciferase
enzyme that allows bioluminescence as seen in fireflies •
found in peroxi of cells in abdomen ◦
luciferase used same mechanism for peroxi protein transport found in mammalian cells because signals and •
transport proteins are same
Protein Transport - How Does Peroxisomal Protein Move from Cytosol to Target?
1) signal sequence on transported protein •
2) receptors for signal sequences •
3) translocation channels •
4) coupling to ATP hydrolysis (energy requirement) •
5) a way of differentially targeting soluble and membrane-associated proteins •
Rule 1 - Peptide Signal Sequence
Ex. signal sequence in luciferase enzyme found on many peroxi proteins in different species •
called PTS1 (peroxisomal-transport sequence 1) ◦
consists of just tripeptide (serine, lysine, leucine) - SKL sequence ◦
Document Summary
Fatty acid metabolism major function of peroxisome - breakdown of long chain fatty acids via beta-oxidation animal cells - first oxidized to medium chain fa"s which are shuttled to mitochondria. Fa"s required for metabolic energy bc produce highest atp yield byproduct = hydrogen peroxide catalase enzyme converts toxic hydrogen peroxide to oxygen and water molecules catalase also useful marker to visualize peroxisomes in cell - because abundant. Peroxisome biogenesis formation of new peroxisomes by division and incorporation of new proteins peroxi proteins synthesized in cytosol and transported to peroxi. Pmp70 - targeted to precursor membranes first to create peroxisomal ghost membrane proteins transport peroxi matrix proteins (catalase) to interior of organelle more proteins = growing peroxi via fission. Pts1 sequence necessary + suf cient for transport of proteins to peroxi. Pex5 recognizes pts1 sequence - binds to pts1 at c-terminus of target protein.
