Peroxisomes PDF

CELLULAR MOLECULAR BIOLOGY (LEC) CELLMOL Term 1 | A.Y. 2024 - 2025 ○ When you say peroxisome, we are referring to the compound hydrogen 1 Peroxisomes...

CELLULAR MOLECULAR BIOLOGY (LEC) CELLMOL Term 1 | A.Y. 2024 - 2025 ○ When you say peroxisome, we are referring to the compound hydrogen 1 Peroxisomes peroxide. ○ When you look at the enzymes in the peroxisomes, you’ll see that they’re very much involved in the hydrogen peroxide metabolism (electron micrograph) Peroxisomes What are peroxisomes? ○ Peroxisomes, just like lysosomes, are bound by a single membrane ○ This is unlike mitochondrion, the nucleus, and the chloroplast, that are bound by three membranes The enzyme called Oxidase In the electron micrograph (see image above), ○ Previously mentioned in the SER, there is a dense center. monooxygenated that are involved in ○ That center is the Crystalline core: the detoxification urate oxidase ○ What is an Oxidase? ○ Take note that this is the signature It always have the oxidase inside the peroxisome participation of an oxygen Prominent in kidney and liver cells, molecule photosynthetic cells, germinating seedlings Recall the Redox Reaction, that store fats in their seeds (According to it’s stated there that oxidation Becker) is the lost of electrons and it Characteristic enzymes: may or may not involved ○ Catalase oxidases The enzyme catalase is When a molecule is unique to peroxisomes reduced, electron or This is confined to the hydrogen atom is peroxisomes in the eukayotic taken up (because the cell hydrogen atom has No peroxisomes can be found an electron) in the prokaryotic cells If a molecule takes up All cells have peroxisomes a hydrogen atom, that except the mature mammalian molecule is reduced RBC When a molecule ○ Oxidases (also in other organelles, loses an electron or e.g. SER, mitochondria etc.) hydrogen atom, then Why are they called peroxisome? the molecule is oxidized PAGE 1 Cellular Molecular Biology Lecture CELLULAR MOLECULAR BIOLOGY (LEC) CELLMOL Term 1 | A.Y. 2024 - 2025 Oxidation does not always ○ In the presence of a catalase, (recall require the presence of the gen micro), we have the enzyme oxygen atom catalase catalyzing the breakdown of We can have oxidations H2O2 to the oxygen molecule and where it requires presence of water oxygen and there are You see here that inside the peroxisomes, oxidations that do not need it. even if it causes production of the toxic H2O2, It’s okay as long as electrons it can clean up its own mess are lost from the molecule ○ The H2O2 can be broken down by the Enzymes that cause oxidation catalase inside the peroxisomes so in the presence of oxygen are that H2O2 does not accumulate called oxidases What will be the other enzymes present in the The RH2 is the reduced peroxisome? substrate (refer to the image ○ The other enzyme is the peroxidase above), it has the hydrogen What does the peroxidase atom that can be given up do? Once it gives up the In peroxidase, you hydrogen atom, it have an additional now becomes R reactant that is the We see that the reduced substrate reduced substrate R’H2 has a hydrogen here is now oxidized electron that can be What happened to the given up hydrogen atoms? The other reactant is The one that the H2O2 accepted the The enzyme reacting hydrogen atom is the is the peroxidase oxygen The oxidizing agent is In this case oxygen is the H2O2 reduced into Looking at the products hydrogen peroxide We see that the In catalyzing the oxidation of a reduced reduced substrate substrate, it converts the oxygen to hydrogen R’H2 has now peroxide become oxidized to So now, we are accumulating hydrogen R’ peroxide in the peroxisome but hydrogen ○ Why do we peroxide is a toxic agent that can kill the cell. say its It is a powerful oxidizing agent. oxidized? ○ What additional functions does the ○ Because it peroxisomes have? doesn’t have The peroxisomes, in addition the hydrogen to having oxidases that can atom resolve in the production of anymore H2O2, it also has the ○ Two H have enzymes catalase and been lost peroxidase PAGE 2 Cellular Molecular Biology Lecture CELLULAR MOLECULAR BIOLOGY (LEC) CELLMOL Term 1 | A.Y. 2024 - 2025 The peroxidase does not catalyze the breakdown of H2O2 The peroxidase here catalyzes the oxidation of a reduced substrate (R’H2) by hydrogen peroxide H2O2 is the oxidizing agent The other reactant is the reduced substrate that will be In relation to detoxification, we’re repeating oxidized by H2O2 what you call ROS The reduced substrate R’H2 is Reactive oxygen species are toxic to the cells oxidized to R’ and the toxic H2O2 is now converted to the non-toxic water (2H2O) Functions 1. Detoxification Fatty acids formula ○ Hydrocarbon chain and COOH When you say Beta-oxidation you have the formation of the acetyl CoA Beta carbon is oxidized please go back to the chemical reaction Imagine R’H2 to be a toxic product, it From one molecule of fatty acid you can have can be methanol, formaldehyde, or so many molecules of acetyl CoA that is any toxic organic product molecule produced through Beta oxidation ○ It is reduced in the presence The acetyl CoA that is produced can enter the of an H2O2 produced by the Krebs cycle. Then it produce NADH and peroxisomes itself using the NADH2. And these two will enter the enzyme peroxidase, by doing respiratory chain and they are oxidized and so it can cause the oxidation you have a series of redox reactions. Then of the toxic product/ toxic you’ll have ATP molecules produced via molecule into the non-toxic oxidation. product/molecule The longer the fatty acid the more acetyl CoA ○ Same applies for the toxic is produced, the more will enter the Kreb H2O2 to the non-toxic 2H2O cycle, the more NADH and NADH2 will be ○ This comprises the function of produced the more will enter the electron detoxification transport chain, then the more ATPs are produced through oxidating phosphorylation That is how important the acetyl CoA is PAGE 3 Cellular Molecular Biology Lecture CELLULAR MOLECULAR BIOLOGY (LEC) CELLMOL Term 1 | A.Y. 2024 - 2025 It is very important that the fatty acids broken down, thus they undergo beta oxidation accumulate in the body fluids There are other functions of acetyl CoA in the and the very particular ones cytosol for example not just for functioning in are the myelin sheaths that the Krebs cycle will be destroyed in the Where does Beta oxidation of fatty acids nervous system occur? ○ If we are talking about plants and yeasts all the beta oxidation takes place in the peroxisome ○ But if we are talking about animal cells, a small part of fatty acid about 25-50% undergo beta oxidation These are long chains of fatty acids that undergo oxidation When we say metabolism, we talk about in peroxisomes in animal cells breakdown or synthesis The other ones are converted The substrate is urate (ionized uric acid) to acetyl CoA in the It is oxidase, we need the participation of mitochondria oxygen Urate is oxidized to allantoin ○ After this Allantoin may be converted to allantoic acid or urea and excreted from the body Urate is toxic ○ It is one of the things being tested for during blood chem ○ If you have a high uric acid then it means it is not being eliminated in your kidneys or it failed to be converted to its non-toxic form Shown above are the 3 groups of fatty acid H2O2 is also produced that undergo beta oxidation in peroxisome of ○ But as previously discussed it can be the animal cell, the rest are in mitochondria rendered non-toxic in the Genetic disease : X-linked peroxisomes adrenoleukodystrophy ○ Can be involved in other chemical ○ What is the problem here? reactions in the peroxisomes if you The long chain fatty acid can’t have your preoxidase enzyme enter the peroxisome. Why? In order for it to go in a transport protein is needed however because of the defect in the integral membrane protein it cannot ○ What happens? Since it can’t enter peroxisomes, it can’t be PAGE 4 Cellular Molecular Biology Lecture CELLULAR MOLECULAR BIOLOGY (LEC) CELLMOL Term 1 | A.Y. 2024 - 2025 What will be the sources of the different components of the molecules inside the peroxisome? ○ How can peroxisomes have additional catalase? They don’t have ribosomes or peptides Where did the polypeptides components inside the peroxide come from? ○ Since they don’t have ribosomes, the proteins should be exported from the outside. ○ Where will the proteins in the peroxisome come from? Our example is the enzyme catalase, this has a polypeptide subunit. We see that the Catalase polypeptides are synthesized in the Cytosolic ribosomes NOT in the ribosomes in the RER The heme portion of the imported from the outside and This image is from Becker thus we now have the Note that a portion of this image was covered assembly of the different by doc to avoid confusion components In this image we have the biogenesis of Once sufficient in size, it will peroxisomes undergo division How are peroxisomes formed? Peroxisome is NOT a component of the ○ New peroxisomes can be formed by Endomembrane System the division of existing peroxisomes or Number 5 (refer to the image) by fusion of vesicles from the ER ○ We know that Peroxisome is NOT a This is similar to the binary component of the Endomembrane fission of the prokaryotic cell System. However, there are new One peroxisome (the one in findings that show exception to the the middle) after it has grown rule in size, it can divide into two They found that some How will they grow in size? proteins in the peroxisomes ○ They have to grow in size first before that are coming from vesicles anything else derived from the RER ○ We have mentioned that the ER is the ○ BUT up until now, the peroxisomes is main source of membrane lipids, and still not considered part of the so we have the phospholipids and endomembrane system membrane proteins that are incorporated into an existing membrane to increase the size PAGE 5 Cellular Molecular Biology Lecture CELLULAR MOLECULAR BIOLOGY (LEC) CELLMOL Term 1 | A.Y. 2024 - 2025 This is because there is only few proteins that are from the RER PAGE 6 Cellular Molecular Biology Lecture