CHAP 12 Chloroplasts, Mitochondria, and Peroxisomes

cell and molecular in class notes

Chloroplasts are similar to mitochondria

Both generate metabolic energy, evolved from endosymbiosis

Chloroplasts are…. (what in size)

larger and more complex

Function: What do Chloroplasts convert CO2 to?

Carbohydrates

• also synthesize amino acids, fatty acids, and lipids

Function: What do Chloroplasts change Nitrite to (reduction)?

Reduce them to ammonia NH3

• useful for incorporation of N into organic compounds

Structure of Chloroplast: What are the membranes that are stacked?

Thylakoid 

• singular: granum (stacked thylakoid membranes) 

• Plural: grana (single thylakoid membranes) 

Structure of Chloroplast: What is the stroma equivalent to in mitochondria?

Matrix 

Structure of Chloroplast: Where does photosynthesis happen?

In the Thylakoid membrane (light reactions)

• taking energy from sun —> chemical energy

Structure of Chloroplast: Where are dark reactions formed?

In the stroma these reactions are formed

• calvin cycle

Structure of Chloroplast: Similarity of mitochondrions with the inner membrane

Both don’t have porous in the inner membranes

Structure of Chloroplast: H+ gradient leads to…

ATP formation

Structure of Chloroplast: Why is it green?

Chlorophylls in the thylakoid membrane

Chloroplasts: Where is the DNA present?

circular DNA is present in the STROMA 

What do Chloroplast genomes code for?

encode both RNAs + proteins for gene expression

• tRNAs are sufficient to translate all mRNA codons with universal code 

What does Chloroplast make? (specific protein)

Rubisco

• catalyzes CO2 to ribulose - 1,5-bisphosphate in the CALVIN CYCLE

What does RUBISCO need to function?

needs high concentration of CO2, for photosynthesis

How many units does RUBISCO have?

8 large and 8 small subunits

• multisubunit protein

• large ones are synthesized nuclear genome

• small units chloroplast genome

What are the 2 transports main ones in the chloroplast?

Co-translational and post-translational transport in the chloroplast

Chloroplasts: All plastids develop from what?

Proplastids

Development of Chloroplasts

Proplastid —>etioplast (dark), intermediate stage—→ chloroplast (move back to light)

What are PEROXISOMES:

single membrane organelles contianing enzymes

Structure of PEROXISOMES:

500 peroxisomes in a cell,

no genome, single membrane

Origin of PEROXISOMES

From the ER and fission from existing peroxisomes

What are the proteins from PEROXISOMES

Peroxins

What are the functions of PEROXISOMES

1. Oxidation of fatty acids - transports chains into liver

2. Biosynthesis of lipids and amino acids

3. Synthesis of bile acids - without it we cannot digest fats & bile salts pro— without it emulsification of fats;; liver 

4. Cholesterol (synthesized in liver + peroxisomes)

5. Synthesis of plasmalogen - synthesized in peroxisomes - found in cardiac cells

6. Catalase (peroxins), changes hydrogen peroxide to water;; oxidizing another organic compound

7. Plants — glyoxylate cycle

What is Glyoxylate cycle?

in seeds…. Peroxisomes convert fatty acids (lipids) to carbohydrates (sugar)

• provides energy and raw materials for the germinating plant 

• PEROXISOMES ARE SOMETIMES CALLED GLYOXYSOMES—- provide raw materials for plant germination

Peroxisomes are also involved in…

photorespiration

Photorespiration equation

C6H12O6 and O2 —→ ATP CO2 + H2O

• break down of sugar in presence of light

• production of glycolate

• happens through the stoma

• wasteful process

• Peroxisomes ALLOW CARBON IN GLYCOLATE TO BE RECOVERED AND UTILIZED

How are peroxisomes created?

Proteins get made in the ER, then turned into vesicles, then bind, then become peroxisomes with imporomer that import matrix proteins

How are matrix proteins from peroxisomes look like in the peroxisomes?

They are folded

Where are membrane proteins of the peroxisome made?

They are synthesized in ER and Free ribosomes on cytosol

Which is the most rapid peroxisome formation?

Growth and division from existing peroxisomes fusion

What are the recessive genetic disorders from peroxisomes?

1. RCDP 1 & Zellweger syndrome

What is the Zellweger syndrome?

CNS, LIVER, KIDNEYS 

• Myelination is low in the neurons

• Liver and kidneys there are cysts 

• If there is a mutation, it nhibits transport proteins

Where does Glycolysis occur in the Mitochondria?

Cytosol

• pyruvate is transported into mitochondria

• complete oxidation to CO2

• Yields bulk of usable energy ATP (from glucose metabolism)

How is energy produced in Mitochondria?

through oxidative phosphorylation

• takes place in inner mitochondrial membrane

• electrons for NADH and FADh2 transferred through series of carriers in the membrane to molecular oxygen

• energy is converted to PE, stored in proton gradient, drives ATP synthesis

What part of the mitochondria generates ATP

inner membrane 

How is the surface area increased in Mitochondria?

Folding into cristae

What is Cristae in Mitochondria impermeable to?

Most ions, small molecules (critical to maintaing proton gradient)

The outer mitochondrial membrane is highly permeable to what types of molecules?

Small

Why is fission important in mitochondria?

For distribution of mitochondria between daughter cells at cell division and facilitating transport of mitochondria to areas of high energy

What subunits are important for translation of proteins in Mitochondria?

16s, 12srRNA & tRNAs

What is a disease caused by mutations in mitochondria genes?

Leber’s hereditary optic neuropathy

• blindness & caused by mutations that encode components of ETC

How many proteins do mammalian mitochondria genomes encode?

13 proteins

Where are the proteins synthesized for mitochondria?

On free ribosomes & imported to mitochondria as complete polypeptides 

• import of proteins is complex due to double membrane structure

How are proteins inserted into the Mitochondria?

1. targeted to the matrix by amino terminal sequences —presequences that are removed by proteolytic cleavage after import

2. Presequences bind to receptors on the mitochondria that are part of a complex—- tom complex—- translocase of the outer membrane

3. Proteins are transferred to another complex into the inner membrane —tim complex—- translocase of the inner membrane **some proteins cross matrix through Tim23 & exit laterally

What does protein translocation into the MItochondria require?

Electrochemical potential across the inner membrane during electron transport

***requires Hsp70 chaperones unfolded proteins*

What are presequences cleaved by when going into the mitochondria?

Matrix processing peptidase

What happens if there are proteins with multiple transmembrane domains, when wanting to come into the mitochondria?

internal import signals instead of presequences

After translocation of proteins into the mitochondria what are they bound to?

Tim9-Tim10 chaperones, bringing them to Tim22 

• protein is transferred laterally into the inner membrane 

How are proteins sorted to the outer membrane and intermembrane space in Mitochondria?

1. Inner membrane proteins encoded by mitochondria genome: synthesized on ribosomes in the mitochondrial matrix

2. targeted to the Oxa1 translocase in the inner membrane

3. Exit Oxa1 laterally to insert into the inner membrane

4. Proteins destined for the outer membrane/intermembrane space pass through the Tom complex

5. Proteins with singler transmembrane domains are inserted through outer membrane protein Mim1

6. Beta barrel proteins pass through TOM and are bound by… Tim9-Tim10

7. Carried to translocon (SAM), sorting and assembly machinery

8. SAM mediates insertion into the outer membrane 

How are lipids imported from the cytosol in Mitochondria?

1. mitochondria catalyzes synthesis of phospholipid cardiolipin

2. improves efficiency of oxidative phosphorylation by restricting proton flow across the membrane 

What is lipid transfer between the ER and mitochondria mediated by?

Phospholipid transfer proteins

• lipids transported through cytosol and released at a new membrane (mitochodnria)

What is transport of ATP and ADP mediated by in the Mitochondria?

Integral membrane protein (adenine nucleotide translocator)

• 1 molecule of ADP moves into mitochondrion in exchange for 1 molecule of ATP transferred to cytosol

• ATP carries a negative charge, exchange is driven by voltage component

Mechanism of Oxidative Phosphorylation in Mitochondria

Pi is brough in as phosphate in exchange for hydroxyl ions (oh)

• exchange is electrically neutral, driven by proton concentration gradient

Higher pH within mitochondria corresponds to a higher concentration of hydroxyl ions (Oh), favoring translocation to the outside 

In Chloroplasts: what is the outer membrane like?

Contains Porins & permeable to small molecules

In Chloroplasts: What is the inner membrane like?

Impermeable to ions and metabolites, required movement through transporters

In Chloroplasts: Where does electron transport and ATP generation take place?

In the Thylakoid membrane

In Chloroplasts: movement of protons?

Stroma —→ thylakoid lumen

What is the thylakoid membrane equivalent to?

To…Inner membrane of the mitochondria

Import of proteins to the Chloroplast stroma?

1. Proteins synthesized on free ribosomes & imported into chloroplasts as completed polypeptides

2. N-Terminal sequences (transit peptides) direct translocation across 2 membranes of the envelope

3. Removed by proteolytic cleavage

4. Transit peptides direct proteins to the translocase of the chloroplast OUTER membrane (TOC COMPLEX)

5. Hsp70 molecules keep polypeptide unfolded

6. Hydrolyze GTP giving energy for translocation

Import of Proteins into the Thylakoid Lumen? … What are the 2 pathways?

1. Sec

2. Tat

Import of Proteins into the Thylakoid Lumen? : What is Sec Pathway

1. ATP dependent ; signal sequence is recognized by SecA protein 

2. Protein translocated as unfolded protein through SEC translocon

Import of Proteins into the Thylakoid Lumen? : What is Tat Pathway?

1. Proteins have a twin-arginine signal sequence —- translocated into fully-folded state

2. Energy comes from proton gradient

3. Signal sequences are cleaved by thylakoid processing protease (TPP)

How are Proteins targeted to the thylakoid membrane : What are the 3 pathways?

1. Sec Pathway

2. SRP Pathway

3. Spontaneous

How are Proteins targeted to the thylakoid membrane : Sec Pathway

Proteins with transmembrane sequence exist Sec Translocon Laterally

How are Proteins targeted to the thylakoid membrane : SRP

Proteins are recognized by chloroplast signal recognition particl (cpSRP) and inserted into the thylakoid membrane by Alb3 translocase

Chloroplasts are a type…

Plastid

Difference between chloroplasts and other plastids …?

Chloroplasts specialized for photosynthesis

Plastids involved other aspects of plant metabolism

• Double membrane evelope but lack thylakoid membranes + other components of photosynthesis apparatus

What are Chromoplasts?

Carotenoids

• yellow , organge, red

What are Leucoplasts?

Nonpigmented, sotrage of energy sources in nonphotosynthetic tissues 

• amyloplasts: starch storage 

• elaioplasts: lipid storage 

Development of plastids is controlled by what?

Environmental signals + intrinsic developmental signals 

In Photosynthetic cells of leaves what do proplastids develop into?

Chloroplasts (only in presence of light)

If proplastids are kept in the dark, what are the proplastids kept at?

Etioplasts (intermediate stage)

• array of tubular internal membranes has been formed, chloroplyll not synthesized

  • when moved to light = chloroplasts

Do peroxisomes have their own genomes?

No, they do not

• most peroxisomal proteins are synthesized on free ribosomes and imported as completed polypeptides

What are plasmalogens in peroxisomes?

Phospholipids w 1 h-c chain joined to glycerol by ether bond than ester bond

What do peroxisomes convert fatty acids to through the glycoxylate cycle?

to carbohydrates

• glyoxysomes

• energy + raw materia = germination

Peroxisomes: Photorespiration in leaves, what is the process?

1. Co2 is added to the sugar ribulose by RUBISCO 

But sometimes O2 is added instead of CO2 = glycolate 

2. Glycolate is transferred to peroxisomes and oxidized to glycine 

3. Glycine goes to mitochondria: 2 glycine converted to serine with CO2 released

4. Serine comes back to peroxisomes—> converted to glycerate 

5. Glycerate goes back to chloroplasts and reenters calvin cycle in the stroma

What is involved in peroxisome assembly?

Peroxins/ pex proteins

• peroxisome transmembrane proteins are transported from the ER

What happens when peroxins are mutated?

Association with peroxisome biogenesis disorders

= defective peroxisome assembly

• Recessive RCDP type 1 & Zellweger disorders

What do the mutations in the peroxins (peroxisomes) create?

Destruction of Pex Protein function

How are peroxisomes assembled?

1. Peroxisome transmembrane proteins are translocated into the ER and inserted into the ER membrane 

2. Proteins are budded in vesicles 

3. 2 types of vesicles contain different peroxins FUSE and form peroxisome

Where are peroxisome matrix proteins synthesized at?

Theese are synthesized on free ribosomes & imported as folded polypeptides

What signal is used for importing peroxisomal matrix proteins?

after er PTS1 and is recognized by Pex5 receptor

• Pex5/cargo complex binds to docking complex on the peroxisome

How are Peroxisomal matrix proteins imported from the cytosol?

1. Peroxisome membrane proteins are synthesized on cytosolic ribosomes 

2. targeting signal mPTS which is recognized by Pex19

3. Pex19/cargo is recognized by Pex3 and Pex16 in the peroxisome membrane 

How are New Peroxisomes formed?

1. Vesicle budding from the ER (new content)

2. Growth and division of existing peroxisomes (more rapid)