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Flashcards in MCP 1-11 Deck (18)
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0
Q

Transport via Protein Translocators

A

Directly transport proteins from cytosol –> organelle

  • Co-translationally for cytosol –> ER
  • Post-translationally for cytosol –> mitochondria and peroxisomes
1
Q

Mechanisms of protein import into organelles

A
  1. through nuclear pores
  2. across membranes
  3. by vesicles (i.e. ER -> Golgi, Golgi -> PM, Golgi -> lysosomes
2
Q

Signal Sequences

A

Purpose: direct proteins to the correct organelles

  • stretch of 3-60 aa within protein
  • may be removed by signal peptidase after transport (N terminus)
  • different types specify different locations
  • functionally interchangeable
  • recognized by specific receptors
3
Q

Function of Mitochondria

A
  • provide ~90% of cell’s energy (ATP)
  • near sites of high ATP use
  • more in cells with higher energy demands
4
Q

Structure of Mitochondria (4 compartments)

A

Two compartments separated by two membranes:

  1. Matrix space: enzymes for beta-oxidation (break down FA) and TCA cycle; location of mito. DNA genome and transcription/translation machinery for mito.genes
  2. Inner membrane: cristae increases surface area, ETC, ATP synthase, transport proteins, H+ electrochemical gradient (drives ATP synthesis)
  3. Outer membrane: porin forms channels
  4. Intermembrane space: between inner/out membranes, cytochrome c
5
Q

Role of Mitochondria in Apoptosis

A

Cytochrome c released from intermembrane space into cytosol –> caspase cascade (proteolytic cascade, cleaves key cellular proteins)

6
Q

Transport of proteins into mitochondria

A

Protein binds TOM (outer membrane) -> moves laterally until it hits TIM complex (inner membrane) -> protein moves across, into matrix -> signal sequence cleaved by mitochondrial signal peptidase -> chaperone proteins fold into final conformation
* Energy requirements: ATP gradient, electrochemical gradient across inner membrane

7
Q

Features of mitochondrial genome

A
  • very small circular dsDNA
  • encodes 2 rRNAs, 22 tRNAs, 13 mRNAs
  • little regulatory sequence
  • no introns
  • genetic code is slightly different (4 codons have different “meanings” from codons in nuclear genome)
  • ~10-20 copies of genome/mitochondrion
8
Q

Replication of Mitochondrial DNA

A
  • occurs throughout cell cycle, not limited to S-phase like nuclear DNA
  • mtDNAs chosen at random to replicate
  • total # mtDNA doubles in every cell cycle
  • Origin of replication on each strand
9
Q

Transcription of Mitochondrial DNA

A
  • both strands of DNA transcribed from single promoter region on each (HSP = heavy strand promoter, LSP = light strand promoter)
  • produces 2 giant RNAs, each a transcript of one full-length DNA strand
  • Each RNA cleaved into 2 rRNAs, 22 tRNAs, and 13 mRNAs
10
Q

Translation of Mitochondrial mRNAs

A
  • occurs in matrix
  • uses tRNAs and rRNA encoded in mtDNA (mt genome)
  • only produces 13 polypeptides (encoded in the 13 mRNAs), all are subunits involved in ET and ox. phosphorylation
11
Q

Functions of Peroxisomes

A
  • Oxidative degradation (use oxygen to oxidize –> hydrogen peroxide), catalse driven reactions (convert left over hydrogen peroxide to water and oxygen)
  • Beta oxidation (very long chain fatty acids that can’t be broken down by mito. –> acetyl CoA)
  • Synthesis of cholesterol, bile acids, and some lipids (ex. plasmalogen synthesis for myelin sheaths)
12
Q

Disorders of peroxisome biogenesis

A
  • defects in proteins required for biogenesis

- lack many peroxisomal enzymes or peroxisomes are absent from cells

13
Q

Zellweger Syndrome

A
  • disorder of peroxisome biogenesis
  • peroxisomal enzymes synthesized normally but not imported
  • empty peroxisomes
  • lethal in early infancy
14
Q

Deficiency of single peroxisomal enzyme

A
  • defect in synthesis, import or function of one peroxisomal protein
  • less severe phenotype
  • partially functional peroxisomes
15
Q

X-linked Adrenoleukodystrophy (ALD)

A
  • deficiency of a single peroxisomal protein
  • peroxisomes lack membrane protein involved in degradation of very long chain FA -> build up -> leads to demyelination of neurons, dysfunction of nervous system, and adrenal insufficiency
  • lethal in mid childhood
16
Q

Treatment of ALD

A
  1. Allogeneic stem cell transplant: high morbidity, compatible donor cells not always available, must be performed at early stage of brain lesions
  2. Gene therapy: recent success in two patients
17
Q

Gene therapy for ALD

A
  • Hematopoietic stem cells (HSCs) collected from two 7 y.o ALD pt
  • HSCs corrected ex-vivo using HIV-derived lentiviral vector expressing wt ALD protein
  • Chemotherapy used to eradicate bone marrow, pt own corrected HSCs were infused
  • Progeny of HSCs distribute throughout body, including brain microglia responsible for maintaining myelin
  • 4 year follow up: 10-11% of hemo. cell lines stably expressed wt protein, results similar to allogeneic stem cell transplant
  • First successful clinical test of HIV-derived vector in HSC-based gene therapy*