Transcript ppt

11 Bioenergetics and Metabolism
Chapter Outline:
• Mitochondria
• Oxidative Phosphorylation
• Chloroplasts and Other Plastids
• Photosynthesis
• Peroxisomes
amyloplast
Mitochondria, chloroplasts, peroxisomes
Student learning outcomes:
Explain similarities, differences structure and function of
mitochondria, chloroplast, peroxisome
Explain process of transport of proteins to organelles:
signals on proteins, complexes that assist
Explain metabolic functions of mitochondria, chloroplast:
membrane compartments, proton gradient and ATP
Mitochondria and chloroplasts have genomes
Figure 10.3** Overview of protein sorting
**
Fig. 10.3
Introduction
Generation of metabolic energy- major cell activity
Mitochondria generate energy from breakdown of
lipids and carbohydrates.
Chloroplasts use sunlight energy to generate ATP
and the reducing power needed to synthesize
carbohydrates from CO2 and H2O.
Peroxisomes contain metabolic enzymes:
fatty acid oxidation, generate peroxides, have catalase
Mitochondria
Mitochondria are surrounded by double membrane:
• Outer membrane permeable to small molecules
• Inner membrane has numerous folds (cristae);
•
extend into interior (matrix).
Fig. 11.1
Fig 11.2 Metabolism in the matrix of mitochondria
Matrix contains small genome (human 17 kb; yeast 80 kb)
Enzymes for oxidative metabolism:
• Pyruvate (from glycolysis) into mitochondria; complete
oxidation to CO2 yields most of energy (ATP) from glucose
• Enzymes of citric acid (Krebs) cycle - in mitochondrial matrix.
• Most of energy produced by oxidative phosphorylation,
occurs on inner mitochondrial membrane
(electron transport chain)
Fig. 11.2
Mitochondria
• High-energy electrons from NADH and FADH2 transferred
through a membrane carriers membrane to molecular oxygen
• Energy of electrons converted to potential energy stored in a
proton gradient, which drives ATP synthesis.
• Inner membrane has many proteins involved in
oxidative metabolism and transport
• Inner membrane impermeable to most ions, small molecules
Mitochondria
Outer mitochondrial membrane highly permeable to
small molecules:
• Porins form channels for free diffusion of small molecules.
• Composition of intermembrane space similar to cytosol
(with pH ~7; matrix pH ~8)
Mitochondria can fuse,
also can divide
Mitochondria have DNA
Genomes reflect endosymbiotic origin:
• usually circular DNA molecules, multiple copies.
• encode only a few proteins (some oxidative phosphorylation).
• encode rRNAs and most tRNAs needed
for translating protein-coding sequences
• Ribosomes are in matrix
• Some different codon usage
Table11.1
Human mtDNA
16-kb
Fig. 11.3
Molecular Medicine 11.1 Diseases of Mitochondria: Leber’s Hereditary Optic
Neuropathy: LHON mutations in mitochondrial DNA
Mutations in mitochondrial genes cause disease
Leber’s hereditary optic neuropathy, blindness;
mutations in mitochondrial genes:
components of
electron transport chain
Mitochondria
• Genes for many mitochondrial proteins in nucleus.
• Some genes transferred from prokaryotic ancestor
• Most proteins are synthesized on free cytosolic
ribosomes, imported to mitochondria as complete
polypeptides.
• Because of double-membrane structure of
mitochondria, import of proteins is complex
• Matrix proteins are targeted by NH2-terminal
sequences (presequences); removed after import
Figure 11.4 Import of mitochondrial matrix proteins
Matrix proteins:
Membrane or free proteins
• Presequences target
• Tom receptors/ channels on
outer membrane (translocase)
• Tim receptors on inner
membrane
• Electrochemical gradient
• Hsp70 Chaperones
• MPP cleavage
• ATP hydrolysis
Fig. 11.4
Compare ER/Golgi
Figure 11.5 Binding cycle of an Hsp70 chaperone
• Presequence cleaved by matrix processing
peptidase (MPP)
• Hsp70 chaperones
facilitate folding.
• Similarity to signal
peptidase for ER
Fig. 11.5
Figure 11.6 Import of small molecule transport proteins into the
mitochondrial inner membrane
Inner membrane proteins are
small molecule transporters.
• multiple internal import signals,
• Hsp90 chaperone , plusTom70,
translocates across channel.
• Intermembrane: proteins escorted
by mobile Tim22, “Tiny Tims”.
• Translocated through Tim22;
internal stop-transfer signals
causes exit insert inner membrane.
Fig. 11.6
Figure 11.7 Sorting of proteins containing presequences to
different mitochondrial compartments
Both presequences, internal signal sequences.
• Translocated in Tom40.
• Some exit channel laterally,
• Some remain in
intermembrane space
• Others transported back to
intermembrane space
• Or inserted into inner
membrane
Fig. 11.7
Figure 11.8 Insertion of β-barrel proteins into the mitochondrial
outer membrane
Outer membrane proteins:
including Tom40 and β-barrel proteins (e.g., porins),
• Pass through Tom
complex into
intermembrane space.
• Carried by Tiny Tims
to a SAM (sorting and
assembly machinery)
complex
• Inserted into outer
membrane
Fig. 11.8
Mitochondria
Phospholipids are imported from cytosol.
Phospholipid transfer proteins:
• take phospholipids from ER membrane,
• transport them through cytosol,
• released at new membrane (e.g. mitochondria)
Mitochondria catalyze
synthesis of cardiolipin
• Phospholipid with
four fatty acid chains..
Figure 10.3** Overview of protein sorting
**
The Mechanism of Oxidative Phosphorylation
2. Mechanism of Oxidative phosphorylation:
• Electrons from NADH and FADH2 combine with O2:
• Energy released from oxidation/reduction reactions
drives ATP synthesis
• Electrons travel through electron transport chain
• Proteins on inner mitochondrial membrane
• Sets up proton gradient across membrane
• Intermembrane space has lower pH (more H+)
• Chemiosmotic mechanism for synthesis of ATP:
• Protons returning to matrix power ATP synthase.
Fig 11.10 Transport of electrons from NADH
Transfer of electrons from NADH:
•
•
•
•
•
Complex I,
Coenzyme Q (ubiquinone)
Complex III
Cytochrome c
Complex IV
(cytochrome oxidase)
• to O2
• 3 H+ transported
across membrane
• V is ATP synthase:
H+ reentry gives ATP
Fig. 11.10
Fig 11.11 Transport of electrons from FADH2
Transfer of electrons from FADH2:
• Complex II (less energy)
• Coenzyme Q (ubiquinone)
• Complex III
• Cytochrome c
• Complex IV
(cytochrome oxidase)
• to O2
• 3 H+ transported
across membrane
• V is ATP synthase:
H+ reentry gives ATP
Fig. 11.11
The Mechanism of Oxidative Phosphorylation
Chemiosmotic coupling mechanism:
• Couples electron transport to ATP generation.
• Electron transport coupled to transport of protons to
intermembrane space
• Proton gradient
across inner membrane
• Also electric potential
• Electrochemical
gradient exists
Fig. 11.12
Fig 11.13 Structure of ATP synthase
ATP synthase:
• Phospholipid bilayer impermeable to ions
• Protons cross through protein channel.
• Energy converted to ATP
in complex V (ATP synthase):
F0 is channel
F1 rotates, makes ATP
• 4 protons to synthesize 1 ATP:
• 1 NADH yields 3 ATP;
Fig. 11.13
• 1 FADH2 yields 2 ATP
Fig 11.14 Transport of metabolites across the mitochondrial inner membrane
Electrochemical gradient drives transport of small
molecules into and out of mitochondria.
• ATP exported; ADP and Pi brought in.
• Integral membrane protein transports 1 ADP in, 1 ATP out
• Pyruvate exchanged for OH-
Fig. 11.14
Chloroplasts and Other Plastids
3. Chloroplasts: organelles for photosynthesis:
•
•
Convert CO2 plus H2O to carbohydrates
Synthesize amino acids, fatty acids, and lipids of
their membranes.
Similar to mitochondria:
• generate metabolic energy,
• evolved by endosymbiosis,
• contain own genome
• replicate by division.
Figure 11.15 Structure of a chloroplast
Chloroplasts are larger and more complex:
• double membrane — chloroplast envelope.
• internal membrane system, thylakoid membrane,
network of flattened discs (thylakoids),
arranged in stacks (grana)
3 internal compartments:
• intermembrane space
• stroma, ~ mitochondrial matrix
• thylakoid lumen
• Electron transport, chemiosmotic
generation of ATP in thylakoid membrane,
not in intermembrane space
Fig. 11.15
Fig 11.16 Chemiosmotic generation of ATP in chloroplasts and mitochondria
**Comparison chemiosmotic mechanism locations
Fig. 11.16
Chloroplasts and Other Plastids
Chloroplast genome reflects evolutionary origins from
photosynthetic bacteria.
• Circular DNA molecules, multiple copies,
• Encode RNAs, proteins for gene expression, photosynthesis
Rubisco catalyzes addition of CO2 to ribulose-1,5-bisphosphate during the
Calvin cycle. Rubisco is critical enzyme for photosynthesis,
Chloroplasts and Other Plastids
Proteins from cytosolic ribosomes imported after
completion
• N-terminal transit peptide
• Guidance complex
• Proteolytic cleavage
• Toc complex
• Hsp70 chaperones
• Tic complex
Fig. 11.17
• SPP stromal processing peptidase
Fig 11.18 Import of proteins into the thylakoid lumen or membrane
Thylakoid proteins have second signal sequence,
(exposed after cleavage of transit peptide).
3 paths:
• Chaperones
• + charge
• SRP (signal
recognition particle)
Fig. 11.18
Chloroplasts and Other Plastids of Plants
Plastids:
• Double-membrane organelles including chloroplasts
• Plastids contain same genome, differ in structure
and function.
• Chloroplasts unique: internal thylakoid membrane
and photosynthesis
• Classified by pigments
Fig 11.19 Electron micrographs of chromoplasts and amyloplasts
• Chloroplasts contain chlorophyll.
• Chromoplasts contain carotenoids: result in yellow,
orange, red colors of flowers and fruits
• Leucoplasts are nonpigmented - store energy
sources in nonphotosynthetic tissues.
– Amyloplasts store starch
– Elaioplasts store lipids
Chloroplasts and Other Plastids
Plastids develop from proplastids,
small undifferentiated organelles
• Mature plastids change.
• Chromoplasts from chloroplasts,
in ripening fruit.
• Proplastids arrested at
intermediate stage (etioplasts).
• In light, etioplasts develop
into chloroplasts.
Fig. 11.20
Photosynthesis
4. Photosynthesis:
• ultimate source of energy for biological systems:
Light reactions:
• energy from sunlight drives synthesis of ATP and
NADPH, coupled to formation of O2 from H2O.
Dark reactions:
• ATP and NADPH drive glucose synthesis
• CO2 plus H2O form sugars
Fig 11.22 Organization of a photocenter
Sunlight absorbed by photosynthetic pigments - chlorophylls.
Photocenters in thylakoid membrane have pigment molecules
Absorption of light excites electron, converts light energy to
potential chemical energy.
Electrons transferred through membrane carrier chain, results in
synthesis of ATP and NADPH
Fig. 11.22
Fig 11.25 Electron transport and ATP synthesis during photosynthesis
Photosynthesis: electron transport chain
•
•
•
•
•
•
4 complexes on thylakoid membrane.
2 photosystems (photosystems I and II); split H2O
Cytochrome bf complex
NADP reductase forms NADPH
H+ gradient in thylakoid lumen
ATP synthase
Fig 11.27 The pathway of cyclic electron flow
Cyclic electron flow uses electrons from
Photosystem I only,
• generates extra ATP but not NADPH
Fig. 11.27
Photosynthesis
Summary photosynthesis:
• Thylakoid membrane impermeable to protons, is
permeable to other ions, particularly Mg2+ and Cl–
• Difference more than 3 pH units between stroma and
thylakoid lumen → lot of energy across membrane.
• Each pair of electrons gives 2 protons at photosystem
II, 2–4 protons cytochrome bf complex.
• 4 protons for synthesis of 1 ATP: each pair electrons
yields 1 to 1.5 ATP.
•
Cyclic electron flow yields 0.5 to 1 ATP per pair electrons.
Peroxisomes
Peroxisomes:
• Single-membrane-enclosed organelles that contain
diverse metabolic enzymes (peroxins)
• no genome
Fig. 11.28
Peroxisomes
• Peroxisomes break down substrates by oxidative
reactions, produce hydrogen peroxide.
• Peroxisomes contain catalase: converts H2O2 to water or uses
it to oxidize other organic compound.
• Peroxisomes synthesize lipids, amino acid lysine.
• In animal cells, cholesterol and dolichol are synthesized in
peroxisomes and in ER.
• In liver, peroxisomes synthesize bile acids from cholesterol
Fig. 11.29
Peroxisomes
Peroxisome assembly
• Begins on rough ER: 2 peroxins localize.
• Pex3/Pex19-containing vesicles bud off ER
• PTS1,2 signals target proteins
from free ribosome to join peroxisome
• Signals recognized by
receptors and protein channels
• Protein import, addition of lipids
results in peroxisome growth, division.
• Enzyme content, metabolic activities
of peroxisomes can change
Fig. 11.33
Peroxisomes
Diseases from deficiencies in peroxisomal enzymes,
or failed import into peroxisome.
Zellweger syndrome,
lethal within first 10 years of life,
results from mutations in at least
10 different genes affecting
peroxisomal protein import.
Peroxisome biogenesis disorders (PBD)
– part of leukodystrophies.
Damage white matter of brain,
affect metabolism in blood and tissues.
Review Questions:
1. What 2 properties of mitochondrial inner membrane
give it unusually high metabolic activity?
4. What roles do molecular chaperones play in
mitochondrial protein import?
Compare/ contrast import of proteins into mitochondria
and into chloroplast – membrane vs. cytoplasm
11. How are proteins targeted to peroxisomes?