Mitochondria Guest lecturer: Chris Moyes, Dept of Biology Contact:

Endosymbiosis Mitochondria formed as a result of an endosymbiotic event around 2 billion years ago.

From: Gerhart and Kirschner: Cells, Embryos and Evolution

Mitochondrial compartments Inner membrane Respiratory chain and ATP synthase impermeable to most charged molecules highly folded into invaginations called cristae. Outer membrane Permeable to larger molecules Matrix Enzymes of the citric acid cycle, mtDNA Intermembrane space space between inner and outer membranes

Mitochondrial compartments

Mitochondrial morphology and movement Mitochondria are dynamic organelles they may exist as individual organelles may become elaborate network move throughout the cell on cytoskeleton Changes in the network are mediated by fission and fusion proteins Dynamin-Related Protein causes fission Fuzzy Onion Protein (FZO) causes fusion

Mitochondrial reticulum

Fusion and fission proteins regulate network

Mitochondrial energy production Three major steps in oxidative phosphorylation 1) Production of reducing equivalents (NADH, FADH 2 ) from glycolysis, fatty acid oxidation, and the citric acid cycle 2) Electron transport and generation of proton motive force 3) Phosphorylation - Synthesis of ATP, driven by the proton motive force

Mitochondria make other products Mitochondria produce biosynthetic precursors OXPHOS also leads to the production of: Superoxide: formed when O 2 steals electrons from the ETC complexes Heat: a by-product of the reactions of OXPHOS

Show 14-10, gen overview Overview of energy production by OXPHOS

Reducing equivalents are produced in the oxidation of carbohydrate and lipid

Oxidation and Electron Transport Electrons from NADH and FADH 2 are passed down respiratory chain to O 2 Electron transport expels protons, creating a proton gradient- the proton motive force (PMF)

Proton motive force (PMF) The PMF is an electrochemical gradient of membrane potential (ΔΨ) and pH (ΔpH)

The PMF supplies the energy for active transport into the mitochondria

Phosphorylation The F 1 F o ATPase (or ATP synthase) is a molecular motor -it uses the PMF to make ATP -it can also be reversed (using ATP hydrolysis to recharge the PMF)

Oxidation and phosphorylation are coupled by a shared dependence on the PMF

Because of this “coupling”, the two processes are interdependent If the PMF is large, what would you predict about oxygen consumption? If you took away oxygen, what would happen to the PMF? What would an increase in [ADP] do to the oxygen consumption? What would happen to ATP synthesis and oxygen consumption if the inner membrane became leaky?

Uncoupling proteins Many mammals warm vital tissues using brown fat Adipose tissue with abundant mitochondria that possess a the protein thermogenin (or uncoupling protein 1). UCP-1 short-circuits the proton gradient, increasing VO 2 and heat production. All eukaryotes have proteins related to UCPs, that are thought to prevent the PMF from “over- charging”, thereby reducing ROS production.

Mitochondrial biogenesis requires proteins encoded in 2 genomes (nucleus and mtDNA) Nucleus encode most proteins 2 copies of each gene per diploid cell genes regulated independently proteins imported by post-translational import from cytoplasm mtDNA encodes few proteins 1000’s of copies per cell genes transcribed as a polycistron transcribed and translated directly in mitochondria)

Peculiarities of mtDNA mtDNA is a very compact genome -genes attached end to end, with mRNA regions interspersed among rRNA and tRNA genes -tRNA excision liberates protein-coding genes -many genes lack a full termination codon (TAA) Diversity -maternal origin (most animals) -many cells have multiple genotypes within a single cell (heteroplasmy) -defects accumulate with age

Editing of mtDNA polycistron

Nuclear gene expression is coordinated by transcription factor networks

Mt enzyme synthesis requires coordinated gene expression and accessory factors