Readings and Objectives Reading – Russell : Chapter 6 – Cooper: Chapter 3, 11 Objectives Sugar metabolism Mitochondrion structure Mitochondrial genome Proteins Mitochondrial function Krebs cycle Oxidative Phosphorylation 2

Mitochondria: structure Generate energy from lipids & carbohydrates Surrounded by a double-membrane system The inner membrane has numerous folds (cristae), which extend into the interior (matrix) 3

Outer membrane: permeable to small molecules Porins, form channels (in o.m.), allows free diffusion of small molecules Intermembrane space: composition similar to the cytosol Inner membrane: impermeable to most ions and small molecules Helps maintain the proton gradient Mitochondria positioned near locations of high-energy use, ie. synapses in nerve cells, muscle cells Continually fusing and dividing, remodels the network of mitochondria in the cell, and affects function and morphology Mitochondria: dynamic organelles 4

Endosymbiotic origin Mitochondria are thought to have evolved from bacteria that began living inside larger cells (endosymbiosis) Living organisms that have genomes most similar to the mitochondrial genome are free-living α-proteobacteria See a review paper for endosymbiosis under literature section 5

Circular DNA 16 kbp, multiple copies Maternal inheritance Map: Origin of replication: D-loop Code for rRNAs, tRNAs, own ribosomes Encode 13 proteins essential for oxidative phosphorylation – Electron transfer chain complexes, including I, III, IV and V Mitochondrial genome Human Mitochondrial genome map (16 kbp) 6

Mitochondrial: Genetics mitochondrial genetic code is different from the universal code U in the tRNA anticodon can pair with any of the four bases in the third codon position of mRNA; thus four codons are recognized by a single tRNA Some codons specify different amino acids in mitochondria than in the universal code 7

Mitochondrial Proteins contain 1000 to 1500 different proteins, but nearly half of them remain unidentified mitochondria from different tissues contain different proteins Genes for many mitochondrial proteins are in the nucleus (95% of mtProteins) Some of these genes were transferred to the nucleus from the original prokaryotic ancestor of mitochondria Cytosolic protein synthesis  mit. Transport All Krebs enzymes, rep/transcrip/translation Complex because of mito. double membrane 8

Presequences, N-terminal positively charged a.a. targets proteins to matrix Partially unfolded by Hsp70 chaperone – Prevent aggregation as emerge from free ribosomes bind to receptors on Tom protein complex (translocase of outer membrane) – First to Tom20 then Tom5 – To import pore  Tom40 – Passage, bind intermembrane tail of Tom22 Bind Tim complex ( translocase of inner membrane) – Bind Tim21/Tim50 of Tim23 complex  matrix Transport and Assembly of Matrix Proteins 9

In the Matrix Presequence/Hsp70/Tom44 works as a ratchet Reversible binding with short hydrophobic amino acids Sequential ATP hydrolysis Powers the binding & dissociation of Hsp70 Might integrate to membrane, or Protein is pulled into matrix Matrix processing peptidase (MPP) cleaves presequence Hsp70 binding assists proper folding Transport and Assembly of Matrix Proteins 10

Mitochondrial Function Oxidative catabolism of glucose and fatty acids The matrix contains the genetic system and enzymes for oxidative metabolism Pyruvate (from glycolysis) is transported to mitochondria, where its complete oxidation to CO 2 yields the bulk of usable energy (ATP) obtained from glucose metabolism 11

Glycolysis Universal pathway Glucose starting substrate sequentially broken down to pyruvate 10 steps (all enzymes are cytosolic) – Early preparatory steps uses ATP – Later steps produces chemical energy Net yeild: 2 ATP (4ATP-2ATP) 2 NADH 2 Pyruvate 12

Glycolysis Glycolysis provides substrates for mitochondrial Krebs cycle 13

Krebs Cycle In eukaryotic cells, glycolysis takes place in the cytosol Pyruvate is then transported into mitochondria, where it is completely oxidized Pyruvate undergoes oxidative decarboxylation in the presence of coenzyme A (CoA-SH), forming acetyl CoAcoenzyme A 14

Krebs Cycle Acetyl CoA enters the citric acid cycle or Krebs cycle The 2-carbon acetyl group combines with oxaloacetate (4C) to yield citrate (6 C) In the remaining reactions, 2 carbons of citrate are completely oxidized to CO 2 and oxaloacetate is regenerated All enzymes are in matrix 15

Krebs Cycle The citric acid cycle completes the oxidation of glucose to six molecules of CO 2 yields one GTP, three NADH, and one reduced flavin adenine dinucleotide (FADH 2 ), which is another electron carrier All enzymes are in matrix 16

Krebs Cycle 17

Electron Transport Chain High-energy electrons from NADH and FADH 2 are transferred through a series of carriers in the membrane e - carriers organized in ET complexes I, II, III, IV Low energy electrons from IV carried on O2 +2H + to form H2O energy from ETC is used to pump protons to intermembrane space Coenzyme Q cyt b 18

Electron Transport Chain Electrons from FADH 2 are transferred through complex II Then carried by Coenzyme Q to complex III and IV 19

Proton gradient and Chemiosmotic coupling proton gradient established across the inner membrane Chemiosmotic coupling: Energy stored in H+ gradient is coupled to ATP synthesis (Peter Mitchel 1961) 20

Oxidative phosphorylation protons can cross the membrane only through a protein channel (complex V) complex V (ATP synthase), has two units, F 0 and F 1, linked by a slender stalk. F 0 spans the inner membrane and forms a channel through which the protons move F1 catalyzes the synthesis of ATP 21

Oxidative phosphorylation flow of protons through F 0 drives the rotation of part of F 1, which acts as a rotary motor to drive ATP synthesis Four protons are required to synthesize one ATP Oxidation of one NADH yields 3 ATP; oxidation of FADH 2 yields 2 ATP Krebs and glycolysis: total 38 ATP per molecule of glucose (ie. 2 pyruvate) 22

23 Coenzyme A An Acyl carrier composed of adenosine 3',5'-diphosphate linked to 4-phosphopantethenic acid (vitamin B 5 ) and thence to β- mercaptoethylamine