Anabolism I Carbohydrates

ANABOLISM I: Carbohydrates Photosynthesis and Carbohydrate Synthesis in Plants Introduction to Anabolic Pathways So far, we were mainly concerned with CATABOLISM: how to extract energy from biomolecules. The rest of the semester we’ll be concerned with ANABOLISM: how to build biomolecules. Biosynthesis Photosynthesis TCA/OxPhos LIGHT ANN Introduction to Anabolic Pathways So far, we were mainly concerned with CATABOLISM: how to extract energy from biomolecules. The rest of the semester we’ll be concerned with ANABOLISM: how to build biomolecules. TCA/OxPhos ANN Introduction to Anabolic Pathways So far, we were mainly concerned with CATABOLISM: how to extract energy from biomolecules. The rest of the semester we’ll be concerned with ANABOLISM: how to build biomolecules. Photosynthesis LIGHT ANN Introduction to Anabolic Pathways So far, we were mainly concerned with CATABOLISM: how to extract energy from biomolecules. The rest of the semester we’ll be concerned with ANABOLISM: how to build biomolecules. Biosynthesis ANN Photosynthesis Metabolism • Carbon/oxygen cycle + CO2 + H2O ⇌ CH2O + O2 • Recall that living organisms are built of complex structures. • Building complex structures means paying a huge entropic price; need a constant input of energy. • The ultimate source of this energy on Earth is sunlight O2 + CH2O ⇌ H2O + CO2 Respiration Photosynthesis Key topics: DE0’= –0.45V à +41 kcal/mol • Photosynthetic electron transfer and photophosphorylation • CO2 assimilation in photosynthetic organisms • Photorespiration in C3 plants • Avoiding photorespiration in C4 plants Robert (Robin) Hill Key experiments: v Light caused O2 (Hill reaction) 4Fe3+ (CN)6 ⇌ 4Fe2+ (CN)6 v Light caused reduction of NADPH (Ochoa reaction) Severo Ochoa ∴light causes electron flow to NADPH (=A) ∴light causes electron flow, and O2 evolution that did not involve any CO2. ….The lack of involvement of CO2 in this light-caused oxygen production was definitively shown in 1941 Photosynthesis hn vLight caused evolution of O2 – Hill reaction: 2H2 O + 4Fe3+ (CN)6 ⇌ 4Fe2+ (CN)6 + 4H+ + O2 light Kornberg Lecture 03.17.17 (33:00-36:17) 3 min Photosynthesis CO2 + H2O ⇌ CH2O + O2 Use heavy isotope of oxygen ( O) to ”label” the oxygen separately in CO2 and H2O, then test the O2 that is produced. 18 Photosynthesis CO2 + H2O ⇌ CH2O + O2 Light Energy is Converted to ATP in Plant Chloroplasts Photophosphorylation 2H2O ⇌ O2 + 4e– + 4H+ (2NADPH + 2H+) 4e– + 4H+ + CO2 ⇌ CH2O + H2O Both of these half-reactions are VERY unfavorable!! Photosynthesis The Electromagnetic Spectrum Photo-pigments Photosynthesis – If Light Energy is Converted to ATP, how much light is needed? – What is the energy in ONE photon of light? – There is enough Light Energy to make >5 ATP – How do you convert photons to chemical energy? – You go through electrons…. – So we have to do re-dox – – ox-phos in reverse! Energy of 1928: Walther Nernst, Albert Einstein, Max Planck, Robert Millikan, and Max Laue Max Planck was a physicist who initiated the particle theory of light; its not just electromagnetic waves. Einstein developed the concept of a photon, or a quanta of light. Thus, quantum mechanics was born. Electromagnetic energy could be emitted only in quantized form, in other words, the energy could only be a multiple of an elementary unit: E = h! ! = c/l = h•c/l where h is Planck's constant, also known as Planck's action quantum, and ! is the frequency of the radiation. Note that the units of energy are represented by h! and not simply by !. So, the energy comes in units and not just a spectrum or continuum. one mole of photons (1 einstein): h = Planck’s constant (6.626 x 10–34 J•s) c = 3 x108 m/s l = 700 x 10–9 m E = 2.84 x 10–19 J/photon × 6.022 x 1023 photons/einstein E = 17.1 x 104 J/einstein E = 171 kJ/einstein E = 41 kcal per mole of photons ….In terms of volts: E = +1.8 volts per mole of photons Photosynthesis – If we have to do re-dox, what is it that must be done? CO2 + H2O ⇌ CH2O + O2 We need an electron acceptor with Eo more positive than water We need an electron donor with Eo more negative than NADPH – First thing is to absorb the light. What does this? ⊝ NADP+ NADPH –0.32 V Need >1.8 V Because we know that oxygen comes from the water, using the energy from the light, and we know that NADPH is the reductant…..we have a problem! DEo for Mito (–1.14V) +0.82 V ⊕ Photosynthesis Various Pigments Harvest the Light Energy Chlorophyll a absorbs higher energies than Chlorophyll b Photosynthesis –Light Absorption The excited state has several choices for dissipating the energy: internal conversion (heat), fluorescence (emit light), or energy transfer. e– hole! Which occurs depends on several factors: ⊕ ⊝ Excited state * ⇀ Energy transfer e– gained! ⇀ special Chl (Acceptor) ⇀ ⇀ ⇀ Vibrational states ⇀ Ground state ⇀ ⇀ ⇀ We have chemistry! special Chl (Donor) Reaction Center (1) The overlap of the fluorescence band of the donor and the absorption band of the acceptor (Förster's overlap integral). (2) The distance between the two molecules; essentially a dipole-dipole interaction obeying Coulomb’s law proportional to r-6. They have to be close. (3) The orientation of the dipoles of the donor and the acceptor molecules. Photosynthesis Organization of LightAbsorbing Molecules in Chloroplasts The probability of energy transfer is ~50% when the "critical distances" are of the order of 50 Å. In a chloroplast, the distance between different pigment molecules much less than 50 Å, so that the probability of energy transfer is high. It is actually >90% efficiency and fast <100 ps Photosynthesis Organization of LightAbsorbing Molecules in Chloroplasts 200 chlorophylls and 50 carotinoids Photosynthesis Harvested Light causes charge separation in the Reaction Center Lets look at this in reality Photosynthesis Harvested Light causes charge separation in the Reaction Center Chlorophyll P* Chlorophyll HA All these distances are ~10 Å QA +1.45 V –0.045 V What is PS II? Is there a PS I? Pheophytin (H+ in place of Mg2+) DEo for PSII≈1.5 V ❶The excited chlorophyll special pair passes an electron through a chlorophyll monomer to pheophytin (very very fast). ❷from which the electron moves rapidly to the tightly bound menaquinone, QA. ❸This quinone passes electrons much more slowly to the diffusible ubiquinone, QB, through the nonheme Fe. Photosynthesis RED-DROP Experiment 680 nm Overall photosynthesis Conclusion is that there are two photosystems: PS I (P700) & PS II (P680) AND, PS II is responsible for O2 production Action Spectrum –––– Keep yellow light on. Give flashes of higher and higher wavelengths Oxygen production – – – – How do we know there were two PSs? Red-Drop Experiment: No O2 after 680 nm ∴ below 680 nm there is photosynthesis system that generates oxygen But, above 680 nm there must be another photosystem because the overall photosynthesis still goes on. Photosynthesis Photosystem II (P680) 11 Photosynthesis ⊕ e– hole! ⊝ e– gained! Photosystem II Evolves Oxygen from Water Clinical Correlations Cycling of Fat and Fatty acids • Recall that adipose hydrolyzes triglycerides to free fatty acids and glycerol. § hormonally controlled: • In liver, FFA (either from blood or de novo) are reesterfied into triglycerides and exported via VLDL • In fasted humans, 65% of FFA are converted this way in the liver and other tissues • Nearly all of the G3P for this is provided by glyceroneogenesis, not glycolysis +insulin ↓ (meal) –insulin ↑ (fast) +glucagon ↑ (fast) –glucagon ↓ (meal) • The homeostasis of [FFA] is involved in type 2 diabetes (T2D) • One class of drugs for T2D modulate the rate of this reesterification by the liver • These are the thiazolidinediones, which increase the rate of reesterification by increasing glyceroneogenesis ADIPOCYTE Clinical Correlations Obesity: A Pandemic • Obesity is defined as having a BMI of >30 [BodyMass Index=weight (kg)/height2 (m2)] • In USA, 40% of population is obese, with another 33% overweight (BMI=25–30) • About 50% of the cases have a genetic component, but that leaves a third are more or less behavioral and/or metabolic causes • Obesity leads to high fat in blood (dyslipidemia), insulin resistance, and hypertension. Any 3 you are said to have METABOLIC SYNDROME • LEPTIN is produced in response to the fed state and reduces appetite. But unlike mice, which are leptin deficient when obese, humans get leptin resistance as well. • GLP-1 agonists are recently been used for T2D, and also have effects for weight loss. Clinical Correlations Protein-Calorie (Energy) Malnutrition • High carbohydrate, but low protein, keeps insulin high and so fat stores are not accessed. • Diminished protein • In addition protein is not accessed for energy synthesis in all tissues; because there is no ketosis. liver can’t make enough VLDL; intestine loses absorption ability. • Similarly seen in older people with liver cirrhosis or loss of appetite. Liver can’t control oxidation of BCAA.