Life ’ s Requirements (in two takes)

1. Life’s Requirements (in two takes) Tori Hoehler, NASA Ames tori.m.hoehler@nasa.gov

2. Take 1: Life as We Know It

3. Complexity, Organization, Order (Among other things) life is an emergent property of big, complicated molecules that interact in an organized way

4. Raw Materials Energy Clement Conditions Solvent H? Requirements for Complexity (SPONCH, e-) (light, chemical) (water) (T, pH, S, etc.)

5. “How would we express in terms of statistical theory that marvelous faculty of a living organism, by which it delays the decay into thermodynamic equilibrium (death)?” “It feeds upon negative entropy”

6. Why Energy? It’s The (2nd) Law . . . Exhibit A (The awesome destructive power of the four year-old boy)

7. “Process” increases entropy in the system (Organized LR + kid → Disorder) 7am 11am Energy expenditure to maintain a complex, ordered state (Disorder + my hard work → Organized LR) 9pm

8. But Energy is Everywhere . . . Earth life uses only a subset of available light and chemical energy, which themselves are a subset of available forms

9. (Our) biology uses only a small part of the EM spectrum for energy There’s a pretty fundamental reason for this!

10. A General Scheme of Energy Transduction (Earth Life) Electron flow Trans-membrane cation gradient High-energy covalent bond Chemical: Electron transfer (redox) reactions with sufficient Gibbs energy to establish cation gradient (approx 10 kJ/mol; ultimately set by energy of covalent bond in final step) Light: Photon energy sufficient to cause electronic transitions of desirable type**. Significantly exceeds energy for ion gradient formation. Wavelength (Voltage) Approx. 400-1025 nm Flux (Power) Approx. 1015 – 1022 photons∙m-2∙s-1 (PAR)

11. A Fundamental Constraint on the use of Light Energy Different wavelengths (energies) of light interact differently with matter Too “Hot” Too “Cold”

12. Life Requires… “An environment capable of maintaining covalent bonds, especially between C, H, and other atoms” This statement encompasses three elements: Covalent bonds (= molecules) Raw materials Clement environmental conditions

13. Life Requires A diverse library of complex molecules thatinteract This requires both covalent and non-covalent interactions. Raw materials, environmental conditions, and solvent (which we’ll come back to) all need to be considered in this context.

14. Covalent vs. Non-Covalent Interactions Non-Covalent Covalent Covalent: Shared electrons. Primary molecular structure and properties. “Marriage” Non-Covalent: No shared electrons. Interactions (molecular recognition, “Dating” tertiary structure, self-organization).

15. Raw Materials: How to make complex, interesting molecules that interact Scaffolding Capable of creating a large, diverse library of molecular skeletons Capable of bonding to multiple heteroatoms Creates structures that are stable, but not too stable Heteroatoms (“sticky bits”) When bonded to scaffold or each other, create potential for an array of non-covalent interactions (incl. directional or hydrogen bonds?) Electrons To “glue” molecules together

16. A lesson from Earth . . .

17. Everything a Body Needs? Scaffolding element (C): Capable of forming 4 bonds Dominantly in intermediate oxidation state*

18. Electrons Needed! … to make (a diverse library of) biomolecules from CO2 Earth: Molecular diversity requires intermediate oxidation state, but elements (esp. C) are available in oxidized form. Life needs electrons and water H—O—H is all around. Molecular Diversity vs. Oxidation State in Terrestrial Biochemistry* Bains & Seager, 2012 Sugars Most available carbon at Earth’s surface

19. Everything a Body Needs? Scaffolding element (C): Capable of forming 4 bonds Dominantly in intermediate oxidation state* Relatively labile covalent bonds Heteroatoms (SPONH): Electrostatic interactions Tertiary structure Molecular recognition Coordination chemistry + “Minerals”: Central to enzyme & cofactor function

20. Heteroatoms and Non-Covalent Interactions

21. Bottom Line Requirements for (our) Life Source of Energy Water Source of Carbon Nutrients Source of Electrons Microbiologists classify organisms based on how they fulfill these needs

22. Clement Conditions? Must be compatible with both covalent bonding and non-covalent interactions

23. What conditions threaten the integrity and interaction of big, complex biomolecules? Heat Radiation Strong Acid/Base Pressure

24. Temperature: -25 to +122˚C (growth)

25. pH: approx. 0 to 12 extracellular

26. Water Activity: approx. 0.6 (incl. NaCl saturation)

27. Pressure: approx. 200 MPa*

28. Take 2: OK, fine, but what about the Horta?

29. The “TerraCentric” Problem There is a famous book published about 1912 by Lawrence J. Henderson . . . in which Henderson concludes that life necessarily must be based on carbon and water, and have its higher forms metabolizing free oxygen. I personally find this conclusion suspect, if only because Lawrence Henderson was made of carbon and water and metabolized free oxygen. Henderson had a vested interest. Carl Sagan

30. “The Limits of Organic Life in Planetary Systems” (“The Weird Life Report”) Theory, data, and experiments suggest that life requires (in decreasing order of certainty): Thermodynamic disequilibrium (Gibbs energy)* An environment capable of maintaining covalent bonds, especially between C, H, and other atoms A liquid environment** A molecular system that can support Darwinian evolution

31. OK, fine, but what about the Horta (Crystalline Entity)? Raw Materials (esp. scaffold) Environmental Conditions Solvent These are Interdependent!

32. Life Requires… Thermodynamic Disequilibrium (Gibbs Energy) “…the requirement for thermodynamic disequilibrium is so deeply rooted in our understanding of physics and chemistry that it is not disputable as a requirement for life. Other criteria are not absolute.” Report of the NRC Committee on the Limits of Organic Life in Planetary Systems (aka “The Weird Life Report”)

33. Creation and replication of information (e.g., generation of specific sequences) Creation of locally ordered states Why Does Life Need Energy? (Take 2) Energy is required to reliably populate states (produce outcomes) of otherwise low probability Production of thermodynamically unfavorable species

34. Addition of energy can drive an otherwise improbable outcome (and we know how much energy is needed) The probability of the outcome, and the energy needed to overcome low probabilities, are affected by physicochemical environment (example: production of thermodynamically unfavorable species) A → B ← ΔGA→B = ΔG˚ + RT∙ln({B}/{A}) ΔGA→B is a quantitative measure of probability (e.g., large positive value = very low probability)

35. Life overcomes the low (!) probability of net forward progress in this reaction by application of 2 visible photons’ worth of energy Other “low probability” properties/processes of biological systems can be addressed in the same way by life, and contemplated in the same way by us. For example . . . 2H2O → O2 + 2H2 ← ΔG˚ = +526 kJ∙(mol O2)-1; Keq = 6 x 10-93 (aqueous at 298K)

36. Information Processing as a Core Attribute of Life (and the importance of doing it with high fidelity) (Weird Life Report: Life requires…a system capable of Darwinian evolution)

37. Shall I Compare Thee…? 1000 monkeys typing on 1000 typewriters for 1000 years . . . Have a probability of 10-1255 of correctly reproducing Sonnet #18 Have about a 10% chance of correctly reproducing “Shall I” Could be expected to correctly reproduce a specified sequence of 10 amino acids (not more), given a keyboard of 20 characters Model Assumptions: 101-key standard keyboard 12-hour shifts (according labor laws) 50 5-letter words per minute Correct punctuation, spaces, and returns, but not capitalization Probability of randomly constructing a specified 200-bit sequence where each bit has 20 possible values (i.e., a protein) is approx. 10-260, requiring energy > 1484 kJ/mol

38. Life Requires… “A Liquid Environment”

39. Pohorille on the Importance of Solvent (see: http://astrobiology.nasa.gov/nai/seminars/detail/161) Much of the business of life is conducted through non-covalent interactions (e.g., molecular recognition) Non-covalent interactions depend heavily on the solvent in which they occur Electrostatic and solvophobic interactions have comparable strength in water, allowing a greater range of organizational possibilities

40. Life requires a solvent capable of mediating life-like chemistry Are there alternatives to liquid water?

41. Non-Covalent Interactions Electrostatic Directional (mostly) Based on complementarity Variable in strength Solvophobic Strength strongly solvent dependent (water > non-polar solvents, for non-polar solutes) Van der Waal’s Weak Non-specific (everything sees everything)

42. ∆ A (kcal/mol) Separation (Å) Strength of Non-Covalent Interactions in Water CH4 - CH4 in water (Hydrophobic) ∆ A (kcal/mol) (Electrostatic) Na+ Cl- in DMSO (Courtesy A. Pohorille)

43. Hypothesis Electrostatic and solvophobic interactions are necessary to confer life-like specificity, but are too stable/weak (respectively) in non-polar solvents to function effectively as arbiters of molecular interaction. (So, water – or something very like water – is necessary for life) Focus/Example: Information processing (molecular recognition) with high fidelity

44. Selectivity in Molecular Recognition Importance of Electrostatic Interactions Discrimination during activation by AAtRNA Synthetase 102:1 non-polar Isoleucine Valine polar >104:1 Electrostatic effect gives 100-fold enhancement in fidelity of molecular recognitiion Tyrosine Phenylalanine

45. Fidelity in Information Processing Information: G-A-T-T-A-C-A Smallest free-living organism = 600 Kbit (Humans = 3 Gbit) 1 “bit” 1 “byte” “Reliable” (>90% successful) replication of 600 Kbit of information requires error rate < 10-7 (> 107-fold discrimination among possible states) Observed error rates during DNA replication are 10-8 to 10-10

46. Selectivity in Chemical Processes Kinetic Thermodynamic the system must be able to sample (interconvert between) possible states a large number of times ΔG = 0 = ΔG˚ + 2.3 RT log (S2/S1) S1 S2 S2/S1 ΔG˚ (kJ/mol) 104 -23 107 -40 at equilibrium at 298K 1010 -57 One H-bond is worth approx. 20kJ/mol. Hmmm….

47. Stability of Electrostatic Interactions Complexed Uncomplexed H H H H H H O O O Water-solvated form approx. as stable as complexed form : : : : : : Less stable than complexed form by 60 kJ/mol) in non-polar solvent correct complex stabilized by 60 kJ/mol relative to alternatives (1 bit at error rate 10-10)

48. Energy in Information Information Content Total Energy (kJ/mol) Stabilization vs. incorrect alternatives Stabilizationvs. solvation in water* Complex dissociation in nps* DNA** 1 bit 50  0 50 1 byte 150  0 150 “protein’s worth” 75000  0 75000 600 Kbit 3x107 3x107  0 Strength/Stability of C-C bond corresponds to about 10 bits worth of information *If stabilization derived entirely from electrostatic interactions **DNA: Average 2.5 H-bond = 50 kJ/mol per bit = hypothetical error rate  10-9

49. OK, fine, but what about the Horta (Crystalline Entity)? Raw Materials (esp. scaffold) Environmental Conditions Solvent These are Interdependent!

50. Ruling Out Alternatives to Liquid Water . . . Allows application of liquid water phase constraints Rules out covalent chemistry that is unstable (or too stable) with respect to: (a) the chemical reactivity of water, or (b) the temperature range in which water is liquid, or (c) both (silicon: gone; carbon: decreased temp range for stability of C-C and C-X relative to non-polar solvents)