BIOC 4520: Metabolic Process Lec 8

Early Earth’s Atmosphere

• Early Earth's atmosphere was anaerobic, rich in hydrogen, methane, CO2, and ammonia
• The presence of these gases was likely essential for the evolution of life
• Haldane proposed that O2 was absent on early Earth, otherwise organic compounds would have been oxidized to CO2 and H2O

Methanogenic Bacteria

• The earliest life forms were methanogenic bacteria (methane-producing Archaea)
• These bacteria are strictly anaerobic and are killed by O2
• They produce methane from hydrogen and CO2, or acetate
  • CO2 + 4 H2 ATP generation CH4 + 2 H2O
  • CH3COOH ATP generation CH4 + CO2
• Acetate and CO2 sustain anaerobic respiration by acting as terminal electron acceptors, allowing the production of ATP (simple “glycolysis”)
• Methanogens thrive in habitats resembling early Earth: rich in CO2 and H2, such as hot volcanic, sulphurous, anaerobic springs

Evolution of Photosynthesis

• Some prokaryotes became autotrophic, generating their energy
• Purple sulfur bacteria are photosynthetic bacteria that reduce carbon to carbohydrates
• They do not release oxygen, and use hydrogen sulfide (H2S) instead of water
• Cyanobacteria evolved oxygenic photosynthesis, releasing oxygen
  • 2H2O O2 + 4H+ + 4e-
• Oxygen initially was absorbed by sediments and Fe2+ was oxidized to Fe3+
• This prevented an immediate increase in atmospheric O2, which would have been lethal to methanogens

The Problem of Oxygen

• Oxygen is a reactive molecule that can extract electrons from other molecules, causing oxidative damage
• This damage is caused by reactive oxygen species (ROS)
• ROS are short-lived but can create a chain reaction
• Plants have developed detoxification mechanisms to combat ROS, such as ascorbate peroxidase (APX) - H2O2 + Asc H2O + MDHA
• High light intensity can overload photosystems and lead to the production of ROS
  • Excess energy electrons cannot be passed to NADP+
  • This leads to electron leakage to molecular oxygen

Photosynthesis and Respiration

• Photosynthesis is the process where plants convert light energy into chemical energy
• Respiration is the process where plants break down glucose to generate energy
• In photosynthetic organisms, there is a significant level of respiration in the light

Photorespiration: The Role of Rubisco

• Rubisco is a bifunctional enzyme that can use both O2 and CO2
• It competes with CO2 in the active site, allowing the enzyme to bind to O2
• O2 binding can reduce productivity by around 30% in plants like wheat, rice, and potato
• This process is called photorespiration
• While photorespiration reduces productivity, it is essential for plant survival
• Mutations in the photorespiratory pathway are harmful
• High CO2 levels can minimize the problem

### The Glycolate Pathway

• The glycolate pathway is a complex, ATP-consuming process that recovers carbon fragments from photorespiration.
• This pathway involves three organelles: chloroplasts, mitochondria, and peroxisomes.
• It loses carbon as CO2 through the mitochondrial decarboxylation of glycine.
• The process converts two moles of P-glycolate to PGA + CO2.

Glycine Decarboxylase (GDC) and Serine Hydroxymethyl Transferase (SHMT)

• Glycine decarboxylase (GDC) is an essential enzyme in the photorespiratory pathway.
• It accounts for a significant portion of mitochondrial protein in plant leaves.
• It is a tetrameric protein with four subunits: P, L, T, and H.
• GDC catalyzes the conversion of glycine to CO2, NADH, and 5,10-methylenetetrahydrofolate.
• Serine hydroxymethyl transferase (SHMT) is another essential enzyme in the pathway.
• It catalyzes the transfer of a one-carbon unit to glycine.
• The one-carbon unit is carried on tetrahydrofolate (THF).

### Folate

• Folate is a vital nutrient for plants, humans, and other organisms.
• It plays a critical role in cell division and growth.
• A deficiency in folate can lead to anemia.
• Folic acid is a synthetic form of folate.
• It was first isolated from spinach leaves.

Photorespiration: Overall Importance

• While photorespiration reduces productivity, it offers several benefits:
  • It recovers carbon from photorespiration
  • It provides a turnover of reducing power and ATP.
  • It helps minimize photoinhibition by removing excess energy.
• C4 photosynthesis is a more efficient mechanism for fixing carbon dioxide in plants and alleviates some challenges associated with photorespiration.