Week 5 - Metabolism - Photosystems

Photophosphorylation

ATP synthesis and the “light” reactions. Photosynthesis combines phototrophy and carbon fixation to produce carbon compounds.

Light reactions capture light energy and use it to create a proton motive force (to synthesize ATP)

In eukarya, the site of light capture is the photosystem in specialized thylakoid structures

Photopigments

Capture photons to transfer light energy to electrons.

Cyanobacteria produce chlorophyll a

Eukaryal algae and plants produce both chlorophyll a and b

Anaerobic photosynthetic bacteria produce bacteriochlorophylls

Each absorbs light in a different range of wavelengths.

Accessory pigments

Help absorb light over a broader range of wavelengths

Photosystems

Composed of numerous antennae that absorbs light energy.

Energy is transferred from pigment to pigment to the reaction centre containing a pair of electron donating chlorophyll molecules.

The electrons are passed to a chlorophyll electron acceptor and onto an associated electron transport system for ATP production

Antenna

Composed of an array of chlorophylls or bacteriochlorophylls and accessory photopigments

Photosystem I (PS I)

Needs an external electron donor to replace the electron removed to form NADH (or NADPH)

Photosystem II (PS II)

Replaces electrons internally but doesn’t produce electron carriers

Anoxygenic photosynthesis (PS I)

Some cells use a molecules other than water to replace lost electrons.

This results in something other than O2 being given off as a waste product.

Energy from light converted to reducing power, as NADPH

Anoxygenic photosynthesis (PS II)

Some cell perform a cyclic version of photosynthesis. This setup doesn’t require a compound to replace lost electrons but also doesn’t produce any reducing power.

Uses electrons excited by light energy to pump protons as they are passed between molecules with increasing reducing potentials.

Does not generate reducing power as NADPH

Oxygenic photosynthesis

A two-photosystem setup that splits water to replace lost electrons.

Releases oxygen as a waste byproduct.

Both PS I and PS II are used together, photons enter PS I and PS II.

Water is used as an electron donor, yielding oxygen.

Proton motive force as well as reducing power as NADPH is generated

The Calvin cycle and carbon fixation

The ATP and NADPH produced in the “light” reactions are used in the “dark” reactions (aka the Calvin cycle) to produce carbon compounds from CO2.

These carbon compounds are then consumed to produce much more energy through the “standard” catabolic reactions.

Key step that incorporates CO2 into organic molecules is catalyzed by ribulose bisphosphate carboxylase: rubisco —> Uses 3 ATP and 2 NADPH per CO2

Carboxylation

Rubisco enzyme adds carbon from CO2 to 5-carbon ribulose 1, 5 - biphosphate, then split into two molecules of 3-phosphoglycerate (3PG)

Reduction

ATP and NADPH are used to convert 3PG to glyceraldehyde 3-phosphate

Regernation

After six gradual turns of the cycle, 6 carbons are stitched together into a glucose molecule and the original 5-carbon ribulose 1,5 - bisphosphate is regenerated

The reductive TCA cycle

Autotrophic microbes use a different cycle. Electron carriers are used, not created. Carbon compounds are reduced, not oxidized.

Nitrogen fixation into ammonia

Plenty of N2. The nitrogenase enzyme can help but, the process is very energy intensive (16 ATP consumed per N2).

The enzyme is very sensitive to oxygen and requires protection.

Ammonia assimilation

Two different system:

• GS-GOGAT works best at low ammonia concentrations

• GDH (glutamate dehydrogenase) works best at high ammonia conc.

• Both systems readily incorporate ammonia to form amino acids for cells

Sulfur metabolism

Sulfur is often found in a highly oxidized state in nature.

Cells need it in a more reduced state to incorporate it into organic molecules.

Since it’s hard to add electrons to sulfate, it’s linked to the remains of an ATP molecule to make the process easier.

Hydrogen sulfide (H2S) is then incorporated into sulfur containing molecules, like cysteine

Nucleotide synthesis

Nucleotides are composed of a nitrogenous base, a 5-carbon sugar, and a phosphate group.

This biosynthesis is complex

• The base must be synthesized or acquired

• The ribose sugar must be synthesized or acquired. All nucleotides start as ribonucleotides, a later reduction step converts some to deoxyribonucleotides.

Finally, the phosphate groups must be added

Lipid synthesis

Fatty acid formation takes place in the cytoplasm. Consists of small multi carbon units added sequentially. Carrier proteins shuttle the units to the growing polymer

Phospholipids synthesis

To form triglycerides and phospholipids, the basic steps are the same

Fatty acids are added to a glycerol 3-phosphate backbone to from triglycerides for storage or to form phospholipids for membrane structures