Chapter 20: Carbohydrate Biosynthesis in Plants and Bacteria

• CO$_2$ assimilation in plants and bacteria

  • Calvin cycle

  • Photorespiration in C3 plants

  • C4 and CAM plants and pathways

• Starch and sucrose biosynthesis in plants

• Integration of carbohydrate and fat metabolism in plants

Overview

• Key concepts in carbohydrate biosynthesis include:

  • Starch

  • Sucrose (transport)

  • Cellulose (storage, cell wall structure)

  • Hexose phosphates

  • Pentose phosphates

  • Triose phosphates

  • ATP, ADP, NADPH, NADP+

  • CO$_2$ fixation (assimilation, carbon fixation)

  • H$_2$O, light reactions, oxygen production

• Carbohydrate metabolism is interconnected with DNA, RNA, protein, and lipid synthesis.

CO$_2$ Fixation in Plants and Bacteria

Calvin Cycle Overview

1. Key Components:

   • Ribulose 1,5-bisphosphate (RuBP) is vital; is continuously regenerated using energy from ATP and reducing power from NADPH.

   • Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase) is possibly the most abundant protein on Earth (40% of leaf protein).

   • Main outcome: Reduction of CO$_2$ with NADPH from light reactions, occurring in the stroma of chloroplasts.

2. Mechanism: Cyclic process known as the Calvin cycle.

Stage 1: Fixation (Carboxylation by Rubisco)

• Process:

  • Rubisco catalyzes the carboxylation of RuBP (5 carbon sugar) with CO$2$, establishing a new carbon-carbon bond using CO$2$ as substrate.

  • Enzyme is a magnesium (Mg$^{2+}$)-containing protein.

  • Each CO$_2$ that enters results in the formation of two molecules of 3-phosphoglycerate (PGA).

  • Two different forms of Rubisco exist:

  • Form I (in vascular plants, algae, cyanobacteria): consists of 8 large and 8 small subunits.

  • Form II (in certain photosynthetic bacteria): consists of 2 large subunits.

Rubisco Structure and Function

• Active Site:

  • Ribulose 1,5-bisphosphate creates an enediolate at the active site that is protonated, generating a branched 6-carbon sugar.

• Mechanism:

  • Mg$^{2+}$ ions promote nucleophilic attack by the enediolate, supporting the reaction that results in 3-phosphoglycerate.

• Key Terms:

  • Carbamoylation: A modification involving the binding of CO$_2$ to the enzyme lysine side chains.

Regulation of Rubisco CO$_2$ Fixation

• RuBP binds to the active site (Lys201) of Rubisco, inhibiting carbamoylation.

• Rubisco activase activates the enzyme by ATP hydrolysis to release RuBP, enabling Lys carbamoylation and Mg$^{2+}$ binding.

• Light and high pH conditions promote activase activity, enhancing Rubisco activation and CO$_2$ fixation efficiency.

Stage 2: Reduction of 3-Phosphoglycerate to Glyceraldehyde 3-Phosphate

• Enzymes Involved:

  • 3-phosphoglycerate kinase and glyceraldehyde 3-phosphate (GA3P) dehydrogenase convert PGA to GA3P.

  • Reduction process requires energy (ATP) and reducing power (NADPH).

• Outcomes:

  • GA3P can be converted into starch or sucrose.

Stage 3: Regeneration of RuBP

• Process:

  • A series of reactions regenerate RuBP from GA3P using intermediates ranging from 3 to 7-carbon sugars.

  • Specifically: 5 triose phosphates needed for every 3 RuBP regenerated, consuming 3 ATP in the process.

• Fates of Triose Phosphates:

  • Converted to starch in chloroplasts, transported to cytoplasm for sucrose or glycolysis.

Stoichiometry of CO$_2$ Fixation

• Net Reaction:
  $3CO<em>2 + 9ATP + 6NADPH + 6H^+ ightarrow C</em>3H<em>6O</em>3 ext{-phosphate} + 9ADP + 8Pi + 6NADP^+ + 3H_2O$

• This equation summarizes the overall input and output during photosynthesis phases, detailing energy and molecule transformation.

Pi-Triose Phosphate Antiport System

• Mechanism:

  • Facilitates a one-for-one exchange of inorganic phosphate (Pi) and triose phosphates (DHAP or PGA).

• Functions:

  • Exports triose phosphates from stroma to cytosol for sucrose synthesis and imports Pi for ATP regeneration, allowing energy transfer between organelles.

Photosynthesis Mechanisms

• Light-Dependent Reactions:

  • $2H<em>2O + 2NADP^+ + 3ADP + 3Pi + ext{Light} ightarrow 3CO</em>2 + 9ATP + 6NADPH + 6H^+ + O_2$

• Light-Independent (Calvin Cycle):

  • Coordinates with light-dependent reactions to produce ATP and NADPH in the stroma.

• Net Photosynthesis Reaction:

  • $6CO<em>2 + 6H</em>2O <br>ightarrow C<em>6H</em>{12}O<em>6 + 6O</em>2$

Photorespiration in C3 Plants

• Effects:

  • Plants utilize water for O$2$ release and convert CO$2$ to carbohydrates; however, C3 plants undergo photorespiration, a wasteful reaction catalyzed by Rubisco, which consumes O$2$ and produces CO$2$.

• Mechanism:

  • Involves the lack of specificity of Rubisco (acts as both carboxylase and oxygenase).

• Glycolate Pathway:

  • Biochemical reactions responding to waste produced during photorespiration.

Strategies to Reduce Photorespiration

C4 Plants

• Separation Mechanism:

  • Physically separate carbon fixation from the Calvin cycle.

  • CO$2$ is captured by PEP carboxylase into 4-carbon (C4) storage compounds, then transported to bundle sheath cells where Rubisco operates without interference from O$2$.

  • Adaptations allow these plants to thrive in warmer regions (e.g., corn, crabgrass).

CAM Plants

• Temporal Separation:

  • Fix CO$_2$ at night (open stomata for gas exchange) and use it during the day in the Calvin cycle (closed stomata to minimize water loss).

  • Adaptative strategies for resilience in hot, dry climates (e.g., cacti, pineapple).

Starch and Sucrose Biosynthesis in Plants

Starch Overview

• Structure:

  • Composed of amylose (20-30%, linear) and amylopectin (70-80%, branched, with molecular weights of up to 200 million).

• Function:

  • Primary storage polysaccharide in plants, synthesized in chloroplasts and amyloplasts.

Starch Biosynthesis Process

• Synthesized by starch synthase with ADP-glucose as substrate, involving two-site insertion mechanisms.

• A branching enzyme introduces α1-6 branches, promoting structure integrity.

Regulation of Starch Biosynthesis

• Controlled primarily by ADP-glucose pyrophosphorylase, activated by 3-phosphoglycerate and inhibited by inorganic phosphate (Pi).

Sucrose Biosynthesis

• Structure:

  • Composed of glucose and fructose linked by an α1→2 bond.

• Synthesis:

  • Occurs in cytosol via condensation of UDP-glucose and fructose 6-phosphate, followed by phosphate group removal.

Regulation of Sucrose Biosynthesis

• Regulated by fructose 1,6-bisphosphatase (FBPase-1) and PPi-dependent phosphofructokinase, which are influenced by fructose 2,6-bisphosphate and light, affecting synthesis rates.

Integration of Carbohydrate and Fat Metabolism in Plants

• Common Pathways:

  • Key metabolic processes include the citric acid cycle, glycolysis, gluconeogenesis, and the pentose phosphate pathway.

• Unique to Plants:

  • CO$_2$ fixation through the Calvin cycle and conversion of acetyl-CoA to C4 compounds (glyoxylate cycle) serve distinct functions in photosynthesis and energy storage.

Conversion of Stored Fatty Acids to Sucrose in Germinating Seeds

• Initiated in glyoxysomes (where fatty acids undergo β-oxidation) producing Acetyl-CoA, then processed through the citric acid cycle or converted to glucose via gluconeogenesis.

Summary of Key Points

• Energy from light (ATP and NADPH) is crucial for CO$_2$ assimilation.

• The Calvin cycle enables the reduction of CO$_2$ and synthesis of carbohydrates such as sucrose and starch.

• Increased concentration of CO$_2$ mitigates the effects of photorespiration exhibited by C3 plants, a condition effectively avoided in C4 and CAM plants.

• Overall, distinct carbohydrates and structures are synthesized from nucleotide-tagged glucose, reinforcing a plant's unique metabolic pathways compared to animals.