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Chapter 7: Metabolism II

Overview of Metabolism II

In this section, we will explore metabolic pathways that do not emphasize oxidation/reduction. These pathways are crucial for catabolism and anabolism associated with proteins, nucleic acids, nitrogen balance, and sugar balance. Despite having lesser energy roles compared to the pathways discussed in the previous chapter, they are still important. These may be thought of as “kitchen sink” pathways, but all metabolic pathways carry significance.

Carbohydrate Storage and Breakdown

Importance of Carbohydrates

Carbohydrates serve as vital cellular energy sources, providing quick energy through glycolysis and intermediates to pathways like the citric acid cycle and amino acid metabolism. Understanding how these molecules are stored and metabolized is essential.

Storage in Plants and Animals

Plants store glucose primarily in the forms of:

• Amylose and amylopectin (for energy storage)

• Cellulose (for structural integrity)
  In cellulose, glucoses are joined by beta-1,4-glycosidic bonds, while amylose and amylopectin have alpha-1,4 and alpha-1,6 bonds, respectively.

Animals primarily store glucose as glycogen, which is structurally similar to amylopectin but features more frequent alpha-1,6 branches, occurring approximately every 10 glucose residues. Glycogen acts not only as an energy reserve but also as a quick supply of glucose when needed and for storing excess glucose when blood sugar levels are high.

Breakdown of Glycogen

The breakdown of glycogen entails three main processes:

1. Release of glucose-1-phosphate (G1P)

2. Rearranging remaining glycogen for continued breakdown

3. Conversion of G1P to glucose-6-phosphate (G6P) for further metabolism

G6P can either be

• Broken down in glycolysis

• Converted back to glucose in gluconeogenesis

• Oxidized in the pentose phosphate pathway
  To prevent futile cycles in metabolism, glycogen synthesis and breakdown utilize distinct mechanisms, helping to control these pathways efficiently.

Glycogen Synthesis

Glycogen synthesis begins with G1P, converted into an activated intermediate, UDP-glucose, which is then added to the growing glycogen chain by glycogen synthase. Branches in the glycogen structure can be formed by the branching enzyme, which introduces additional alpha-1,6 linkages.

Enzymes Involved in Glycogen Breakdown

1. Glycogen Phosphorylase: Catalyzes the production of G1P from glycogen using phosphorolysis (not hydrolysis).

2. Glycogen Debranching Enzyme (GDE): Acts on glycogen branches, converting alpha(1-6) links to alpha(1-4), releasing a trisaccharide and allowing continued breakdown of glycogen.

3. Phosphoglucomutase: Converts G1P to G6P, facilitating interconversion based on substrate concentrations.

Regulation of Glycogen Metabolism

Regulation occurs through both allosteric factors and covalent modifications (phosphorylation/dephosphorylation). Glycogen phosphorylase is regulated by:

• Allosteric Factors: ATP, G6P, AMP, glucose

• Covalent Modification: Phosphorylase kinase transforms GPb (less active) into GPa (more active).

GPb can exist in both R state (active) and T state (inactive). AMP activates GPb, while ATP and G6P inhibit it. Conversely, GPa's only negative allosteric effector is glucose.

Turning Off Glycogen Breakdown

Turning off glycogen breakdown is crucial to cellular regulation. This can occur through:

• Dissociation of ligand from receptor

• GTPase activity of G-proteins

• Action of phosphodiesterase on cAMP

• Dephosphorylation of enzymes by phosphoprotein phosphatase

Glycogen Synthesis Steps

1. Conversion of Glucose: Phosphorylation of glucose leads to G6P, isomers to G1P (via phosphoglucomutase).

2. UDP-glucose Formation: Using UTP, G1P is processed into UDP-glucose.

3. Glycogen Chain Formation: Glycogen synthase joins glucose to the growing glycogen chain, with branching facilitated by the branching enzyme.

Reciprocal Regulation of Glycogen

Glycogen synthesis and breakdown are reciprocally regulated to prevent simultaneous activation. Insulin stimulates glycogen synthase and inhibits glycogen phosphorylase, facilitating the incorporation of glucose into glycogen post-meal.

Pentose Phosphate Pathway

The Pentose Phosphate Pathway (PPP) is critical for producing NADPH, ribose-5-phosphate, and erythrose-4-phosphate, which are essential for anabolic reactions and nucleotide synthesis. The pathway is divided into two phases:

• Oxidative Phase: Produces NADPH through the oxidation of G6P via glucose-6-phosphate dehydrogenase.

• Non-Oxidative Phase: Involves the interconversion of sugar phosphates.

Reversible reactions allow intermediates from glycolysis to funnel into and out of the PPP, with the direction dictated by cellular needs.

Calvin Cycle

The Calvin Cycle is a light-independent reaction of photosynthesis occurring in plants, wherein carbon dioxide is assimilated into carbohydrates. The cycle requires multiple ATPs and NADPH molecules for the reduction of carbon dioxide into glucose. The key steps include:

• Fixation of CO2 to ribulose-1,5-bisphosphate catalyzed by RUBISCO.

• Formation of 3-phosphoglycerate (3-PG) and subsequent steps leading to glucose production. The additional three-carbon molecules produced are primarily used in gluconeogenesis.

C4 Plants

C4 plants possess an adaptation that allows them to concentrate CO2 before assimilation, reducing water loss and preventing photorespiration. Initial fixation occurs in special mesophyll cells, and subsequent steps occur in bundle sheath cells.

Urea Cycle

The Urea Cycle plays a pivotal role in nitrogen metabolism, particularly in animals, and occurs predominantly in the liver and kidneys. The cycle converts toxic ammonia to urea for excretion, involving several key steps:

1. Formation of carbamoyl phosphate from ammonia and bicarbonate, utilizing ATP.

2. Creation of citrulline by combining carbamoyl phosphate with ornithine.

3. Formation of argininosuccinate, which cleaves to produce arginine and fumarate, resulting in urea synthesis upon the hydrolysis of arginine.

Nitrogen Fixation

Nitrogen fixation is the conversion of atmospheric nitrogen to a usable form for organisms. It is catalyzed by the enzyme nitrogenase found in specific anaerobic bacteria. The overall reaction for nitrogen fixation is:
$N<em>2 + 6H^+ + 6e^- ightarrow 2NH</em>3$
This process requires significant ATP consumption and is foundational for building amines in proteins and nucleic acids.

Amino Acid Metabolism

Amino acid metabolism encompasses pathways for the synthesis and degradation of amino acids. Essential amino acids cannot be synthesized by the body and must be obtained from the diet. The three categories of amino acid pathways involve glucogenic substrates, ketogenic substrates, or both:

• Transamination: A key process where amino groups are exchanged between an amino acid and an alpha-keto acid, enabled by transaminases. An example is:
  $Pyruvate + Aspartic ext{ acid} <br>ightleftharpoons Alanine + Oxaloacetate$

• An important reaction in amino acid metabolism is the conversion of alpha-ketoglutarate to glutamate using ammonium ions.

Amino Acid Catabolism

The breakdown of amino acids generates substrates for energy production. An important overview includes:

• Glutamine is catabolized by glutaminase, yielding ammonium and glutamate.

• Each amino acid undergoes distinct catabolic pathways leading to various metabolic intermediates (e.g., pyruvate, acetyl-CoA).

Nucleotide Metabolism

The metabolism of nucleotides involves complex synthesis pathways for both purines and pyrimidines, with de novo synthesis processes for each:

Pyrimidine De Novo Biosynthesis

Starting materials include bicarbonate, glutamine, and ATP, generating carbamoyl phosphate. The key regulatory enzyme, aspartate transcarbamoylase, is activated by its substrates and inhibited by CTP. The pathway progresses to UMP, UDP, and UTP.

Purine De Novo Biosynthesis

Purines are synthesized from ribose-5-phosphate via PRPP. Regulation is influenced by the levels of GMP and AMP. The branch point at IMP diverges to pathways leading to AMP and GMP, both of which regulate their production based on cellular needs.

Deoxyribonucleotide Synthesis

Deoxyribonucleotides are synthesized from ribonucleotides using ribonucleotide reductase. This enzyme is tightly regulated to balance the production of different nucleotide forms.
The final product formation involves multi-step pathways and substrate interactions ensuring the appropriate nucleotides are synthesized as required for DNA and RNA.