Biochem Oct. 2nd

Introduction

• Apology for deviation from the norm in the presentation.

• Announcement of Topic Eight, focusing on a deep dive into biological energy processes and the chemical basis of life.

• Mention of a comic about sugar, introducing the overarching theme of metabolism and the meticulous process by which organisms derive energy from food, specifically sugar and other carbohydrates.

Overview of Monosaccharides and Linkages

• Glucose and Fructose:

  • Glucose is an aldohexose that typically exists in the alpha configuration when forming cyclic structures (e.g., alpha-D-glucopyranose).

  • Fructose is a ketohexose that predominately forms a five-membered ring (furanose) and is often found in the beta configuration (e.g., beta-D-fructofuranose).

  • Linkage: In sucrose, the anomeric carbon (Carbon 1) of alpha-glucose forms an acetal bond with the anomeric carbon (Carbon 2) of beta-fructose, termed an alpha 1, beta 2 glycosidic linkage.

• Nutrition Labels:

  • Define sugar broadly as any mono or disaccharide, which are simple sugars that are quickly absorbed and metabolized.

  • The most common form of sugar encountered daily is sucrose, a disaccharide composed of one glucose unit and one fructose unit.

Polysaccharides

• Polysaccharides are complex carbohydrates formed when many monosaccharides join together via glycosidic bonds, serving various structural and energy storage roles.

• Energy Storage:

  • In animals, the primary storage form of glucose is glycogen, a highly branched polymer suitable for rapid glucose mobilization.

  • Represented by a structure with glycogenin as a central protein primer molecule, around which glycogen chains are synthesized.

  • Glucose monomers are joined to glycogenin and within chains via alpha 1,4 glycosidic linkages.

  • These linear chains vary in length, typically between 12 to 14 glucose units before a branch point.

  • Each chain features alpha 1,6 branch points which occur roughly every 8-12 glucose residues, producing a highly branched, tree-like structure.

  • This extensive branching maximizes the number of non-reducing ends, allowing for rapid enzymatic breakdown and glucose release.

  • Overall structure resembles multiple layers or shells of glycogen, with up to 12 layers possible in living cells, forming a dense polymer that can efficiently store a large amount of glucose in a compact space.

• Energy Storage Locations:

  • Glycogen is stored mainly in:

  • Muscle cells: Provides a ready and localized source of energy for muscle contraction during activity.

  • Liver: Large reserves of glycogen are used to maintain blood glucose homeostasis, releasing glucose into the bloodstream as needed.

Starch as Plant Storage

• Plants store glucose as polysaccharides called starch, which is a crucial energy source for humans.

• Two primary forms of starch:

  • Amylose: A minor component (typically 20-30% of starch), consists solely of long, unbranched alpha 1,4 polyglucose chains with very few or no branch points. It tends to form a helical structure.

  • Amylopectin: The predominant component (70-80% of starch), is based on amylose's linear structure but is highly branched. Branch points occur roughly every 25 glucose monomers, also via alpha 1,4 linkages for the linear segments and alpha 1,6 linkages at the branch points. This structure is similar to glycogen but less branched.

• Enzymatic Degradation of Starch:

  • The human body can effectively degrade starch using enzymes like amylase (in saliva and pancreas) and glucoamylase, breaking it down into glucose monomers.

  • This facilitates efficient glucose absorption into the bloodstream, making starch-rich foods like potatoes, rice, and corn excellent sources of energy.

Functions beyond Energy Storage

• Cellulose: A major structural polysaccharide in plant cell walls, composed entirely of beta 1,4 glucose linkages. These linkages create long, linear fibrils that are highly stable and organized, making it difficult for most animals (including humans) to digest due to the absence of cellulase enzymes. Its structural integrity provides hardness, rigidity, and support to plant walls.

  • This explains why humans cannot derive energy from abundant plant materials like wood or cotton.

• Pectin: Found prominently in the cell walls of fruits, consisting of polymers related to galacturonic acid (a derivative of galactose) rather than glucose, also characterized by alpha 1,4 linkages. Pectin is responsible for the gelling properties in jams and jellies.

• Examples of Dietary Fiber:

  • Defined as polysaccharides (and lignins) that humans cannot efficiently digest. These fibers play crucial roles in digestive health, even without providing direct caloric energy.

  • Includes cellulose, chitin (a structural polysaccharide in fungal cell walls and arthropod exoskeletons, composed of N-acetylglucosamine units), and various non-starch polysaccharides.

Inulin and Its Cultural Significance

• Inulin is a complex fructan (a polymer of fructose) rich in certain root vegetables (e.g., chicory, Jerusalem artichokes). It serves as a low-starch alternative beneficial for diabetics due to its minimal impact on blood glucose levels and its prebiotic properties.

• Found in indigenous diets (like those of the Anishinaabe), highlighting traditional knowledge on food preparation techniques (e.g., extended cooking, fermentation) for easier digestion and absorption of inulin, which is otherwise difficult to break down.

• Inulin is high in certain tubers and must be cooked for extended periods to become digestible and prevent digestive discomfort.

Nutritional Labels and Missing Components

• Discussion about what might be missing or underrepresented from nutritional labels, particularly focusing on the category of “total carbohydrates.” There's an argument that starches, despite being digestible carbohydrates, are often not explicitly broken out alongside sugars and fibers, leading to a less complete picture of caloric intake and glycemic impact.

Oligosaccharides in Cellular Function

• The crucial role of oligosaccharides (short chains of monosaccharides, typically 3-10 units) in the formation of cell surfaces, particularly as components of glycolipids and glycoproteins. These surface carbohydrates influence cell-cell recognition, adhesion, and provide physical protection.

• Examples of carbohydrates attracting water, creating a slippery, protective surface (e.g., mucus layers) and mediating highly specific interactions between cells (e.g., the species-specific recognition between sperm and egg cells, and blood group antigens).

Transition to Energy Metabolism

• Previous discussion focused on the diverse structures and roles of polysaccharides, from energy storage to structural support. This section marks a crucial transition into the intricate mechanisms of energy metabolism within cells, specifically how these stored molecules are utilized.

• Reminder about the forthcoming quiz and the relevance of reviewing materials on carbohydrate structures and their immediate implications for energy processes.

Introduction to Thermodynamics of Reactions

• Basics from chemistry reviewed, focusing on Gibbs free energy ($\Delta G$) as the primary determinant of reaction spontaneity.

• Spontaneous (exergonic) reactions occur with a negative $\Delta G$ value, meaning they release energy and proceed without external energy input.

• Reactions with a positive $\Delta G$ (endergonic) are not favored and require an input of energy to proceed, but they can be driven by coupling them with highly favorable (exergonic) reactions, a common strategy in biological systems.

Introduction to ATP as Energy Carrier

• Adenosine Triphosphate (ATP): The universal and main energy currency or carrier in cells, facilitating energy transfer for various biological processes. It is fundamentally different from energy storage molecules (e.g., glycogen or fat), functioning as an immediate energy shuttle.

• Structure of ATP involves a ribose sugar, an adenine base (forming adenosine), and three phosphate groups linked in series: alpha, beta, and gamma. These phosphate groups are connected by two 'high-energy' phosphoanhydride bonds.

• The hydrolysis of the terminal (gamma) phosphate group from ATP releases a significant amount of chemical energy ($\Delta G \approx -30.5\text{ kJ/mol}$ under standard conditions), which is crucial for driving otherwise unfavorable biological reactions forward.

Favorability of ATP Hydrolysis

• Hydrolysis of ATP typically results in ADP (Adenosine Diphosphate) and inorganic phosphate (Pi), with a standard free energy change ( \Delta G^\circ' ) being significantly negative, indicating a highly exergonic reaction.

• Explaining the concept of “high energy bonds” within ATP: these are not unusually strong bonds, but rather the chemical environment and molecular rearrangements upon hydrolysis lead to a substantial release of free energy.

• This favorability stems from several key factors:

  1. Relief of charge repulsion: ATP's three phosphate groups carry significant negative charges, closely packed together. Hydrolysis reduces this electrostatic repulsion.

  2. Resonance stabilization: The inorganic phosphate (Pi) product has more resonance stabilization possibilities than the phosphate groups within ATP, contributing to a lower energy state for the products.

  3. Increased entropy: Breaking one ATP molecule into two separate molecules (ADP and Pi) increases the disorder (entropy) of the system, which is thermodynamically favorable.

  4. Stabilization by hydration: The products (ADP and Pi) are more extensively hydrated (stabilized by interaction with water molecules) than ATP itself, further contributing to their lower free energy.

Reaction Coupling via ATP Hydrolysis

• The energy released from ATP hydrolysis is harnessed to drive energetically unfavorable reactions by a process called reaction coupling, often involving a shared intermediate.

• Example of glutamine synthesis from glutamate illustrated:

  • The direct synthesis of glutamine from glutamate and ammonia is energetically unfavorable (positive $\Delta G$).

  • This unfavorable condition is overcome through coupling to ATP hydrolysis, catalyzed by glutamine synthetase.

  • Detailed pathway involves:

  1. Phosphorylation of glutamate: ATP donates its terminal phosphate group to glutamate, forming a high-energy intermediate, glutamyl phosphate, and releasing ADP. This step activates the glutamate.

  2. Ammonia attack: The ammonia molecule then attacks the activated glutamyl phosphate, displacing the phosphate group and forming glutamine.

  • The net reaction, when coupled, has a negative overall $\Delta G$, allowing glutamine synthesis to proceed efficiently.

Additional Energy Sources Beyond ATP

• Mention of phosphocreatine (also known as creatine phosphate) as another high-energy phosphate compound found predominantly in muscle and nerve cells. It serves as a rapidly mobilizable reserve of high-energy phosphates to quickly regenerate ATP.

• During intense physical activity, when ATP demand increases rapidly, phosphocreatine donates its phosphate group to ADP to form ATP via the creatine kinase reaction. This quick ATP re-phosphorylation allows for a burst of energy and significantly increases stamina and power output for short durations.

Conclusion and Further Discussion

• Wrap-up of the biochemical principles discussed, emphasizing the critical relationship between carbohydrate