Lecture 9

Absorptive State

  • Definition: The absorptive state refers to the period during digestion when nutrients are absorbed from food, and the body utilizes them for energy and storage.

  • Processes Involved:

    • Glycogen Synthesis: The conversion of glucose into glycogen for storage.

    • Triglyceride Synthesis: Formation of triglycerides from fatty acids and glycerol.

    • Metabolic Fuel: Glucose serves as a primary metabolic fuel during this state.

Learning Outcomes

  • Goals of the Biochemistry Course (BM210):

    • Glycolysis Reactions: Understanding the enzymatic steps involved in glycolysis.

    • Cori Cycle: Comprehending the process and significance of the Cori cycle in energy metabolism.

    • Glycolysis Regulation: Exploring mechanisms through which glycolysis is regulated.

    • Glycogenin and Glycogen Synthase Functions: Learning the roles of these enzymes in carbohydrate metabolism.

    • Fatty Acid Synthesis: Understanding the biosynthesis of fatty acids from carbohydrates.

    • Triacylglyceride (Triglyceride) Synthesis: Appreciating the synthesis and role of triglycerides in energy storage.

Part A: Glycolysis

Definition of Glycolysis

  • Glycolysis is defined as the sequence of biochemical reactions that metabolizes one molecule of glucose into two molecules of pyruvate, yielding a net production of two ATP molecules.

  • Net Reaction Equation:
    extD−glucose+2extNAD++2extADP+2extPi<br>ightarrow2extpyruvate+2extATP+2extNADH+2extH++2extH2extOext{D-glucose} + 2 ext{NAD}^+ + 2 ext{ADP} + 2 ext{Pi} <br>ightarrow 2 ext{pyruvate} + 2 ext{ATP} + 2 ext{NADH} + 2 ext{H}^+ + 2 ext{H}_2 ext{O}

Pathway Overview

  • Key steps involved in glycolysis include:

    • Phosphorylation Reactions: Two phosphorylation reactions produce fructose 1,6-bisphosphate.

    • Cleavage: Fructose 1,6-bisphosphate is cleaved into two triose phosphates.

    • Molecular Rearrangements: A series of rearrangements occur:

    • Two ADP molecules are phosphorylated to form two ATP per triose phosphate (total of four ATP per glucose).

    • Each triose phosphate undergoes oxidation, yielding one NADH (total of two NADH per glucose).

    • Pyruvate Formation: The final product, pyruvate, is formed.

Anaerobic Glycolysis

  • Glycolysis occurs anaerobically, meaning it does not require oxygen.

  • Redox Balance Maintenance: The regeneration of NAD+ is crucial, as depletion of NAD+ halts glycolysis.

Control of the Glycolytic Pathway

Dual Role of Glycolysis

  • Glycolysis serves two primary functions:

    1. ATP Generation: Degradation of glucose to generate ATP for energy.

    2. Building Blocks Provision: Supplies intermediates for synthetic reactions like fatty acid synthesis.

Control Sites in Glycolysis

  • Key enzymes catalyzing irreversible reactions in glycolysis are potential control points:

    • Hexokinase: Inhibited by its product glucose 6-phosphate.

    • Phosphofructokinase:

    • Committed step in glycolysis.

    • Inhibition factors: ATP, low pH, citrate.

    • Activation factors: AMP, fructose 2,6-bisphosphate.

    • Pyruvate Kinase:

    • Inhibition by ATP and alanine.

    • Activation by fructose 1,6-bisphosphate.

Regeneration of Fructose 2,6-Bisphosphate

  • Fructose 2,6-bisphosphate is synthesized by the enzyme phosphofructokinase 2.

Phosphofructokinase 2 Regulation

  • Bifunctional Enzyme: Contains both kinase and phosphatase activity.

  • Regulation: Reciprocal control via phosphorylation of serine 460 by protein kinase A.

    • Phosphorylated form promotes phosphatase activity.

    • Dephosphorylated form promotes kinase activity.

Fate of Pyruvate

Diverse Fates

  • Pyruvate can undergo various metabolic fates depending on the oxygen availability:

    • In the presence of oxygen: Enters the TCA cycle and electron transport chain.

    • In the absence of oxygen: Undergoes fermentation processes, leading to lactate production in mammals and ethanol in yeast.

Lactic Acid Fermentation

  • Occurs when oxygen is scarce, resulting in the regeneration of NAD+ from NADH, allowing glycolysis to continue.

    • Consequences: Leads to lactic acid buildup, creating an oxygen debt that must be resolved post-exercise.

Cori Cycle

  • The Cori cycle illustrates the recycling of lactate originating from anaerobic glycolysis in muscles:

    • Lactate is converted back to pyruvate in cardiac muscle cells.

    • In the liver, lactate is converted to glucose via gluconeogenesis—a critical process for maintaining blood glucose levels.

Part B: Glycogen Synthesis

Overview of Glycogen

  • Glycogen serves as a readily mobilized storage form of glucose.

  • Functions:

    • Controlled release of glucose helps maintain blood glucose levels.

    • Major storage locations are the liver (8-10% of fresh weight) and skeletal muscles (1-2% of muscle mass).

    • In pregnant women, glycogen stores in the uterus provide nourishment for the embryo.

Similarity to Starch

  • Glycogen closely resembles amylopectin but has a higher frequency of branching, occurring every 10th glucose unit.

Stages of Glycogen Synthesis

  • Initiation Stage: Autocatalytic synthesis initiated by glycogenin.

  • Elongation Stage: Catalyzed by glycogen synthase with the help of a branching enzyme.

Glycogenin Function

  • Glycogenin is a glycosyl-transferase homodimer that initiates glycogen biosynthesis.

  • It catalyzes the addition of glucose monomers derived from UDP-glucose to the hydroxyl group of tyrosine 194 via autocatalysis.

Regulation of Glycogen Synthase

  • Glycogen synthase's activity is modulated by:

    • Covalent Modifications: It is phosphorylated by protein kinase A and glycogen synthase kinase 3 (GSK3), converting it from the active a form to the inactive b form.

    • Allosteric Regulation: The b form remains active in the presence of high concentrations of glucose 6-phosphate.

UDP-Glucose in Glycogen Synthesis

  • The immediate precursor for glycogen synthesis is uridine diphosphate glucose (UDP-glucose).

  • Formation of UDP-glucose occurs through:
    extglucose−1−phosphate+extUTP<br>ightleftharpoonsextUDP−glucose+extPPiext{glucose-1-phosphate} + ext{UTP} <br>ightleftharpoons ext{UDP-glucose} + ext{PPi}

  • Hydrolysis of pyrophosphate (PPi) into two molecules of inorganic phosphate assures the irreversibility of this reaction.

Branching Enzyme in Glycogen Formation

  • The enzyme that facilitates branching is known as amylo (1,4→1,6) transglycosylase, which transfers blocks typically composed of 7 residues to internal sites within the glycan chain, creating α(1,6) linkages.

Importance of Glycogen Branching

  • Increased solubility of glycogen.

  • Creation of a large storage form while allowing rapid synthesis and degradation.

Key Terms Related to Glycogen Bioenergetics

  • Glycogenin, Glycogen Synthase: Enzymes critical in glycogen metabolism.

  • UDP-Glucose: Key precursor in glycogen synthesis.

  • Pyrophosphate hydrolysis: A crucial step providing energy for the synthesis pathway.

Part C: Fatty Acid and Triacylglycerol (Triglyceride) Synthesis

Overview of Fatty Acid Synthesis

  • Fatty acids are synthesized in the cytoplasm during the fed state when excess carbohydrate and acetyl CoA are available.

  • They serve as precursors for triacylglycerides (TG), which are the main storage form of fats.

Fatty Acid Chain Growth

  • Fatty acid elongation involves the sequential addition of two-carbon units from acetyl CoA, with malonyl ACP serving as the activated donor, driven by decarboxylation.

Mammalian Fatty Acid Synthase Structure

  • Composed of multiple domains within a single polypeptide chain, functioning as a dimer comprising two 272 kDa subunits.

Overview of Triacylglycerol Synthesis

  • Synthesis requires transporting fatty acids made from glucose or amino acids involved in lipid metabolism and occurs in tissues such as the liver and adipose tissue.

  • Triacylglycerol Characteristics:

    • Cannot cross cell membranes without breakdown by lipases, necessitating transport via lipoproteins (e.g., VLDL, chylomicrons).

Activation of Fatty Acids

  • Fatty acids are activated before utilization by forming high energy thioester bonds through the action of acyl CoA synthetase.

Pathway Yield and Energy Involvement

  • Energy requirements for fatty acid synthesis necessitate high energy investment, requiring ATP for the attachment of the acyl group, making it a complex and regulated process of energy storage.