Glycogenesis Notes

Glycogenesis

Glycogenesis refers to the beginning of glycogen.

• "Glyco" (prefix) = glycogen
• "Genesis" (suffix) = beginning of

Glycogen

Glycogen is the stored form of glucose in animals, analogous to starch in plants. Glucose is a monosaccharide, a simple sugar, used to create ATP (adenosine triphosphate). One molecule of glucose can produce approximately 34-36 ATP molecules, making it an efficient energy source.

When Glycogenesis Occurs

Glycogenesis primarily happens during fed states, particularly in the absorptive state (0-4 hours after eating). During this period, the body stores ingested glucose as glycogen.

Glucose Structure

Glucose has the molecular formula C6H{12}O_6. It consists of a ring-shaped structure with six carbons, labeled C1 through C6.

Steps of Glycogenesis

1. Glucose to Glucose 6-Phosphate:

   • The first step involves converting glucose to glucose 6-phosphate, which is also the first step in glycolysis.
   • This is achieved by using the enzyme hexokinase (in most tissues) or glucokinase (in liver and pancreatic cells, which is not explicitly mentioned in the transcript but relevant).
   • ATP (adenosine triphosphate) is used to donate a phosphate group to the sixth carbon of glucose, forming glucose 6-phosphate and ADP (adenosine diphosphate).
   • Glucose + ATP \rightarrow Glucose-6-Phosphate + ADP

2. Glucose 6-Phosphate to Glucose 1-Phosphate:

   • The enzyme phosphoglucomutase transfers the phosphate group from the sixth carbon to the first carbon, creating glucose 1-phosphate.

3. Glucose 1-Phosphate to UDP-Glucose:

   • Uridine triphosphate (UTP), a nucleotide, reacts with glucose 1-phosphate.
   • Uridine diphosphate (UDP) is attached to the first carbon of glucose, forming UDP-glucose and inorganic pyrophosphate (two phosphates).
   • Glucose-1-Phosphate + UTP \rightarrow UDP-Glucose + PPi

4. Glycogen Synthesis:

   • The enzyme glycogen synthase links UDP-glucose molecules together, releasing UDP.
   • Glycogen synthase facilitates the linking of glucose molecules at the first and fourth carbons, forming α-1,4-glycosidic bonds.
   • UDP-Glucose + Glycogen{(n ewline -1) glucose ewline units} \rightarrow UDP + Glycogen{(n
     ewline glucose
     ewline units)}

5. Branching:

   • Branching occurs when the 6th carbon binds to the first carbon via α-1,6-glycosidic bonds. Glycogen branching enzyme creates these branches, which is crucial for glycogen's solubility and efficient synthesis/degradation.
   • This results in a highly branched molecule of glycogen.
   • The types of bonds present in glycogen are 1-4 (primary) and 1-6 (branching).

Glycogen vs. Starch vs. Cellulose

• Glycogen: Storage form of glucose in animals, highly branched.
• Starch: Storage form of glucose in plants, similar to glycogen but less branched. Contains α-1,4-glycosidic bonds.
• Cellulose: Structural component of plant cell walls. Features 1-4 bonds, but every second glucose molecule is flipped upside down. Has β-1,4-glycosidic bonds which are indigestible by humans.

Digestion

• Amylase: Enzyme that breaks down glycogen and starch by cleaving 1-4 and 1-6 bonds.
• Humans cannot digest cellulose because they lack the enzymes to break the specific β-1,4-glycosidic bonds that occur when every second glucose is flipped.
• Indigestible carbohydrates become fiber.

Additional Notes

• Glycogenesis is stimulated by insulin, which is released in response to high blood glucose levels after a meal. This was not explicitly mentioned in the transcript, but important to note.
• The liver and skeletal muscles are the primary sites of glycogen storage.