Diabetes and Insulin

What does the literal name Diabetes Mellitus mean, and what are the two core clinical tests used to diagnose it according to this lecture?

Literal Meaning: Diabetes = increased urine output; Mellitus = honey sweet.

• Core Diagnostic Tests: 1. Fasting plasma glucose test. 2. Oral Glucose Tolerance Test (OGTT).

Contrast Type 1 and Type 2 Diabetes Mellitus based on their underlying etiology, typical onset window, and cellular defects.

Type 1 Diabetes: Insufficient insulin production driven by the autoimmune destruction of pancreatic beta-cells. It typically develops early in life (formerly juvenile/insulin-dependent diabetes).

Type 2 Diabetes: Insulin resistance where target cells fail to respond appropriately to circulating insulin. It typically develops in late adulthood and is strongly associated with obesity.

Match the clinical symptom of diabetes to its physiological mechanism/term:

1. Excessive thirst

2. Frequent urination

3. Excessive hunger

4. Fruity-smelling breath

1. Polydipsia: Driven by intracellular dehydration as high blood glucose draws water out of cells via osmosis.

2. Polyuria: Occurs when blood glucose exceeds the renal threshold, spilling into urine and causing osmotic diuresis.

3. Polyphagia: Triggered because cells are structurally starving without insulin to facilitate glucose entry.

4. Ketoacidosis: Caused by alternative fat metabolism producing volatile acetone as a byproduct.

Describe the exact peptide structure of active insulin, including its chain lengths, precursor format, and chemical bonds.

Precursor: Synthesised as a single-chain precursor called proinsulin.

• Active Molecule: Composed of two distinct polypeptide chains: the A chain (21 amino acids) and the B chain (30 amino acids).

• Bonds: The two chains are held together covalently by two interchain disulfide bonds (and one additional intrachain disulfide bond within the A chain).

Detail the physical structure of the Insulin Receptor (IR) complex and describe its immediate conformational action upon insulin binding.

Structure: A heterotetrameric transmembrane glycoprotein consisting of two extracellular alpha-subunits (which contain the insulin-binding domains) and two transmembrane beta-subunits (which contain intrinsic tyrosine kinase activity).

Immediate Action: Insulin binding to the alpha-subunits triggers a conformational change that causes the internal beta-subunits to autophosphorylate specific tyrosine residues, turning on its kinase activity.

Following receptor autophosphorylation, how does Insulin Receptor Substrate-1 (IRS-1) propagate the metabolic signal downstream?

1. The activated beta-subunits phosphorylate tyrosine residues on IRS-

2. Phosphorylated IRS-1 acts as a docking platform that recruits and binds Phosphoinositide 3-Kinase (PI3K) via its SH2 domains.

3. 3. PI3K phosphorylates the membrane phospholipid PIP₂ to convert it into PIP₃

Trace the signaling pathway from membrane-bound PIP₃ to the activation of Protein Kinase B (PKB/Akt).

1. Accumulation of PIP₃ recruits both PDK1 (Phosphoinositide-dependent kinase-1) and Akt/PKB to the plasma membrane.

2. PDK1 physically phosphorylates and activates Akt/PKB.

3. Activated Akt diffuses into the cytoplasm to phosphorylate downstream targets that govern glycogen synthesis, protein synthesis, and cell survival.

What primary molecular mechanism turns off the insulin signaling cascade to prevent over-signaling?

Protein tyrosine phosphatases physically remove the activating phosphate groups from the tyrosine residues of the insulin receptor and IRS-1, returning them to their inactive, unphosphorylated states.

State the anatomical site, physiological mechanism, and therapeutic outcome of SGLT2 inhibitors like Dapagliflozin.

Site: Proximal convoluted tubules of the kidneys.

• Mechanism: Blocks the Sodium-Glucose Linked Transporter 2 (SGLT2), which is responsible for reabsorbing ~90% of filtered glucose back into the blood.

• Outcome: Lowers blood glucose by forcing excess glucose out of the body through the urine (glucuresis), entirely independent of insulin action.

What is the "Incretin Effect," and how do drugs like Semaglutide exploit this pathway?

Incretin Effect: Oral glucose causes a significantly higher surge of insulin release than an identical dose given intravenously, because oral food triggers gut hormones called incretins (GLP-1 and GIP).

• Semaglutide Mechanism: Acts as a synthetic GLP-1 Receptor Agonist that mimics GLP-1. It binds to pancreatic receptors to enhance glucose-dependent insulin secretion, suppress glucagon release, and delay gastric emptying.

Why does the body's natural GLP-1 hormone have an incredibly short half-life, and how do drugs like Sitagliptin or Vildagliptin fix this?

The Problem: Natural endogenous GLP-1 is rapidly broken down and inactivated within minutes by the enzyme Dipeptidyl peptidase-4 (DPP4).

• The Fix: DPP4 Inhibitors (e.g., Sitagliptin, Vildagliptin) bind to and block this enzyme. This stabilizes endogenous GLP-1, lengthening its circulatory half-life so it can continue stimulating the first-phase insulin response.

What naturally occurring plant alkaloid is explicitly highlighted in the lecture slides for its anti-hyperglycemic activity via DPP4 inhibition?

Berberine. It operates as a herbal remedy that inhibits the DPP4 enzyme pathway to help stabilise incretins and lower blood glucose.

Classify the multi-organ damage caused by uncontrolled diabetes into macrovascular vs. microvascular pathologies, and state the primary clinical endpoint of each as emphasized in the lecture.

• Macrovascular Disease (Large Blood Vessels): Driven by extra lipid accumulation and accelerated atherogenesis in large arteries.

  • Clinical Endpoints: Heart Attack (Myocardial Infarction) and Stroke.

• Microvascular Disease (Small Blood Vessels): Driven by basement membrane thickening and advanced glycation end-product (AGE) tissue toxicity.

  • Clinical Endpoints: Kidney Failure (Diabetic Nephropathy)—Note: Diabetes is the leading cause of kidney failure—Blindness (Diabetic Retinopathy), and Nerve Damage (Diabetic Neuropathy).

What specific metabolic error or biochemical imbalance is the cause of acute blurry vision in diabetic patients?

It is driven by a functional error in sucrose metabolism and acute osmotic fluid shifts within the crystalline lens of the eye, caused by high ambient glucose concentrations altering the refractive index.

Trace the systemic biochemical transition that causes a patient to develop diabetic ketoacidosis and display classic fruity-smelling breath.

Because cells lack insulin signaling, they cannot utilise glucose and enter a state of functional starvation.

1. The body compensates by drastically accelerating lipolysis (fat breakdown) to release free fatty acids.

2. The liver metabolises these fatty acids into ketone bodies (acetoacetate and beta-hydroxybutyrate) for energy, which are acidic and lower blood pH.

3. A volatile metabolic byproduct of this pathway, acetone, is excreted via the lungs, creating the characteristic sweet/fruity-smelling breath.

In the insulin signaling cascade, how do downstream enzymes structurally recognise and attach to a phosphorylated IRS-1 molecule?

Phosphorylated tyrosine residues on IRS-1 create highly specific docking sites that are structurally recognised and bound by the SH2 domains (Src Homology 2 domains) present on downstream signaling proteins, specifically the regulatory subunit of Phosphoinositide 3-Kinase (PI3K).

State the exact biochemical substrate and product of the enzyme PI3K within the inner leaflet of the plasma membrane.

PI3K phosphorylates the membrane-bound phospholipid PIP₂ (Phosphatidylinositol 4,5-bisphosphate) to convert it into the crucial second messenger PIP₃ (Phosphatidylinositol 3,4,5-trisphosphate).

How does the physical accumulation of PIP₃ at the membrane cause PDK1 to find and activate Akt/PKB?

• Both PDK1 and Akt/PKB contain highly specialised structural motifs called PH domains (Pleckstrin Homology domains).

• These PH domains have a high, specific affinity for binding directly to PIP₃

• This chemical attraction pulls both enzymes out of the fluid cytoplasm and holds them right next to each other at the membrane interface, allowing PDK1 to physically phosphorylate and activate Akt.

Once Protein Kinase B (Akt/PKB) is fully phosphorylated and activated, what three broad intracellular processes does it immediately orchestrate to handle a postprandial glucose spike?

Glucose Uptake: Triggers the mechanical translocation and fusion of GLUT4 storage vesicles with the cell membrane.

1. Glycogen Synthesis: Phosphorylates and inactivates Glycogen Synthase Kinase 3 (GSK3), which removes the inhibitory brake on Glycogen Synthase, allowing glucose to be packed away into glycogen polymers.

2. Protein Synthesis & Survival: Promotes cellular growth, protein translation pathways, and blocks pro-apoptotic signaling.

While SGLT2 inhibitors like Dapagliflozin successfully lower blood glucose via insulin-independent glucuresis, what specific clinical side-effect risk must be monitored, and what therapeutic advantage do they offer?

Risk: Increased risk of urinary tract infections (UTIs) and genital mycotic infections due to the high concentration of sugar left passing through the urinary tract, which provides a rich breeding ground for microbes.

• Advantage: They lower blood glucose safely without inducing hypoglycemia because their mechanism of action bypasses pancreatic insulin secretion entirely.

What long-term microvascular risk was noted in a referenced 2016 study following rapid glycemic correction with the GLP-1 receptor agonist Semaglutide?

The 2016 study suggested a transient or localised increased rate of retinopathy complications (damage to retina vessels), alongside minor intestinal/gastrointestinal issues, during the initial phases of intensive therapy.

What advanced pharmacological combination strategy is suggested to maximise glycemic control while minimising individual drug toxicities?

The development and clinical deployment of combination therapies combining both GLP-1R agonists and SGLT2 inhibitors.

• This pairs the insulin-sensitising, satiety-inducing actions of incretins with the active renal clearance of glucose from the bloodstream

Which domain binds PIP₃

PH domain

which domain binds phosphorylated IRS-1

SH2 domain