Biochemistry of Diabetes

Bruno SopkoAlice Skoumalová

Regulation of Carbohydrate metabolism Hormonal Regulation Markers Additional effects

Content

Regulation of Carbohydrate metabolism - Glycolysis

Regulation of Carbohydrate metabolism - Gluconeogenesis

Metabolism of glycogen

Overview of the major pathways of glucose metabolism

Regulation of enzymes:Types of regulations Mechanism Example

Substrate concentration Saturation kinetics (Michaelis-Menten equation)

Glucokinase (activation after a meal - high Km)

Allosterically A conformational change after an allosteric activator binding

Enzymes of glycolysis and gluconeogenesis (allosteric efectors: ATP, AMP, citrate)

Covalent modification A conformational change after phosphorylation by a protein kinase

Phosphorylation of glycogen synthase and glycogen phosphorylase (glucagon)

Protein-protein interaction

A conformational change after a modulator protein binding

Muscle glycogen phosphorylase (activation by Ca2+-calmodulin)

Zymogen cleavage Activation by proteolysis of a precursor molecule

Blood clotting proteins

Enzyme synthesis Induction or represion of enzyme synthesis

Enzymes of gluconeogenesis (induction during fasting)

Hormonal RegulationGlucose homeostasis: maintenance of blood glucose levels near 80 to 100 mg/dL (4.4-5.6

mmol/l)

insulin and glucagon (regulate fuel mobilization and storage)

Hypoglycemia prevention:

1. release of glucose from the large glycogen stores in the liver (glycogenolysis)

2. synthesis of glucose from lactate, glycerol, and amino acids in liver (gluconeogenesis)

3. release of fatty acids from adipose tissue (lipolysis)

Hyperglycemia prevention:

4. conversion of glucose to glycogen (glycogen synthesis)

5. conversion of glucose to triacylglycerols in liver and adipose tissue (lipogenesis)

Hormonal Regulation

Pathways regulated by the release of:

glucagon (in response to a lowering of blood glucose levels)

insulin (in response to an elevation of blood glucose levels)

Synthesis and secretion of insulin and glucagon: the islets of Langerhans (β- and α-cells)

preprohormone (modification - in ER, GC, SV)

Major sites of insulin action on fuel metabolism:

The storage of nutriens

• glucose transport into muscle and adipose tissue

• glucose storage as glycogen (liver, muscle)

• conversion of glucose to TG (liver) and their storage (adipose tissue)

• protein synthesis (liver, muscle)

• inhibition of fuel mobilization

Insulin receptor signaling: the tyrosine kinase activity

a dimer (α and ß subunits)

Signal transduction:

1. the ß-subunits autophosphorylate each other when insulin binds (activating the receptor)

2. the activated receptor binds and phosphorylates IRS (insulin receptor substrate)

3. multiple binding sites for different proteins

Major sites of glucagone action on fuel metabolism:

Mobilization of energy stores

1. release of glucose from liver glycogen

2. stimulating gluconeogenesis from lactate, glycerol, and amino acids (liver)

3. mobilizing fatty acids (adipose tissue)

Regulators of insulin and glucagon release:

Glucose Insulin Amino acids

Insulin + +

Glucagon - - +

Hormonal RegulationHormone Function Major metabolic pathways affected

Insulin • Promotes fuel storage after a meal• Promote growth

• Stimulates glucose storage as glycogen (muscle,liver)• Stimulates FA synthesis and storage after a high-carbohydrate meal• Stimulates amino acids uptake and protein synthesis

Glucagon • Mobilizes fuels• Maintains blood glucose levels during fasting

• Activates gluconeogenesis and glycogenolysis (liver) during fasting• Activates FA release from adipose tissue

Epinephrine • Mobilizes fuels during acute stress

• Stimulates glucose production from glycogen (muscle, liver)• Stimulates FA release from adipose tissue

Cortisol • Provides for changing requirements over the long-term

• Stimulates amino acid mobilization from muscle protein• Stimulates gluconeogenesis• Stimulates FA release from adipose tissue

Glucose receptors

Transporter Tissue distribution Comments

GLUT 1 ErythrocytesBlood-brain barierBlood-placentar barier

Present in high concentrations

GLUT 2 LiverKidneyPancreatic β-cellsIntestinal mucosa cells

A high Km for glucose

The glucose sensor in the pancreas

GLUT 3 Brain Major transporter in the brain

GLUT 4 Adipose tissueSceletal muscleHeart muscle

Insulin-sensitive transporter! The number increases on the cell surface.

GLUT 5 Intestinal epithelium A fructose transporter

Stimulation by insulin of glucose transport into muscle and adipose cells:

Binding of insulin to its cell membrane receptor causes vesicles containing glucose transport proteins to move from inside the cell to the cell membrane

Pathways affected by insulin

1. Carbohydrate metabolismstimulation of glucose utilization:

glycogen synthase ↑glycolysis ↑

inhibition of gluconeogenesisthe transport of glucose into tissues (muscle, adipose tissue)

2. Lipid metabolismstimulation of the glucose conversion into FA:

acetyl CoA carboxylase ↑NADPH (PPP ↑)

storage of fat:lipoprotein lipase ↑

inhibition of the degradation of fat:hormone sensitive lipase ↓

Pathways affected by insulin

1. Carbohydrate metabolismstimulation of glucose utilization:

glycogen synthase ↑glycolysis ↑

inhibition of gluconeogenesisthe transport of glucose into tissues (muscle, adipose tissue)

2. Lipid metabolismstimulation of the glucose conversion into FA:

acetyl CoA carboxylase ↑NADPH (PPP ↑)

storage of fat:lipoprotein lipase ↑

inhibition of the degradation of fat:hormone sensitive lipase ↓

Effects of insulin deficiency

1. Glucose uptake and utilization↓

2. Proteolysis↑

3. Gluconeogenesis↑

3. Degradation of fat↑

Hyperglycemia (≥ 9mmol/l)

Glucosuria

Hyperlipidemia

Metabolic acidosis

Ketonuria

Type I (insulin-dependent) Type II (non-insulin-dependent)

Incidence 10-20% 80-90%

Age childhood, the teens Middle-aged, older

Cause An autoimmune diseaseComplete absence of insulin

Unknown

Relative insulin deficiency

Symptoms Hyperglycemia, hypertriglyceridemia, ketoacidosis

Hyperglycemia, hypertriglyceridemia

Habitus Thinness Obesity

Ketoacidosis Yes No

Insulin Very low or absent Normal (increased)

Therapy Insulin Diet, drugs, insulin

Types of diabetes:

The oral glucose tolerance test (oGTT):

Used if:

elevated fasting levels of glucose - 5,3-6,7 mmol/l (for diagnosis of diabetes, screening of patients with impaired glucose tolerance)

screening of gestational diabetes

Procedure:

administration of 75g glucose in an aqueous solution

after overnight fasting (10h)

„common“ diet and physical activity during previous three days

be seated and do not smoke during the test

determination of the glucose levels in the capillary blood before the glucose load and after 60 and 120 minutes

Factors affecting oGTT: previous diet, infection, stress

The oral glucose tolerance test (oGTT):

The blood glucose level returns to the basal level by 2 hours

Diagnostic criteria for diabetes mellitus (according to WHO)

1.

or

diabetic symptoms (polyuria, polydipsia, weight loss)+ plasmatic glucose level ≥ 11,1 mmol/l

2.

or

plasmatic glucose level after fasting ≥ 7 mmol/l

3.

or

plasmatic glucose level 2h after glucose intake (oGTT) ≥ 11,1 mmol/l

4. Glycated haemoglobin 38-47 mmol/mol (prediabetes)Glycated haemoglobin over 48 mmol/mol (diabetes)

Factors influencing glucose measurements:

Biological material Metoda Cause of interference

Explanation of interference

Blood All Delay Erythrocytes oxidize glucose

Blood Glucose-oxidase Acetaminophen Increase measured values

Plasma Spectrophotometry Haemolysis or hyperlipidemia increases opacity

Change in measurement linearity

Urine All Bacteriuria Glucose consumption

Urine Benedict's test Reducing factors (vit. C, saccharides, ↑urates, ↑creatinine)

False positive

Urine Glucose-oxidase DetergentsVit. C, keton bodies

False positiveFalse negative

Diabetes Screening(ADA recommendation):

1. over 45 years old, mainly in case BMI > 25kg/m2 repeat every 3 years

2. younger overweight people (BMI > 25kg/m2) with further risk factors: physical inactivity parents or siblings with diagnosed diabetes previous diagnosis of IGT or IFG syndrome of polycystic ovaria hypertension or dyslipidemia gestational diabetes or newborn > 4kg vascular diseases

The chronic diabetes complications:

A. Microvascular (diabetic retinopathy, nefropathy, neuropathy)

nonenzymatic glycation of proteins in vascular tissue

B. Macrovascular (atherosclerosis)

nonenzymatic glycation of proteins in vascular tissue and lipoproteins

C. Diabetic cataract:

increased osmolarity of the lens (increased activity of the polyol pathway → ↑sorbitol)

nonenzymatic glycation of proteins of lens

Hyperglycemia - protein glycation: hemoglobin

vascular tissue proteins → contribute to the diabetic complications (cataracta, atherosclerosis, retinopathy, nephropathy)

Glycated proteins:

- impaired structure and fucntion

The importance of the maintance of low glucose levels in diabetic patients !

Diabetic cataract :

↑glucose concentration in the lens → ↑aldose reductase activity → sorbitol accumulation → ↑osmolarity, structural changes of proteins

Lens metabolism:

Protein kinase C and DAG

Protein kinase C and DAG

Protein kinase C and DAG

Literature:Marks´ Basic Medical Biochemistry, A Clinical Approach, third edition, 2009 (M. Lieberman, A.D. Marks)

Color Atlas of Biochemistry (J. Koolman, K.H. Roehm)

Francois R. Jornayvaz1 and Gerald I. Shulman, Diacylglycerol Activation of Protein Kinase Cε and Hepatic Insulin Resistance, Cell Metabolism 15, 2012