CARBOHYDRATE OVERVIEW

MLAB 2401: Clinical Chemistry

Keri Brophy-Martinez

Introduction2

Organisms rely on the oxidation of complex organic compounds to obtain energy

Three general types of compounds provide chemical energy to our cells

Lipids=Fats Amino acids = Proteins Carbohydrates= Sugars, starches

Carbohydrates3

Major food source & energy supply of body

Primary source of energy for brain, erythrocytes, retinal cells

Depending on individual diet, 50-90% of the body's carbohydrate intake is in the form of

Grains - cereals, bread Starchy vegetables - potatoes Legumes - beans, peanuts other sources = sugar, molasses, lactose from milk, fructose from fruit

Stored primarily as liver and muscle glycogen

Description and Classification of Carbohydrates

Contain C, H and O moleculesContain a C=O (ketone) and an –

OH(aldehyde) functional group

Classification Based on certain properties

The size of the base carbon chain Location of the CO functional group Number of sugar units Stereochemistry of compound

Chemical Properties5

Some ( not all ) carbs are reducing substances (donate electrons) Chemical reduction of other substances These sugars must contain an aldehyde or ketone group Reducing sugars

Glucose Maltose Lactose Fructose Galactose

Sucrose is not a reducing substance

Carbohydrate Metabolism

Glucose is primary energy sourceNervous tissue can not concentrate or store

carbohydrates, so a steady supply of glucose is needed

Once the level of glucose falls below a certain range, normal function is impaired

Carbohydrate Breakdown

Dietary

Carbohydra

tes

•Mouth•Salivar

y amylase

Dextrins/

Maltose

•Stomach/Intestines

•Pancreatic amylase

Monosaccharide

•Absorption into intestinal mucosa

•Delivered to liver

Carbohydrate Breakdown

Ultimate Goal Convert glucose to CO2 and water with ATP as a by-

product

Possible channels Converted to liver glycogen and stored Metabolized to CO2 and H2O Converted to keto-acids, amino acids, and proteins Converted to fats and stored in adipose tissue

Biochemical Pathways in Carbohydrate Breakdown

Embden-Meyerhoff pathway Converts glucose to pyruvate/lactate Primary energy source for humans

Hexose monophosphate shunt Oxidizes glucose to ribose and CO2 Produces NADPH as an energy source

Glycogenesis Converts glucose to glycogen

Carbohydrate Metabolism10

Glycolysis – the conversion of glucose and other hexoses into lactate or pyruvate

Breakdown of glucose for energy production

Glycogenesis – the conversion of glucose to glycogen usually in liver & muscle Excess glucose is converted and stored as

glycogen High concentrations of glycogen in liver and

skeletal muscle Glycogen is a quickly accessible storage form of

glucose

Carbohydrate Metabolism11

Glycogenolysis – the breakdown of glycogen to form glucose Glycogenolysis occurs when plasma glucose is decreased Occurs quickly if additional glucose is needed Controlled by hormones & enzymes

Gluconeogenesis – the formation of glucose from non-carbohydrate sources, such as amino acids, glycerol & fatty acids into glucose

Occurs mainly in the liver

Glucose

Glycogenesis

Glycogenolysis

Glycolysis

Gluconeogenesis

Carbohydrate Metabolism13

Also related: Lipogenesis – the conversion of carbohydrates to fatty

acids Fat is another energy storage form, but not as quickly

accessible as glycogen Lipolysis – the decomposition of fat

The sum or net of all of these processes determines the level of blood glucose.

Regulation of Plasma Glucose14

Organs / systems involved in glucose regulation

Liver : Glucose Glycogen Glucose

Muscle Skeletal & heart

Pancreas Synthesizes hormones Insulin and Glucagon,

somatostatin

Other Endocrine glands Anterior pituitary gland ( growth hormone) Adrenal gland (epinephrine and cortisol) Thyroid gland (thyroxine)

Regulation of Plasma Glucose15

If plasma glucose is decreased : Glycogenolysis

The liver releases glucose into the plasma (quick response) Gluconeogenesis and lipolysis

If plasma glucose is increased :

Glycogenesis Liver stores glucose as glycogen

Lipogenesis Formation of lipids

Hormones that Regulate Glucose

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Insulin Most important & only one to decrease

glucose level Synthesized in the Beta cells of the Islets of

Langerhans (in the pancreas) Released when plasma glucose is increased

Action / Effects of insulin

Facilitates glucose entry into cells cell membranes need insulin to be present for glucose to enter

Promotes liver glycogenesis glucose to glycogen

Promotes glycolysis speeds up utilization of glucose in cells

Promotes synthesis of lipids from glucose Such as the formation of Triglycerides

Promotes amino acid synthesis from glucose intermediates

Decreases / inhibits glycogenolysis and gluconeogenesis

Insulin Control18

Insulin secretion controlled by: Blood glucose level Certain Amino Acids ie. leucine, & arginine

Counterregulatory Hormones 19

Glucagon 2nd most important glucose regulatory hormone

Referred to as a hyperglycemic agent

Synthesized in alpha cells of the islets of Langerhans

Action/Effect of Glucagon20

Stimuli – decreased plasma glucoseAction

Increases glycogenolysis & gluconeogenesis Promotes breakdown of fatty acids Promotes breakdown of proteins to form amino

acids Increases plasma glucose concentration

Other Regulatory Hormones 21

Epinephrine One of two glucose regulating hormones from the

adrenal gland Origin – adrenal medulla Action/effect

Inhibits insulin secretion & release Promotes lipolysis Stimulates glycogenolysis Immediate release of glucose

Stimuli Neurogenic - based on physical / emotional stress. Adrenal tumors

Other Regulatory Hormones 22

Glucocorticoids - such as cortisol Origin – adrenal cortex

Effect – antagonistic to insulin increases blood glucose promotes gluconeogenesis from breakdown of proteins inhibits the entry of glucose into muscle cells

Stimuli – anterior pituitary’s ACTH

Other Regulatory Hormones 23

Growth Hormone (GH) and Adrenocorticotropic Hormone (ACTH) Origin – anterior pituitary gland Effect – antagonistic to insulin

Increases plasma glucose levels inhibits insulin secretion inhibits entry of glucose into muscle cells inhibits glycolysis inhibits formation of triglycerides from glucose

Stimuli decreased glucose stimulates its release increased glucose inhibits its release

Other Regulatory Hormones 24

Thyroid hormones (such as thyroxine) Origin – thyroid gland Effect

increases absorption of glucose from intestines Promotes comversion of liver glycogen to glucose

Stimuli – pituitary gland’s TSH

Other Regulatory Hormones

Somatostatin Origin-Delta cells of the islets of Langerhans in the

pancreas Effect - increase plasma glucose Actions

antagonistic to insulin, inhibits endocrine hormones including glucagon &

growth hormone

References

Bishop, M., Fody, E., & Schoeff, l. (2010). Clinical Chemistry: Techniques, principles, Correlations. Baltimore: Wolters Kluwer Lippincott Williams & Wilkins.

Sunheimer, R., & Graves, L. (2010). Clinical Laboratory Chemistry. Upper Saddle River: Pearson .

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