Carbohydrates

Carbohydrates

Compounds containing C, H, O Classification of CHO is based on four

different properties: The size of the base carbon chain The location of the CO functional group. The number of sugar units The stereochemistry of the compound.

Major sources of energy for the body. 4 classifications: based on the number of

sugar units in the chain Monosaccharide Disaccharide Polysaccharide oligosaccharide

Monosaccharide

CHO derivative formed by the addition of chemical group: phosphate, sulfate, and amines.

Example: glyceraldehydes (3 carbon compound- smallest CHO)

CHO is aldehyde: called aldose CHO is ketone: called ketose D- and L- form used to describe possible

isomers of glucose. Ex: ( D-glucose and L-glucose.)

CHO forms depend on: Fisher projections Hawarth Projections Most CHO are of the D- forms and are

monosaccharide such as D-glucose, D-fructose, etc.

Disaccharides

Formed from two monosaccharide with the production water.

Most common form is sucrose (table sugar), which is glucose and fructose and is a non reducing sugar. Other forms include: Lactose (glucose and galactose) and maltose (malt product) and are both reducing agents.

Polysaccharides

Starch- plants (cellulose); not digested by humans.

Glycogen: stored form of CHO in the liver. Formed by the combination of

monosaccharide. Two CHO molecules join to generate water. Two CHO molecules spilt to loose water-

hydrolysis.

Starch: principle CHO (polysaccharide) storage product of plants

Glycogen: principle CHO storage product in animal.

Glycoside linkage of CHO involves many CHO- some carbons favor linking depending on the CHO.

All monosaccharide and many disaccharides are reducing agents because they contain a free aldehyde or ketone that can be oxidized.

Example of reducing agents is maltose and lactose are reducing agents because they contain a free aldehyde or ketone that can be oxidized.

Starch and glycogen: storage products, range in different sizes, similar because of their main chain composed of 1,4 glycoside linkage.

Glycogen is more branched than starch and is shorter (branching permits more larger amount of CHO in small volume)

Glucose Metabolism

Glucose is a primary source of energy. Various tissues and muscles throughout

the body including ECF depend on glucose for energy.

If glucose levels fall below certain levels the nervous tissue lose its primary energy source and is incapable of maintaining normal function.

Fate of glucose

CHO is digested as starch and glycogen. Amylase digest the no absorbable forms of

CHO to dextrin and disaccharide which are hydrolyzed to monosaccharide by maltose.

Maltase is an enzyme released by intestinal mucosa.

Sucrase and lactase: enzymes that hydrolyze sucrose to glucose; fructose and lactose to glucose and galactose.

Lactose intolerance: due to a deficiency of lactase enzyme on or in the intestinal lumens, which is need to metabolize lactose. Results in an accumulation of lactose in stomach as waste lactic acid- causing the stomach upset and discomfort.

Glucose metabolism : disaccharides are converted into monosaccharide – absorbed by the stomach transported to the liver by the hepatic portal venous blood supply.

Glucose is the only CHO to be directly use for energy or stored as glycogen. Others have to be broken down then utilized for energy and storage.

After glucose is absorbed it can go into one of three metabolic pathways based on (1) availability of substrate and (2) nutritional status of cell.

Ultimate goal to convert glucose to CO2 and H2O.

Requires ATP and ADP, O2 in the final step.

NADH acts as intermediate – ATP is gained.

1st step in all pathways is Glucose is converted to glucose -6 phosphate using ATP- catalyzed by hexokinase.

Glucose-6- phosphate enters the pathway s to generate energy from glucose by:

Glycolysis (Embden-Meyerhof ) Hexose Monophosphate Shunt or (PPP) Glycogenesis (storage of glucose as

glycogen)

Embden-Meyerhof (EM)

Glucose is broken down into two- 3 carbon molecules of pyruvic acid. This enters TCA-cycle and oxidized to 2 molecules of lactic acid.

Enters anaerobic glycolysis- no O2 required; this important for body function and tissue function that required little or no oxygen supply for energy production.

2 molecules of ATP for each mole of glucose 4 molecules of ATP- net gain of 2 moles of ATP.

EM continue

Glycerol released from the hydrolysis of triglyceride; which enters at 3-phosphoglycerate.

Beta- oxidation: where fatty acids, ketones and some amino acids are catabolized to acetyl CoA.

Most amino acids enter as pyruvate.

Hexose Monophosphate Shunt

2nd energy pathway Adetour for glucose -6-phosphate from glycolytic

pathway to convert and become 6-phosphogluconic acid.

Formation of ribose-5-phosphate and nicotinamide dinucleotide phosphate.

Allows pentose (ribose) to enter glycolytic pathway.

If energy requirements met within the body – the glucose goes to storage as glycogen.

3rd pathway: Final stage Conversion of glycerol, lactate, pyruvate to

glucose- occurs by amino acid conversion by the liver and kidneys.

Glucose-6-phosphate converted to glucose-1-phosphate to uridine diphosphoglucose then to glycogen.

Liver and muscle synthesize glycogen. Within the liver, heptocyte release glucose

to maintain blood glucose levels. Glucose-6-phophate is necessary, if

glucose is absent it is not metabolize.

Regulation of carbohydrate metabolism:

The liver, pancreas and endocrine gland keep blood glucose levels within a narrow range.

During brief fasting states (between meals) glucose supplied to ECF from the liver through glycogenolysis.

Long fasting states- glucose is synthesized from tissue by glycogenolysis.

Glycogensis: process if glycogen is converted back to glucose-6-phosphate for entry into glycolytic path.

2 major hormones involved: Insulin and glucagon; these hormones allow the body to respond on as needed bases.

Hormone regulation

Hormones effect the entry of glucose into cells and fate in the cells within the body.

As needed hormones regulate release of glucose. (exp: after meals glucose increase, without hormones to shut off secretion, the mechanism of glucose release would steadily increase.

Hormones and endocrine systems work together to meet 3 requirements:

Steady supply of glucose. Store excess glucose Use stored glucose as needed

Insulin

Primary hormone responsible for the entry of glucose in the cell.

Synthesized in the beta cells of islets of langerhans in the pancreas.

As the beta cells detect in increase in body glucose, they release insulin.

Insulin release cause increase movement of glucose into the cells and increase glucose metabolism

Is the only hormone that decreases glucose levels and is referred as a hypoglycemic agent.

Glucagon

Peptide hormone that is synthesized by the alpha cells of the islets cells of the pancreas and released during stress and fasting states.

Released in response to decreased body glucose.

Main function is to increase hepatic glycogenolysis, inhibit glycolysis and increase gluconeogenesis.

Hyperglycemic agent

Epinephrine

Hormone produced by the adrenal gland Increases plasma glucose by inhibiting

insulin secretion, increasing glycogenolysis and promotes lipolysis.

Release during times of stress

Glucocorticoids

Cortisol is released when stimulated by ACTH.

Cortisol increases plasma glucose by decreasing intestinal entry into the cells and increasing glycogenesis, liver glycogen and lipolysis.

Released during extended increase of glucose

Insulin antagonist

Thyroxine

The thyroid gland is stimulated by TSH to release thyroxin.

Increases glucose levels by increasing glycogenolysis. and intestinal absorption of glucose.

Somatostatin

Produced by the delta cells of the lslets of langerhans of the pancreas.

Increases plasma glucose levels by the inhibition of insulin, growth hormone and other endocrine hormones.

Hyperglycemia

Increased in plasma glucose levels. During a hyperglycemia state, insulin is

secreted by the beta cells of the pancreatic islets of langerhans.

Insulin enhances membrane permeability to cells in the liver, muscle, and adipose tissue.

Due to hormone imbalance

Diabetes Mellitus

Metabolic diseases charaterized by hyperglycemia resulting from defect in insulin secretion, insulin action or both.

Two major types: Type I, insulin dependent and Type 2, non insulin dependent.

1995: further categories by WHO/ADA:

Type 1 diabetes, type 2 diabetes, other specific types and gestation diabetes mellitus.

Type 1 diabetes

Deficiency or loss of insulin production due to beta cell destruction.

Commonly occurs in children (juvenile diabetes) Genetics play a minimal role, can be due to

exposure to environmental substances or viruses.

Clinical picture: less than 20 yrs old, polyuria, weight loss, increased glucose levels

Treatment: give insulin

Type 2 diabetes mellitus

Due to lack of or no insulin production, insulin resistant.

Seen adults greater than 20 yrs old, most common adult form.

Genetics play a larger role in addition to diet.

Relative insulin deficiency

Other specific types

Secondary condition, genetic defect in beta cell function or insulin action, pancreatic disease, disease of endocrine origin, drug or chemical induced.

Characteristics of the disease depends on the primary disorder.

Gestational diabetes mellitus

Glucose intolerance that is induced by pregnancy

Caused by metabolic and hormonal changes related to the pregnancy.

Glucose tolerance usually returns to normal after delivery.

Infants are at a high risk for developing respiratory stress disorder, and hyperbilirubinuria.

Pathophysiology of Diabetes Mellitus

Type 1 and Type 2 diabetes: there is an increase in blood glucose levels (hyperglycemic). There is also elevation of glucose in urine (glucosuria) if glucose levels in blood exceed 180 mg/dl.

Type 1: tend to produce ketones because of the difference in glucagon and insulin concentration through increased beta-oxidation. Absence of insulin and with increased glucagon which leads to gluconeogenesis and lipolysis.

Type 2: have very little ketone production.

Lab findings Type 1

Ketoacidosis that tend to reflect dehydration, electrolyte imbalance, acidosis's and oxidation fatty acids producing acetoacetate, Beta-hydroxybutyrate and acetone. Beta-hydroxybutyrate and acetone contribute to acidosis condition.

Bicarbonate and total carbon dioxide are decreased due to deep respiration- body trying to compensate for acidosis by blowing off CO2 and removing H ions.

Lab findings with Type 2

Over production of glucose: > 300-500 mg/dl Dehydration due to the inability to excrete

glucose in urine. No ketones bodies formed because of the lack

of lipolysis. Can lead to coma, in addition to N to elevated

sodium and potassium, slight decrease in bicarbonate and increase in BUN:Creat ratio.

Hypoglycemia

Decreased glucose levels Most effective on the CNS- why there is

shaking and tremors, heart rate increases- dizziness, cold sweat, if not corrected can result in slurred speech, loss of motor skills-unconsciousness-coma-death.

Glucose measurements

Use serum, plasma or whole blood Sample needs to refrigerated and

separated from cells with one hour of collection.

Fluoride is the anticoagulant of choice.

Glucose and other carbohydrates are capable of converting cupric ions in an alkaline solution to form cuprous ions.

Benedict and Fehlings reagent: uses cuprous /cupric methodology forming a deep blue to red color when cuprous ions are present. Reagent contains alkaline solution of cupric ions stabilized by citrate or tartrate- which detects the reducing substance.

Methods

Glucose oxidase method: converts beta-d- glucose to gluconic acid. Mutarotase may be added to facilitate to conversion to alpha-d-glucose to beta-D-glucose. Oxygen is consumed and hydrogen peroxide is produced. Can measure the amount of oxygen loss or H2O2 produced. Horseradish peroxidase is used as a catalyst. Chromogens used for color change: 3-methyl-2-benzothiazolinone hydrozone and N,N – dimethylaniline- this is a coupled reaction known as Trinder’s reaction

Hexokinase: more accurate less interference from uric acid, bilirubin and ascorbic acid.

In the presence of ATP- hexokinas converts glucose to glucose-6-phosphate.

Glucose-6-phophate and NADP converted to 6-phosphogluconate and NADPH by glucose-6-phosphate dehydrogenase- produces a red color measured at 340 nm.

Glucose monitoring and 2 hr test

2 hour test utilizes the knowledge that normally a glucose level will return to normal after 2 hrs if no disease or impairment involved.

GTT most sensitive, more accurate. Utilizes fasting along with set time intervals.

Glycosylated Hemoglobulin (HbA1c)

Is a term used to describe the formation of Hgb compound formed when glucose reacts with the amino group of Hgb.

Used to monitor and manage diabetes, monitors blood glucose levels over the last 60-90 days.

Specimen of choice is EDTA whole blood

Methods

2 major categories Based on charge difference between

glycosylated and nonglycosylated Hgb. (cation-exchange chromatography, electrophoresis, and isoelectric focusing)

Structural characteristics of glycogroups on Hgb. (affinity chromatography and immunoassay)

Ketones

Ketone bodies are produced by the liver through the metabolism of fatty acids to provide energy to provide ready energy from stored lipids in low CHO available.

Acetone (2%), Beta-hydroxybutyrate (78 %) and acetoacetic acid (20%).

Low levels present all the time, but when the body is deprived if CHO (diet, vomiting, and glycogen storage disease) ketones levels increase.

Ketonemia and ketonuria

Microalbuminuria

Because Diabetes mellitus cause progressive disease in the kidneys (nephropathy), the lab will monitor urinary albumin through measuring microalbumin in the urine.

3 methods: Spot random urine test (albumin to creatinine

ratio.) 24 hour (timed) 4 hour over night