Clinical Chemistry: Principles, Techniques, and Correlations, Enhanced Edition
Diabetes diagnostics are crucial for timely and accurate identification of diabetes mellitus, a chronic condition characterized by elevated blood glucose levels. Early diagnosis through tests like fasting plasma glucose, HbA1c, and oral glucose tolerance tests enables prompt intervention and manage...
DIAGNOSTIC METHODS IN DIABETES MELLITUS
CARBOHYDRATE METABOLISM
Heterotrophs need energy from the oxidation of organic molecules. Imp ones are:
➔ Carbohydrate
➔ Amino acids
➔ Lipids
Disease states involving carbohydrates: They are split into two groups
➔ Hyperglycemia
➔ Hypoglycemia
GENERAL DESCRIPTION OF CARBOHYDRATES
Compounds containing C, H, and O with formula as Cx(H2O)y.
All contain C=O and –OH functional groups
There are some carb derivatives from the basic formula (Cx(H2O)y) because carbohydrate
derivatives can be formed by the addition of other chemical groups, such as:
➔ Phosphates
➔ Sulfates
➔ Amines
The classification of carbohydrates is based on four different properties: These are:
1. The size of the base carbon chain
2. The location of the CO function group
3. The number of sugar units
4. The stereochemistry of the compound
CLASSIFICATION OF CARBOHYDRATE
Classification is based on the number of carbons in the molecule
For example: Trioses contain three carbons, tetroses contain four, pentoses contain five, and
hexoses contain six.
The smallest carbohydrate is glyceraldehyde (3C)
Carbohydrates are hydrates of aldehyde or ketone derivatives based on the location of the CO
functional group (Fig. 14.1). The two forms of carbohydrates are aldose and ketose (Fig. 14.2).
,The aldose form has a terminal carbonyl group (O=CH–) called an aldehyde group, whereas the
ketose form has a carbonyl group (O=C) in the middle linked to two other carbon atoms (called a
ketone group).
Fischer and Haworth projection: Fischer projections are used for sugars in their open-chain
form, Haworth projections are often used to depict sugars in their cyclic forms
,Stereoisomers
The central carbons of a carbohydrate are asymmetric (chiral)—four different groups are
attached to the carbon atoms. This allows for various spatial arrangements around each
asymmetric carbon (also called stereo genic centers) forming molecules called stereoisomers.
➔ Stereoisomers have the same order and types of bonds but different spatial
arrangements and different properties
For each asymmetric carbon, there are 2n possible isomers. For example if c-atoms are 4 then it
has 16 different isomers.
Assigning D/L to monosaccharides: assigned based on configuration at the highest numbered
asymmetric carbon. If OH persists to the right in Fischer projection then its D and if to the left it
is L (called enantiomers- a pair of molecules that exist in two forms that are mirror images of
one another but cannot be superimposed one upon the other)
THE 2nd CLASSIFICATION OF CARBOHYDRATES
It is based on the number of sugar units in the chain. Chaining relies on no of glycosidic bonds
that serve as bridges between oxygen atoms. When 2 carb join, water is produced and when
they split a molecule of water is consumed (called hydrolysis)
1- Monosaccharides are simple sugars that cannot be hydrolyzed to a simpler form. These
sugars can contain three, four, five, and six or more carbon atoms
For example: Glucose, fructose, and galactose
2- Disaccharides: Formed when two monosaccharide units are joined by a glycosidic
linkage. On hydrolysis, disaccharides will be split into two monosaccharides by
disaccharide enzymes (e.g., lactase) located on the microvilli of the intestine. These
monosaccharides are then actively absorbed.
For example: The most common disaccharides are maltose (comprising two D-glucose
molecules in a 1 → 4 linkage), lactose, and sucros.
, 3- Oligosaccharides are the chaining of 2 to 10 sugar units
For example: Raffinose (18C)—plants like cabbage, sprouts etc.
4- Polysaccharides are formed by the linkage of many monosaccharide units. On
hydrolysis, polysaccharides will yield more than 10 monosaccharides.
For example: Amylase hydrolyzes starch to disaccharides in the duodenum. The most common
polysaccharides are starch (glucose molecules) and glycogen
CHEMICAL PROPERTIES OF CARBOHYDRATES
Some carbs are reducing substances (must have aldehyde or ketone group) e.g. glucose and
sucrose
Can link other carbs via glycosidic bond (a bond between hemiacetal group of one carb and OH
group of the other carb)
What decides the fate of the reducing sugar?
If a bond forms with C atoms in a sugar other than anomeric (A carbon that is aldehyde or
ketone in open chain and becomes a stereocenter in closed system) carbon---it becomes a
reducing sugar and if it binds to anomeric carbs, it no longer remains in reducing state.
All monosaccharides and many disaccharides are reducing agents. This is because a free
aldehyde or ketone (the open-chain form) can be oxidized under the proper conditions. A
disaccharide remains a reducing agent when the hemiacetal or ketal hydroxyl group is not linked
to another molecule, both maltose and lactose are reducing agents, whereas sucrose is not.
GLUCOSE METABOLISM
1- Glucose (provides energy for life processes) is the main product of dietary carbohydrate
metabolism. Oxidation of glucose by TCA and glycolysis yields cellular activity
2- After a carbohydrate-containing meal excess glucose is:
➔ Stored as glycogen in liver and muscle (glycogenesis)
➔ Converted to fat and stored in adipose tissue (glycogenolysis)
Muscle glycogen provides the glucose for muscular activity.
3- During fasting:
➔ Glycogen breakdown in the liver and, to a lesser extent, in the kidney releases
glucose into the plasma
➔ Excess glucose is oxidized to fatty acids and stored as fat in the tissues. If
needed (in fasting state) glucose can also be formed from fats and protein
(gluconeogenesis)
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