Carbohydrates are carbonyl compounds (aldehydes , ketones)that occur widely in living organisms as part of biomolecules . Carbohydrates are one of the most important classes of biomolecules . Their main functions can be classified into three groups:(i)to deliver and store energy,(ii)to help provide the basic framework for DNA and RNA , the information carrying molecules , and (iii)to form structural elements of cell walls of bacteria and plants (polysaccharides) . In addition , they form cell surface structures (receptors)used in conducting signals from cell to cell . Combined with numerous proteins and lipids , carbohydrates are important components of numerous internal cell structures.
Monosaccharides (simple sugars)are aldehydes(–C =O,–H)or ketones (C =O)with two or more hydroxy groups (general structural formula:CH2On).The aldehyde or ketone group can react with one of the hydroxy groups to form a ring.
This is the usual configuration of sugars that have five or six C atoms (pentoses and hexoses).
The C atoms are numbered sequentially . The D- and the L-forms of sugars are mirror-image isomers of the same molecule.
The naturally occurring forms are the D-(dextro) forms . These further include b -and α -forms as stereoisomers . In the cyclic forms , the C atoms of sugars are not on a plane , but three dimensionally take the shape of a chair or a boat . The b -D- glucopyranose configuration (glucose)is the energetically favored one , since
all the axial positions are occupied by H atoms.
The arrangement of the –OH groups can differ, so that stereoisomers such as mannose or galactose are formed.
These are compounds of two monosaccharides . The aldehyde or ketone group of one can bind to an α -hydroxy or a b -hydroxy group of the other.
Sucrose and lactose are frequently occurring
C.Derivatives of sugars
When certain hydroxy groups are replaced by other groups,sugar derivatives are formed.
These occur especially in polysaccharides . In a large group of genetically determined syndromes , complex polysaccharides cannot be degraded owing to reduced or absent enzyme
function (mucopolysaccharidoses , mucolipidoses).
Short (oligosaccharides)and long chains of sugars and sugar derivatives (polysaccharides) form essential structural elements of the cell.
Complex oligosaccharides with bonds to proteins or lipids are part of cell surface structures, e.g., blood group antigens.
Medical relevance examples of human hereditary disorders in the
metabolism of carbohydrates are: Diabetes mellitus (MIM 125850):a heterogeneous group of disorders characterized by
elevated levels of blood glucose , with complex clinical and genetic features .
Disorders of fructose metabolism : Benign fructosuria (MIM 229800),hereditary fructose intolerance with hypoglycemia and vomiting
(MIM 229600),and hereditary fructose 1.6-bisphosphatase deficiency with hypoglycemia, apnea , lactic acidosis , and often with lethal outcome in newborn infants (MIM 229700).
Galactose metabolism : Inherited disorders with acute galactose toxicity and long-term effects (Galactosemia,MIM 230400;galactokinase deficiency,MIM 230200;Galactose epimerase deficiency,MIM 230350),and others.
Glycogen storage diseases : Eight types of disorders of glycogen metabolism that differ in clinical symptoms and the genes and enzymes involved (MIM 232200,232210 –232800).