UNKLAB NURSING PORTAL

Home » Nursing-Biomedic Subjets » Biochemsitry

Category Archives: Biochemsitry

Cortisol Hormone Effects

Advertisements

Carbohydrates

SOURCE: http://themedicalbiochemistrypage.org

Image Courtesy: http://kidshealth.org

Introduction

Carbohydrates are carbon compounds that contain large quantities of hydroxyl groups. The simplest carbohydrates also contain either an aldehyde moiety (these are termed polyhydroxyaldehydes) or a ketone moiety (polyhydroxyketones). All carbohydrates can be classified as either monosaccharidesoligosaccharides orpolysaccharides. Anywhere from two to ten monosaccharide units, linked by glycosidic bonds, make up an oligosaccharide. Polysaccharides are much larger, containing hundreds of monosaccharide units. The presence of the hydroxyl groups allows carbohydrates to interact with the aqueous environment and to participate in hydrogen bonding, both within and between chains. Derivatives of the carbohydrates can contain nitrogens, phosphates and sulfur compounds. Carbohydrates also can combine with lipid to form glycolipids or with protein to form glycoproteins.

Carbohydrate Nomenclature

The predominant carbohydrates encountered in the body are structurally related to the aldotriose glyceraldehydeand to the ketotriose dihydroxyacetone. All carbohydrates contain at least one asymmetrical (chiral) carbon and are, therefore, optically active. In addition, carbohydrates can exist in either of two conformations, as determined by the orientation of the hydroxyl group about the asymmetric carbon farthest from the carbonyl. With a few exceptions, those carbohydrates that are of physiological significance exist in the D-conformation. The mirror-image conformations, called enantiomers, are in the L-conformation.

 

Structure glyceraldehyde enantiomers

Monosaccharides

The monosaccharides commonly found in humans are classified according to the number of carbons they contain in their backbone structures. The major monosaccharides contain four to six carbon atoms.

Carbohydrate Classifications

# Carbons

Category Name

Relevant examples

3

Triose

Glyceraldehyde, Dihydroxyacetone

4

Tetrose

Erythrose

5

Pentose

Ribose, Ribulose, Xylulose

6

Hexose

Glucose, Galactose, Mannose, Fructose

7

Heptose

Sedoheptulose

9

Nonose

Neuraminic acid, also called sialic acid

The aldehyde and ketone moieties of the carbohydrates with five and six carbons will spontaneously react with alcohol groups present in neighboring carbons to produce intramolecular hemiacetals or hemiketals, respectively. This results in the formation of five- or six-membered rings. Because the five-membered ring structure resembles the organic molecule furan, derivatives with this structure are termed furanoses. Those with six-membered rings resemble the organic molecule pyran and are termed pyranoses

Such structures can be depicted by either Fischer or Haworth style diagrams. The numbering of the carbons in carbohydrates proceeds from the carbonyl carbon, for aldoses, or the carbon nearest the carbonyl, for ketoses.

Cyclic Fischer projection of glucose Haworth projection of glucose

Cyclic Fischer Projection of α-D-Glucose

Haworth Projection of α-D-Glucose

The rings can open and re-close, allowing rotation to occur about the carbon bearing the reactive carbonyl yielding two distinct configurations (α and β) of the hemiacetals and hemiketals. The carbon about which this rotation occurs is the anomeric carbon and the two forms are termed anomers. Carbohydrates can change spontaneously between the α and β configurations: a process known as mutarotation. When drawn in the Fischer projection, the α configuration places the hydroxyl attached to the anomeric carbon to the right, towards the ring. When drawn in the Haworth projection, the α configuration places the hydroxyl downward.

The spatial relationships of the atoms of the furanose and pyranose ring structures are more correctly described by the two conformations identified as the chair form and the boat form. The chair form is the more stable of the two. Constituents of the ring that project above or below the plane of the ring are axial and those that project parallel to the plane are equatorial. In the chair conformation, the orientation of the hydroxyl group about the anomeric carbon of α-D-glucose is axial and equatorial in β-D-glucose.

Chair form of glucose

Chair form of α-D-Glucose

Disaccharides

Covalent bonds between the anomeric hydroxyl of a cyclic sugar and the hydroxyl of a second sugar (or another alcohol containing compound) are termed glycosidic bonds, and the resultant molecules are glycosides. The linkage of two monosaccharides to form disaccharides involves a glycosidic bond. Several physiogically important disaccharides are sucrose, lactose and maltose.

Sucrose: prevalent in sugar cane and sugar beets, is composed of glucose and fructose through an α–(1,2)–β-glycosidic bond.

Structure of sucrose

Sucrose

Lactose: is found exclusively in the milk of mammals and consists of galactose and glucose in a β–(1,4) glycosidic bond.

Structure of lactose

Lactose

Maltose: the major degradation product of starch, is composed of 2 glucose monomers in an α–(1,4) glycosidic bond.

Structure of maltose

Maltose

Polysaccharides

Most of the carbohydrates found in nature occur in the form of high molecular weight polymers calledpolysaccharides. The monomeric building blocks used to generate polysaccharides can be varied; in all cases, however, the predominant monosaccharide found in polysaccharides is D-glucose. When polysaccharides are composed of a single monosaccharide building block, they are termed homopolysaccharides. Polysaccharides composed of more than one type of monosaccharide are termed heteropolysaccharides.

Glycogen

Structure of glycogen

INTRODUCTION TO BIOCHEMISTRY

SUMMARY: Biological macromolecule structure and function can be examined and understood by applying principles learned from the study of small molecules to macromolecules. The macromolecules we will discuss in class, (proteins, complex carbohydrates, and nucleic acids) can be understood using principles of organic, physical, analytical, and inorganic chemistry.

The links below will take you to a description of many of the handouts I have distributed in class. This online guide is meant to augment your understanding of what we have discussed in class, and not a replacement for class attendance.

PROTEINS

Proteins are polymers of the bifunctional monomer, amino acids. Amino acids form polymers through a nucleophilic attack by the amino group of an amino acid at the electrophilic carbonyl carbon of the carboxyl group of another amino acid. The carboxyl group of the amino acid must first be activated to provide a better leaving group than OH.  (We will discuss this activation by ATP latter in the course.) The resulting link between the amino acids is an amide link which biochemists call a peptide bond. In this reaction, water is released. In a reverse reaction, the peptide bond can hence be cleaved by water (hydrolysis).  

 

COMPLEX CARBOHYDRATES – POLYSACCHARIDES

Monosaccharides are the monomeric units which polymerize to form a polysaccharide. Monosaccharides are simple sugars (polyhydroxyaldehydes or ketones) , which can cyclize by an intramolecular nucleophilic addition of one of the OH groups on the sugar with the aldehyde or ketone, resulting in a hemiacetal or hemiketal. Addition of another alcohol, from an exogenous alcohol under acidic/anhydrous condition results in an acetal. If the added alcohol is from another sugar, a covalent, acetal link between the two sugars results. As with peptides, the process can be reversed under aqueous conditions through hydrolysis of the acetal link, resulting in monomeric sugars.  Biochemist name the acetal link between sugars a glycosidic bond. Click to see figures showing these reactions.

NUCLEIC ACIDS

The monomeric units of nucleic acids consist of a 5-ring cyclized sugar, ribose (for RNA) and deoxyribose (for DNA), which has been attached at C1 to an organic base (pyrimidine or purine) through an “acetal-like” link. (i.e. Instead of a nucleophilic alcohol reacting with the ribose hemiacetal, a nucleophilic N on the base reacts) The link between the monomers requires the activation of the O on C5 through a triphosphate group. The actual link between the monomers is a phosphodiester link, which likewise can be hydrolyzed. See the figures below.

 

Monomers

 

Polymers

 

Chemical Nature of the Amino Acids

Image Ref: http://supersuga.wordpress.com

All peptides and polypeptides are polymers of α-amino acids. There are 20 α-amino acids that are relevant to the make-up of mammalian proteins (see below). Several other amino acids are found in the body free or in combined states (i.e. not associated with peptides or proteins). These non-protein associated amino acids perform specialized functions. Several of the amino acids found in proteins also serve functions distinct from the formation of peptides and proteins, e.g., tyrosine in the formation of thyroid hormones or glutamate acting as a neurotransmitter. (more…)

Membrane Channels

structure of a typical aquaporin

The definition of a channel (or a pore) is that of a protein structure that facilitates the translocation of molecules or ions across the membrane through the creation of a central aqueous channel in the protein. This central channel facilitates diffusion in both directions dependent upon the direction of the concentration gradient. Channel proteins do not bind or sequester the molecule or ion that is moving through the channel. Specificity of channels for ions or molecules is a function of the size and charge of the substance. The flow of molecules through a channel can be regulated by various mechanisms that result in opening or closing of the passageway.

(more…)

Activities of Biological Membranes

Image Ref: http://www.mansfield.ohio-state.edu

Although biological membranes contain various types of lipids and proteins, their distribution between the two different sides of the bilayer is asymmetric. As a general example the outer surface of the bilayer is enriched in phosphatidylethanolamine, whereas the intracellular surface is enriched in phosphatidylcholine. Carbohydrates, whether attached to lipid or protein, are almost exclusively found on the external surfaces of membranes. The asymmetric distribution of lipids and proteins in membranes results in the generation of highly specialized sub-domains within membranes. In addition, there are highly specialized membrane structures such as the endoplasmic reticulum (ER), the Golgi apparatus and vesicles. The most important vesicles are those that contain secreted factors. Membrane bound proteins (e.g. growth factor receptors) are processed as they transit through the ER to the Golgi apparatus and finally to the plasma membrane. As these proteins transit to the surface of the cell they undergo a series of processing events that includes glycosylation.

(more…)

Composition and Structure of Biological Membranes

As indicated above, biological membranes are composed of lipids, proteins, and carbohydrates. The carbohydrates of membranes are attached either to lipid forming glycolipids of various classes, or to proteins formingglycoproteins. The lipid and protein compositions of membranes vary from cell type to cell type as well as within the various intracellular compartments that are defined by intracellular membranes. Protein concentrations can range from around 20% to as much as 70% of the total mass of a particular membrane.

structure of the lipid bilayer of a typical plasma membrane

(more…)

Introduction to Biological Membranes

Biological membranes are composed of lipid, protein and carbohydrate that exist in a fluid state. Biological membranes are the structures that define and control the composition of the space that they enclose. All membranes exist as dynamic structures whose composition changes throughout the life of a cell. In addition to the outer membrane that results in the formation of a typical cell (this membrane is often referred to as the plasma membrane), cells contain intracellular membranes that serve distinct functions in the formation of the various intracellular organelles, e.g. the nucleus and the mitochondria.

Ref: http://themedicalbiochemistrypage.org

The Periodic Table

Copyright: http://www.chemicool.com/

Explore more interesting chemical portal to study general and biochemistry especially understanding periodic table of elements by visit: 

Blood Chemistry Tests and Haematology Reference Values

Image courtesy: http://latorrewellnesscenter.com

(more…)