Biomolecules: Disaccharides, Nucleic Acids, and Protein Structure
Disaccharides
Disaccharides are formed by joining two monosaccharides. Examples discussed:
Sucrose: formed by joining glucose and fructose; circulates in plant sap; obtained from sugarcane and sugar beets; used as table sugar.
Lactose: formed by joining galactose and glucose; lactose is the disaccharide that gives milk its sweet taste.
Maltose: consists of two linked glucose molecules; produced by digestion of starch and by sprouting seeds or in the intestine of an animal.
Nucleic Acids (DNA and RNA)
DNA and RNA are nucleic acids, polymers made of subunits called nucleotides.
A key difference between DNA and RNA is the type of sugar in their nucleotides:
DNA contains deoxyribose. The sugar is deoxyribose.
RNA contains ribose. The sugar is ribose, which has one more oxygen atom than deoxyribose.
Sugars:
Each of DNA and RNA is composed of four different nucleotides, which differ in their nitrogenous bases.
Three of the four bases are the same in DNA and RNA: adenine (A), guanine (G), and cytosine (C).
The fourth base differs:
In DNA, the base is thymine (T).
In RNA, the base is uracil (U).
The purines (two-ring structures) are adenine (A) and guanine (G).
The pyrimidines (one-ring structures) are cytosine (C), thymine (T), and uracil (U).
Nucleotides are linked by phosphate groups at the 5' (five-prime) position of the next nucleotide, forming the sugar-phosphate backbone.
RNA usually consists of a single polynucleotide chain.
DNA consists of two polynucleotide chains.
The two DNA strands are oriented in opposite directions (antiparallel) and are held together by hydrogen bonds between complementary nitrogenous bases on opposite strands.
Because of their sizes, shapes, and arrangement of polar groups, the DNA bases form complementary pairs: adenine pairs with thymine, and cytosine pairs with guanine.
The two polynucleotides in DNA wind around each other to form the familiar double helix.
Base sequences can be arranged in many orders, like letters of the alphabet, but the base sequence is significant because sequences of bases called genes encode instructions for the structure and function of an organism.
A note on presentation: the material will be followed by animations summarizing protein structure.
Protein Structure: Overview
A protein molecule consists of one or more chains of amino acid monomers.
Amino acids are linked by peptide bonds, so a protein polymer is often called a polypeptide.
Proteins are described in four levels of structure because of their complexity: primary, secondary, tertiary, and quaternary (although the material here covers the first two levels).
Primary structure:
Each protein has a unique primary structure, defined by the number and sequence of amino acids making up the polypeptide chain.
There are different amino acids used to build proteins.
The amino acids could theoretically be linked in almost any sequence, yielding a vast variety of proteins.
An illustration shows some of the amino acids making up the primary structure of a protein; a generalized amino acid structure is depicted.
Backbone and R group:
The main backbone of every amino acid is the same, forming the polypeptide backbone.
The R group (side chain) projects out from the backbone and makes each of the amino acids unique.
Different amino acids have different properties that affect how a protein folds.
Consequently, the primary structure determines the shape of the protein, which in turn determines its function.
Protein Secondary Structure
Secondary structure refers to regions where the polypeptide chain is coiled or folded into regular patterns, forming twists and corrugations.
Two common types of secondary structure:
Alpha helix: the chain twists into a helical shape.
Beta (pleated) sheet: the chain folds back on itself, or two regions lie parallel to one another.
Secondary structure arises from hydrogen bonding between atoms along the polypeptide backbone.
The backbone contains highly electronegative oxygen and nitrogen atoms, which create partial negative charges on the electronegative atoms and partial positive charges on adjacent hydrogens.
These partial charges enable attractions between suitably positioned atoms along the chain, promoting the twists or folds that characterize secondary structure.
Summary note on structure levels: proteins are commonly described in four levels of structure, with primary structure determining sequence, and secondary structure arising from backbone hydrogen bonding, among other interactions, ultimately contributing to the overall 3D conformation and function.