Molecules of Life

Chapter 3 Part 2

3.5 Proteins – Diversity in Structure and Function

Proteins are the most diverse biological molecule (structural, nutritious, enzyme, transport, communication, and defense proteins)

Proteins and Amino Acids

Protein• An organic compound composed of one or more

chains of amino acids

Amino Acid Structure

Polypeptides

Protein synthesis involves the formation of amino acid chains called polypeptides

Polypeptide• A chain of amino acids bonded together by

peptide bonds in a condensation reaction between the amine group of one amino acid and the carboxyl group of another amino acid

Fig. 3-16a, p. 44

A DNA encodes the order of amino acids in a new polypeptide chain. Methionine (met) is typically the first amino acid.

B In a condensation reaction, a peptide bond forms between the methionine and the next amino acid, alanine (ala) in this example.

Leucine (leu) will be next.

Stepped Art

Fig. 3-16b, p. 45

D The sequence of amino acid subunits in this newly forming peptide chain is now met–ala–leu–trp. The process may continue until there are hundreds or thousands of amino acids in the chain.

C A peptide bond forms between the alanine and leucine.

Tryptophan (trp) will be next. The chain is starting to twist and fold as atoms swivel around some bonds and attract or repel their neighbors.

Stepped Art

Levels of Protein Structure

Primary structure• The sequence of amino acids in a protein

Secondary structure• The polypeptide chain folds and forms hydrogen

bonds between amino acids

Levels of Protein Structure

Tertiary structure• A secondary structure is compacted into

structurally stable units called domains• Forms a functional protein

Quaternary structure• Some proteins consist of two or more folded

polypeptide chains in close association

Fig. 3-17, p. 45

a) Protein primary structure: Amino acids bonded as a polypeptide chain.

helix (coil) sheet

b) Protein secondary structure: A coiled (helical) or sheetlike array held in place by hydrogen bonds (dotted lines) between different parts of the polypeptide chain.

barrel

c) Protein tertiary structure: A chain’s coils, sheets, or both fold and twist into stable, functional domains such as barrels or pockets.

d) Protein quaternary structure: two or more polypeptide chains associated as one molecule.

Stepped Art

Just One Wrong Amino Acid…

Hemoglobin contains four globin chains, each with an iron-containing heme group that binds oxygen and carries it to body cells

In sickle cell anemia, a DNA mutation changes a single amino acid (glutamate to valine) in a beta chain, which changes the shape of the hemoglobin molecule, causing it to clump and deform red blood cells

Globin Chains in Hemoglobin

Fig. 3-19a, p. 47

valine histidine leucine threonine proline glutamic acid

A Normal amino acid sequence at the start of the hemoglobin beta chain.

glutamic acid

Fig. 3-19b, p. 47

valine histidine leucine threonine proline valine glutamic acid

B One amino acid substitution results in the abnormal beta chain of HbS molecules. The sixth amino acid in such chains is valine, not glutamic acid.

Animation: Sickle-cell anemia

Proteins Undone – Denaturation

Heat, changes in pH, and salts can disrupt the hydrogen bonds that maintain a protein’s shape

When a protein loses its shape and no longer functions, it is denatured

Ex. Albumin (egg white)

3.7 Nucleic Acids

Some nucleotides are subunits (monomers) of nucleic acids (polymers) ; DNA and RNA

Nucleotides

Nucleotide• A small organic molecule consisting of a sugar

with a five-carbon ring, a nitrogen-containing base, and one or more phosphate groups

ATP (Energy Currency of the Cell)• A nucleotide with three phosphate groups• Important in phosphate-group (energy) transfer

Nucleic Acids

Nucleic acids• Polymers of nucleotides in which the sugar of one

nucleotide is attached to the phosphate group of the next

• RNA and DNA are nucleic acids

RNA

RNA (ribonucleic acid)• Contains four kinds of nucleotide bases, including

ATP• Important in protein synthesis

DNA

DNA (deoxyribonucleic acid)• Two chains of nucleotides twisted together into a

double helix and held by hydrogen bonds• Contains all inherited information necessary to

build an organism

Four Nucleotides of DNA

Fig. 3-22, p. 49

covalent bonding in sugar–phosphate backbone

hydrogen bonding between bases

Video: Effects of trans fats