AP Biology

Ch. 5 Macromolecules:Lipids, Proteins, and Nucleic

Acids

Lipids Fats store large amounts of

energy. Triacylglycerols (triglycerides) are

constructed by the joining of a glycerol molecule to three fatty acids.

Formed by dehydration synthesis reactions.

Saturated vs. Unsaturated Fats Saturated fatty acids have the maximum

number of hydrogen atoms. These tend to be solid at room temp., such as butter and meat fat.

Unsaturated fatty acids have one or more double bonds in their hydrocarbon chains. These tend to be liquid at room temp., such as olive oil and canola oil.

Unsaturated fats are generally considered to be healthier when used in moderation.

Phospholipids Where fats have a third fatty acid linked to

glycerol, phospholipids have a negatively charged phosphate group.

This makes the “head” of the phospholipid hydrophilic; the hydrocarbon “tails” are hydrophobic.

Phospholipids are the major components of cell membranes. In a cell membrane, the hydrophobic tails are orientated inward, while the hydrophilic head face outward.

Steroids Steroids include

cholesterol and certain hormones, such as testosterone and estrogen.

Steroids have a basic structure of four fused rings of carbon atoms.

Proteins Proteins are macromolecules of

polypeptide chains. Polypeptide are polymers of amino

acids arranged in a specific linear sequence linked by peptide bonds.

Proteins are one or more polypeptide chains folded and coiled into specific conformations (3-D shapes).

Proteins, cont. Proteins make up 50% of cellular

dry weight. Each type has a unique 3-D shape. Vary in structure and function, but

all are made from the same 20 amino acids.

Proteins: Cellular function Structural support Storage of amino acids Transport, such as hemoglobin Signaling, chemical messengers Cellular response to chemical stimuli

(receptor proteins) Movement (contractile proteins) Defense (antibodies) Catalysis of biochemical reactions

(enzymes)

Amino Acids: Building blocks of proteins

Amino acids consist of an asymmetric carbon bonded to a hydrogen atom, carboxyl group, amino group, and a side chain (R-group) specific for each amino acid.

Physical and chemical properties of the side chain determine the uniqueness of each amino acid.

Groups of Amino Acids Amino acids are put into groups based

on the side chains the molecule contains, and its properties.

Nonpolar, hydrophobic side groups make amino acids less soluble in water.

Polar, hydrophilic side groups make amino acids soluble in water. These can be uncharged polar side groups, or charged (acidic or basic) groups.

NONPOLAR AMINO ACIDS

POLAR AMINO ACIDS

Peptide bonds Peptide bonds are covalent bonds formed by

a condensation reaction that links the carboxyl group of one amino acid to the amino group of another.

Has polarity with an amino group one end (N-terminus) and a carboxyl group on the other (C-terminus).

Has a backbone of repeating N-C-C-N-C-C Polypeptide chains range in length from a

few monomers to more than a thousand, and a unique linear sequence of amino acids.

Protein conformation: 3-D structure

Each protein molecule has a unique native conformation (shape of the protein under normal biological conditions) that reflect its function.

Enables a protein to recognize and bind specifically to another molecule (enzyme/substrate, hormone/receptor, antibody/antigen)

3-D structure, cont. A protein’s shape is produced

when a newly formed polypeptide chain coils and folds spontaneously, mostly in response to hydrophobic interactions.

Is stabilized by chemical bonds and weak forces between neighboring regions of the folded protein.

Overview of Protein Structure

Four levels of protein structure Primary- amino acid sequence Secondary- regular repeated coiling and

folding of a protein’s amino acid chain Tertiary- 3-dimensional shape of a protein

due to bonding between side chains Quaternary- results from interactions

between several polypeptide chain (protein has subunits, like hemoglobin and collagen)

PrimaryStructure

Secondary Structure Coiling and folding of the polypeptide backbone Stabilized by hydrogen bonds between peptide

linkages in the amino acid chain Two major types: Alpha helix- helical coil stabilized by hydrogen

bonds between every 4th peptide bond. Found in fibrous proteins such as collagen.

Beta pleated sheet- antiparallel chains fold into accordian-like pleats, held by hydrogen bonds. Form the dense core of globular proteins (lysozyme) and some fibrous proteins (silk)

SecondaryStructure:Alpha helix or Beta-pleated sheet

Spider silk: a structural protein

Tertiary structure 3-D shape of a protein due to bonding between

side chains, and interactions with the aqueous environment.

Protein shape is stabilized by: Weak interactions such as hydrogen

bonding between side chains, ionic bonds between charged side chains, and hydrophobic interactions between nonpolar side chains

Covalent linkages such as disulfide bridges between two cysteine monomers brought together by protein folding

Quaternary structure Occurs in proteins made up of two or

more polypeptides, resulting from the interactions between them.

Collagen is a fibrous protein with three helical polypeptides supercoiled into a triple helix; makes it very strong connective tissue in animals

Hemoglobin is a globular protein with four subunits that fit together.

Protein conformation Protein’s 3-D shape is a consequence

of the interactions responsible for secondary and tertiary structure.

Protein conformation is influenced by the physical and chemical environment

If a protein’s environment is changed, it may become denatured and lose its shape.

Denaturation Process that changes a protein’s

structure, therefore affecting its biological function. Can be caused by:

1) Heat- disrupts weak interactions2) Chemical agents that disrupt H-bonds,

ionic bonds, and disulfide bridges (pH for example)

3) Transfer to an organic solvent causes hydrophobic chains to move toward the outside, while the hydrophilic side chains turn toward the interior.

Protein folding 3-D shape is hard to predict from amino

acid sequence alone. Protein’s native conformation may

alternate between several shapes with folding occurring in stages.

Biochemists can now track a protein as it goes through the folding process.

Chaperone proteins temporarily brace a folding protein.

Nucleic Acids: Informational Polymers

Nucleic acids store and transmit hereditary information.

DNA stores information for the synthesis of specific proteins.

RNA, specifically mRNA, carries this genetic information to the protein synthesizing machinery (ribosomes).

Nucleic acids, cont.

Nucleic acids are polymers of nucleotides. Each nucleotide monomer consists of a

pentose (5-C sugar) covalently bonded to a phosphate group and to one of four nitrogenous bases (A,G,C, T or U).

In making a chain, nucleotides join to form a sugar-phosphate backbone from which the nitrogenous bases project.

The sequence of bases along a gene specifies the amino acid sequence of a particular protein.

DNA: Deoxyribonucleic acid

DNA is a helical, double-stranded macromolecule with bases projecting into the interior of the molecule.

DNA has the pentose, deoxyribose. Adenine hydrogen bonds to thymine, and cytosine to guanine (Chargaff’s rule).

One DNA strand serves a template for a new strand.

X-ray evidence of DNA structure In the early 1950’s, Rosalind Franklin

studied DNA using X-ray diffraction. The patterns in her pictures showed

the DNA formed a coil shape (helix). Her studies indicated that there

were two strands, and that the nucleotides were toward the center of the molecule.

Franklin’s X-ray diffraction of DNA

Watson and Crick James Watson and Frances Crick were

working on the structure of DNA in the 1950’s also.

Using information from Chargaff, Franklin, and other scientists, they put together a 3-D model of DNA.

Their model was a double helix, with H-bonded nitrogenous bases holding the strands together.

The won the Nobel Prize for their work.

Ribonucleic Acid (RNA) RNA is a single-stranded

macromolecule with the 5-C sugar ribose.

In RNA, adenine binds to uracil instead of thymine, and guanine binds to cytosine.

RNA uses the information from DNA to assemble protein.

Types of RNA Messenger RNA (mRNA)-carries messages

from DNA for assembling amino acids into proteins (made during transcription).

Ribosomal RNA (rRNA)-Proteins and rRNA make up ribosomes, the site of protein synthesis.

Transfer RNA (tRNA)- transfers each amino acid to the ribosome as specified by codes in the mRNA.