Proteins

Polypeptide chains in specific conformations

Protein Graphic Design video

Proteins

• Monomer: Amino acids

• Functions:– Structural support– Transport – hemoglobin– Signaling – chemical messengers– Defense – antibodies– Catalysis - enzymes

20 amino acids each composed of

• Amino group – acts as weak base

• Carboxyl group – acts as weak acid

• Variable R group (radical)

• Properties of R group determine uniqueness of each amino acid

Variation in R groups

What process is this?

Peptide bonds

• Between carboxyl group of one amino acid and amino group of another

• Dehydration synthesis

• N-C-C-N-C-C-N-C-C-

Formation of peptide bond

Function depends upon conformation

• Polypeptide like long piece of yarn, protein like a sweater knitted from yarn

• Structure enables protein to recognize and bind to another molecule

• Consequence of linear sequence of a.a.

• Stabilized by interactions between side chains

Four levels of Protein Structure

• Primary

• Secondary

• Tertiary

• Quaternary

Primary

• Unique sequence of amino acids

• Determined by genes

• Slight change can affect function and conformation

• Peptide bonds connect a.a’s

• Primary dictates secondary and tertiary

• Sickle cell anemia-a.a. substitution in hemoglobin protein, abnormal hemoglobin crystallize, deforming some cells

Secondary

• Regular, repeated coiling and folding

• Stabilized by repeated hydrogen bonds between peptide linkages (between oxygen of one amino acid and amino group hydrogens on another amino acid)

• Two types: Alpha helix, Beta pleated sheet

1. Alpha helix

• coil

• In fibrous proteins:

– Keratin and collagen ( inside hair, connective tissue)

Alpha helix

2. Beta pleated sheet

• 2 or more strands of the polypeptide chain lying side by side

• Connected by H-bonding between adjacent polypeptides

• Globular proteins

• Fibrous proteins –silk spider web

• B-keratin in feathers, hooves, claws, beaks, scales and horns

Beta sheet

3. Tertiary Structure• Contortions due to bonding between

side chains (R groups)

• Overall shape

Weak Interactions

• Stabilized by the cumulative effect of:

• H-bonding between polar side chains

• Ionic bonds between charged side chains

• Hydrophobic interactions cause nonpolar side chains to cluster in the center away from water, weak van der Waals interactions hold them together

Covalent Linkage

• Disulfide bridges –S-S- bond– Bonds between sulfur groups on Cysteine (an

amino acid)

– Rivets the protein together

Quaternary Structure• Interaction between multiple polypeptide

chains (2 separate proteins)• Most enzymes • Collagen – 3 helical polypeptides super

coiled into a triple helix• Connective tissue strength• Hemoglobin – 4 polypeptide chains,

globular protein

Conformation determined by:

• primary structure• Consequence of interactions

responsible for secondary and tertiary• Influenced by physical and chemical

environment• May be dynamic – alternating between

several shapes

Denaturation –a process that alters protein’s conformation

• Excessive heat

• Change in pH

• Change in salt concentration

• Exposure to different chemicals

• Transfer to organic solvent

Nucleic acids

• Store and transmit hereditary info

• Monomer – nucleotide:– Phosphate

– 5C- sugar

– Nitrogenous base

Two families of bases

• Pyrimidine – 1 ring (6 membered ring)

• Cytosine

• Thymine

• Uracil – only in RNA

Purine

• 2 rings

• 5 membered ring fused with a six membered ring

• Adenine

• guanine

Functions of nucleotides

• Monomers of nucleic acids

• Phosphodiester linkages between phosphate of one and sugar of another

• Transfer chemical energy in ATP

• Electron acceptors - NAD

DNA

• Double stranded helix• Contains coded information for

proteins• Programs all cellular activity• Can replicate itself• Makes up genes• Directs RNA to make proteins

RNA

• Single stranded

• Synthesis of proteins coded in DNA

• mRNA – copies code of DNA

• tRNA – transfers amino acids to ribosome

• rRNA – makes up ribosomes

Types of RNA