Chapter 5 Proteins

Proteins More than 50% of the dry weight of most cells Humans have tens of thousands of different proteins, each with a specific structure and function All proteins are polymers (polypeptides) built from the same 20 amino acids Used for structural support, storage, transport of substances, signaling from one part of the organisms to another, movement, defense against foreign substances, and as enzymes (see Table 5.1 on Page 72)

Amino Acids There are only 20 amino acids Organic molecules possessing both carboxyl and amino groups Amino acids are grouped according to the properties of their side chains Some are polar, some nonpolar, some acidic and some basic

Nonpolar amino acids - hydrophobic

Polar amino acids - hydrophilic

Amino Acids Electrically charged amino acids – side chains are negatively or positively charged causing them to act as acids or bases and to dissociate in water. They are hydrophilic.

Peptide Bond Peptide bond – covalent bond linking amino acids together to form polypeptides Polypeptides vary in length from a few monomers to thousands or more

Proteins Each specific polypeptide has a unique linear sequence of amino acids Chains of polypeptides make a protein A protein’s specific conformation determines how it works A functional protein consists of one or more polypeptides precisely twisted, folded, and coiled into a molecule of unique shape Four Levels of Structure Primary, Secondary, Tertiary, Quaternary

The primary structure of a protein is the unique amino acid sequence

Primary Structure A change in the primary structure can affect a protein’s conformation and ability to function

Secondary Structure Segments of the polypeptide chain repeatedly coiled or folded in unique patters that contribute to the protein’s overall conformation Results from the hydrogen bonds at regular intervals along the polypeptide backbone, not the amino acid side chains Can be in the form of an α helix or β pleated sheet

Tertiary Structure Irregular contortions from interactions between side chains of the various amino acids.

Quaternary Structure The association between two or more polypeptides that make up a protein

What Determines Protein Conformation? Dependent upon the interactions among the amino acids making up the polypeptide chain Usually arise spontaneously as soon as the protein is synthesized in the cell These interactions can be disrupted by changes in pH, salt concentration, temperature, etc. causing the protein to denature, losing its native conformation and thus its function

The Protein-Folding Problem Chaperonins, chaperone proteins that assist the folding of other proteins.

X-Ray Crystallography