Two or more polypeptide chains can cometogether to form one functional moleculewith several subunits. The four subunits ofhemoglobin cooperate so that the complexpicks up and delivers more oxygen than ispossible with single subunits.

Tertiary StructureMany functional proteins fold into a compactglobular shape, with many carbon-rich aminoacids sheltered inside away from the surroundingwater. The folded structure of hemoglobin includes a pocket to hold heme, which is themolecule that carries oxygen as it is transportedthroughout the body.

Hydrogen bonds between amino acids formtwo particularly stable structural elements inproteins: alpha helices and beta sheets.Alpha helices (shown in blue) are the basicstructural elements found in hemoglobin,but many other proteins also include betasheets. The inset highlights the pattern of hydrogen bonds (shown in green) that stabilizes alpha helices.

Secondary Structure

What is aProtein?

Quaternary Structure

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PrimaryStructure

one amino acid

heme

Primary structure is the linear sequence of amino acids as encodedby the DNA. This sequence defines how the protein will fold andtherefore also defines how it will function. A single change in theamino acid sequence of hemoglobin can cause the proteins toclump together, resulting in the disease sickle cell anemia.

Proteins play countless roles throughout the biological world, from catalyzing chemical reactions to building the structures of all living things.

Despite this wide range of functions all proteins are made out of thesame twenty amino acids, but combined in different ways. The waythese twenty amino acids are arranged dictates the folding of the proteininto its unique final shape. Since protein function is based on theability to recognize and bind to specific molecules, having the correctshape is critical for proteins to do their jobs correctly.

PDB ID: 1hho

ProteinShape & Function

Enzymes Alpha amylase is an enzyme with a specific catalytic site that begins thebreakdown of carbohydrates in our saliva.

Storage Ferritin forms a hollow shell that stores iron from our food.

Specific amino acid sequences give proteins their distinct shapes and chemical characteristics. Proteinshape is important because many proteins rely onthe recognition of specific 3D molecular shapes tofunction correctly.

Communication Insulin is a small, stable protein thatcan easily maintain its shape whiletraveling through the blood to regulateblood sugar levels.

To learn more about these and other proteins please visit the PDB-101 at pdb101.rcsb.org

PDB ID: 1igt

PDB ID: 2plv

PDB ID: 1ppi

PDB ID: 1fha

PDB IDs: 1su4 and 1iwo

PDB ID: 4ins

Structure Collagen forms a strong and flexible triplehelix that is widely used throughout thebody for structural support.

Text and illustrations by Jessica May and David S. Goodsell. This flyer is accompanied by an animation available at bit.ly/1JkBKgZ.

TransportThe calcium pump moves ions across cell membranesallowing the synchronized contraction of muscle cells.

Defense The flexible arms of antibodies havebinding sites that can protect the bodyfrom disease by recognizing and bindingto foreign molecules.

PDB ID: 1bkv