Year 12 Biology Macromolecules Unit Chapter 1: M5

M5: 3D Proteins (Intro) As mentioned before, proteins are large molecules constructed from amino acids. They’re all made up of varying combinations of the twenty amino acids. Because the combinations can be of any length, and in any order, the variety of proteins is potentially infinite. In much the same way as we have many thousands of words produced from the same 26 letters. (Though we don’t generally make words past a certain length, while a protein could theoretically keep going long after our tongue would get tied saying the word.)

Intro. Continued Proteins can have a variety of purposes: They can have structural roles like hair and muscle proteins. They can act as enzymes, to speed up otherwise slow or awkward chemical reactions. They can also have other roles critical to cellular functions. These proteins are formed from one or more folded amino acid chains, and it is the shape into which they are folded which determines their purpose.

Protein Structure Protein structure is divided into four levels: Primary Secondary Tertiary Quarternary Each step represents an important stage in folding and shaping the protein to fulfil its ultimate purpose.

Primary Structure The primary structure is simply the order of the amino acids which make it up. A single change can make a radically different protein. E.g. Sickle Cell Anaemia is caused by a single amino acid substitution in haemoglobin and is a very serious condition.

Secondary Structure This is the initial folding caused by the hydrogen bonding between amino acids. The polypeptide chains may form a variety of shapes: Coils Helices Fibrils Pleated Sheets Alpha Helix Beta Sheet

Tertiary Structure The tertiary structure is the actual three dimensional shape of the polypeptide chain. This structure forms spontaneously due to the attractions and repulsions between various amino acids (some like each other better than others). This is very important for globular proteins such as enzymes and hormones which don’t work if they’re not exactly the right shape.

Quarternary Structure The Quarternary structure doesn’t apply to all proteins, only to those made up of more than 1 polypeptide chain. The chains first fold into their 3D shapes before connecting to each other using the same attractions and repulsions.

Examples of Importance To be discussed on the next slides: Proteins in the Cell Membrane Enzymes Antigen-Antibody Binding

Proteins in the Cell Membrane Proteins exist on both the inner and outer surfaces of the cell membrane, as well as in the space between. Most functions of the cell membrane are carried out by the proteins embedded within it. Some act as chemical receptors for messages from other cells. These messages are usually hormones, and the message they carry is determined by the specific 3D shape of the hormone, which bonds only to a matching protein. Each protein is then designed to transmit a specifically matched message through the cell. Other proteins are designed to allow one-way transport of certain molecules either into or out of the cell.

Enzymes Enzymes are catalysts designed to speed up reactions in cells that would otherwise take a long time. Each enzyme has a specific shape designed to match specific products. Only the molecules with the complementary shape can fit with the enzyme. However, the reaction can run both ways if needed.

Antigen-Antibody Binding Antigens are foreign molecules which the body would like to get rid of, they’re usually found stuck in the membranes of cells. Antibodies are the bodies defence to antigens. While it’s starting to sound like a broken record it’s once again the specific shape which is important for the antibody to match to the antigen. The antibody is designed to deactivate the antigen when it bonds to it so that the body can safely dispose of it.