Proteins comprise about 50% of a cell's dry weight and make up material in the cell wall. Proteins are peptidoglycans and help to transport chemicals into and out of a cell. In addition, proteins are part of cell structures and cytoplasmic components. Some proteins are antibodies that kill bacteria and play a role in muscle contractions and provide movement of microorganisms. Proteins are made up of polypeptides that bond together using peptide bonds. There are four structural levels of proteins. These are:

• Primary. The primary structure is the sequence in which amino acids are linked to form the polypeptide. Sequences are genetically determined and even the slightest alteration within the sequence may have a dramatic effect on the way the protein functions.

• Secondary. The secondary structure is locally folded and is the repeated twisting of the polypeptide chain that links together the amino acids. There are two types of secondary structures. These are a helix and a pleated sheet. The alpha-helix is a clockwise spiral structure. The pleated sheet forms the parallel portion of the polypeptide chain.

• Tertiary. The tertiary structure is the three-dimensional active structure of the polypeptide chain. Tertiary structure is the minimal level of structure for biological activity.

• Quarternary. Is where the proteins, in order to be functional, contain sub-units of polypeptide chains. An example would be DNA polymerase.

Proteins have many roles in a living organism. They are found in bone collagen and connective tissue and provide protection in the form of immunoglobins, which are antibodies. Some of the other important proteins are:

• Myosin. Muscle contraction.

• Actin. Muscle contraction.

• Hemoglobin transports oxygen (O2) and carbon dioxide (CO2) in blood.

• Enzymes. An enzyme that is a biological catalyst that increases the rate of chemical reactions in cells by reducing the energy required to begin the reaction. The reaction does not change the enzyme. The name of an enzyme typically ends with "-ase."

• Flagellin. Protein in flagella.

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