14

౪

1 Introduction

gregate to form one, big complex, this is called the quaternary structure of a protein.

Hemoglobin is one example, it contains four myoglobins, which aggregate and act

together as one large protein complex to transport the oxygen molecules in blood.

Proteins function mainly either as structures or as enzymes. Many of the fibers in

cells are made of protein, such as the actin fiber that is part of the cytoskeleton and

muscles. Some proteins are active in membrane transport; e. g. a lot of transmembrane

ion channels have a barrel-shape made from β-sheets (Figure 1.11 b). Some proteins are

antibodies, making use of the specificity of the amino acid side chains to recognize struc-

tures from invading bacteria or viruses. A lot of proteins are enzymes, i. e., the catalysts

of the cell. These proteins also make use of the specificity to bind a specific metabolite,

or substrate, in such a way that the following reaction is lower in energy than it would

be unbound (Figure 1.12).

Figure 1.12: The enzyme and its substrate fit together like lock

and key. The specificity comes from specific interdisciplinary forces

(H-bonding, positive and negative charges) at exactly the right three-

dimensional position. The substrate is always positioned so that the

bonds that will be broken and the new ones that will be formed are

activated.

It is important to mention one specific protein in more detail: the G-protein transmem-

brane receptors (Figure 1.13) mentioned before in the signal transduction pathways (Fig-

ures 1.3 and 1.4). Here, a signal has to be transferred from the outside to the inside of the

cell membrane, while at the same time activating an associated enzyme. This is done

by lever action. Basically, the transmembrane part of the protein is stiff, so when the

signal on the outside binds, that binding is transferred by the force of a lever to the in-

side, changing the three-dimensional structure of that inside domain. That force moves

one of the stiff parts of the associated enzyme and thus creates the active form of the

enzyme. This type of lever action is seen in several instances of nanotechnological reg-

ulation. Analogous structure-function relationships of ion channels will be discussed in

more detail in the neuron section (see Section 1.4).

Structure and function of molecules – monomers and polymers. So far, we discussed

the main polymers in the body, or biopolymers. In nanotechnology, artificial, or synthe-

sized, polymers are used as well. Here, the variability of structures is even larger than in

biopolymers, since any reactive repeating unit can be used to create a polymer. Common

polymers are shown in Figure 1.15. The bonds between the repeating units can simply be

carbon-carbon covalent bonds, making the backbone much more hydrophobic in com-

parison to, for example, the amides of a protein backbone. This is possible since most