PROTEINS AS MOLECULAR MACHINES

Proteins are molecular machines. They have distinct specialized functions, which are entirely dependent on their three-dimensional shapes.

Functional Protein Categories

Proteins can be sorted into approximately nine broad functional categories. These include enzymes, transporters, sensory, signal, structural, defense, motor, storage, and regulatory proteins (see Table 2.1). These groups can be further divided into families and classes. The amino acid sequence, but more importantly the 3D structure of a given protein, dictates its function. DNA sequences of genes can be used to predict the function of an encoded protein. Homologs are proteins that function in a similar manner. Evolutionarily speaking, they originate from a single gene, and therefore have similar coding sequences. Thus, putative homologs may be identified using bioinformatic analysis of DNA sequences. The results may provide a clue to the function of an otherwise uncharacterized protein.

Table 2.1. Broad functional categories of proteins

Functional

category

Function

Example

Enzymes

Catalyze chemical reactions

RNA Pol, metabolic enzymes, nucleases

Transport

Assist in the movement of molecules across cellular membranes

Gated channels, porins

Sensory

Bind to ligands in order to sense molecular cues

Cell surface receptors

Signal

Transduce signals through conformational changes or the passage of post-translational modifications such as phosphate groups

Kinases and phosphatases

Structural

Link together to create rigid or semi-rigid cellular structures

Actin, collagen

Defense

Negatively influence the function of nonself cells or other entities

Antibodies, toxins

Motor

Assist in cellular locomotion and physically translocate cellular payloads around a cell

Dynein, myosin

Storage

Bind to and store important metabolites or vitamins

Albumin

Regulatory

Influence gene expression

Transcriptional activators or repressors

 
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