3.4 Protein classification and diversity

Proteins come in various shapes and sizes, each with unique functions. From globular enzymes to fibrous structural proteins, their diverse forms enable them to perform a wide range of tasks in living organisms.

Protein classification helps us understand how these molecular machines work together. By grouping proteins into families and studying their domains, you can uncover the relationships between structure and function in biological systems.

Protein Structure and Shape

Globular, Fibrous, and Membrane Proteins

The three major structural categories of proteins reflect where they work and what they do. Shape and solubility are tightly linked to function here.

Globular proteins have a compact, roughly spherical shape. Their hydrophobic residues are buried in the interior while hydrophilic residues face outward, making them soluble in aqueous environments like the cytoplasm. This solubility allows them to move freely and carry out dynamic functions: enzymes, antibodies, and many signaling proteins are globular.

Fibrous proteins have elongated, fiber-like structures built from repeating structural motifs. They're generally insoluble in water, which makes sense given their role providing structural support and protection. Collagen, the most abundant protein in mammals, reinforces connective tissues like skin, bone, and tendons. Keratin forms tough, protective structures like hair, nails, and feathers.

Membrane proteins are embedded in or associated with biological membranes.

• Integral membrane proteins span part or all of the lipid bilayer. Their transmembrane regions are rich in hydrophobic residues that interact with the lipid core. Examples include ion channels and G protein-coupled receptors.
• Peripheral membrane proteins associate with the membrane surface through electrostatic interactions or by binding to integral proteins. They often function as enzymes or components of signaling pathways.

Protein Domains

Protein domains are distinct structural and functional units within a protein. Each domain typically folds independently into a stable three-dimensional structure, even when separated from the rest of the polypeptide.

• Catalytic domains in enzymes contain the active site and carry out chemical reactions.
• Binding domains recognize and bind to specific ligands, DNA sequences, or other proteins.

Many proteins are multi-domain, consisting of several domains linked together. Transcription factors, for example, often have a separate DNA-binding domain and a transcriptional activation domain. This modularity is a major source of functional diversity: evolution can shuffle, duplicate, and recombine domains to create proteins with new combinations of activities.

Protein Functions

Enzymes and Structural Proteins

Enzymes are biological catalysts that accelerate chemical reactions without being consumed in the process. They work by lowering the activation energy ($E_a$) required for a reaction to proceed. Enzymes exhibit high specificity for their substrates. The classic lock-and-key model describes rigid complementarity between enzyme and substrate, though the induced-fit model is more accurate: the enzyme's active site adjusts its shape slightly upon substrate binding. Examples include DNA polymerase (replicates DNA) and kinases (transfer phosphate groups).

Structural proteins provide mechanical support and maintain cell and tissue shape.

• Collagen is the most abundant protein in the human body and the main component of connective tissues (skin, bones, tendons). It forms a triple-helix structure from three polypeptide chains wound around each other.
• Keratin forms tough, protective structures like hair, nails, and feathers. It's rich in disulfide bonds, which contribute to its rigidity.
• Actin and myosin are the filament proteins that enable muscle contraction and cell movement through their sliding interaction.

Transport Proteins and Receptors

Transport proteins facilitate the movement of molecules across biological membranes or through the bloodstream.

• Ion channels allow selective passage of specific ions ($Na^+$, $K^+$, $Ca^{2+}$) down their concentration gradients. Selectivity depends on the channel's pore size and charge distribution.
• Carrier proteins bind and transport specific substances. Glucose transporters (GLUTs) move glucose across cell membranes. Hemoglobin, while not a membrane protein, is a soluble carrier that transports $O_2$ in red blood cells.
• ATP-powered pumps actively transport molecules against their concentration gradients. The $Na^+/K^+$-ATPase pumps 3 $Na^+$ out and 2 $K^+$ in per ATP hydrolyzed, maintaining the electrochemical gradient essential for nerve signaling.

Receptors are proteins that bind to specific ligands and initiate cellular responses.

• Cell surface receptors recognize extracellular signals like hormones and neurotransmitters. Ligand binding on the extracellular side triggers conformational changes that relay the signal inside the cell.
• Intracellular receptors bind to molecules that can cross the membrane on their own, such as steroid hormones. These receptor-ligand complexes often act directly as transcription factors.
• Receptor activation triggers signaling cascades that ultimately regulate gene expression, metabolism, and cell behavior.

Protein Classification

Protein Families

Protein families are groups of evolutionarily related proteins that share similar amino acid sequences and three-dimensional structures. Members of a family typically descended from a common ancestral protein through gene duplication and divergence.

• The globin family includes hemoglobin and myoglobin, both of which use a heme prosthetic group to bind oxygen. Despite differences in quaternary structure (hemoglobin is a tetramer, myoglobin is a monomer), their individual subunits share a highly similar fold.
• The kinase family consists of enzymes that transfer phosphate groups from ATP to substrate molecules. Protein kinases regulate nearly every aspect of cell signaling, and the human genome encodes over 500 of them.

Protein families are identified through sequence alignment and structural comparisons.

• Conserved regions across family members point to residues that are functionally critical, since mutations at those positions would be selected against.
• Variable regions allow for functional diversity and evolutionary adaptation, enabling family members to recognize different substrates or respond to different signals while sharing a common catalytic mechanism.