4.4: Protein with Alpha, Alpha-Beta, Beta and Little Secondary Structure

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Learning Goals (ChatGPT o1, 1/26/55)

(Learning goals written by Claude, Sonnet 4.6, Anthropic)

Classification of Proteins by Secondary Structure Content

• Classify proteins into the four major CATH structural classes — all-α, all-β, α/β, and proteins with little secondary structure — based on the predominant type and arrangement of secondary structure elements in their core, and explain why α/β proteins are the most abundant class in known proteomes.
• Distinguish orthogonal from up-down α-helical bundle architectures in all-α proteins in terms of the geometric relationship between helix axes, and recognize that these packing arrangements reflect different solutions to the problem of maximizing hydrophobic burial and backbone hydrogen bond satisfaction in helix-only proteins.
• Identify the major subtypes of all-β proteins — rolls, sandwiches, barrels, propellers, and clams — distinguish them by the arrangement and connectivity of their antiparallel β-strands, and explain why antiparallel rather than parallel β-sheets predominate in this class.
• Describe the structural logic of α/β sandwich architectures — two-layer (β-sheet flanked on one side by helices) and three-layer aba (β-sheet sandwiched between two helix layers) — connecting each arrangement to the β-α-β supersecondary motif as the repeating unit and explaining how layered packing maximizes hydrophobic core burial for parallel-strand sheets.
• Explain why proteins classified as having little secondary structure are likely intrinsically disordered or conditionally folded, predict that such proteins may adopt defined conformations upon binding to a target molecule (coupled folding and binding), and identify biological contexts — such as transcriptional activation (HIV TAT) and metal ion sequestration (metallothionein) — where conformational flexibility or metal-induced structure is functionally essential.

Proteins can also be classified according to the type and extent of secondary structure they contain. A detailed description of protein classes can be found at CATH. Here, we will describe the basic types with a few examples.

Alpha proteins

The core of these proteins is composed of alpha helices. The CATH classification shows two major types

a. Orthogonal bundles. Example: the Z[beta] Domain of the RNA-editing Enzyme ADAR1 (1xmk), shown in the interactive iCn3D model in Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): Alpha protein, Orthogonal bundles: Z[beta] Domain of the RNA-editing Enzyme ADAR1 (1xmk) (Copyright; author via source).
Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...fxHFM9ErUptjY7

b. Updown bundles. Example: Phospholipase A2 from Agkistrodon acutus venom (1mc2), shown in the interactive iCn3D model in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): Alpha protein, Updown bundles: Phospholipase A2 from Agkistrodon acutus venom (1mc2) (Copyright; author via source).
Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...NGSeK9JAMe3Qh9

Beta proteins

In these proteins, the core is typically an antiparallel beta sheet. There are many types of these, including single sheets, rolls, beta barrels, clams, sandwiches, propellers, etc (some of which we have already discussed). Here are a few interesting examples.

a. Roll. Example: The second SH3 domain from ponsin (2O9S), shown in the interactive iCn3D model in Figure \(\PageIndex{3}\).

Figure \(\PageIndex{3}\): Beta protein Roll: Second SH3 domain from ponsin (2O9S), (Copyright; author via source).
Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...xYZBXHWpt3njf7

b. Sandwich. Example: Mcg immunoglobulin light chain variable domain (4unu), shown in the interactive iCn3D model in Figure \(\PageIndex{4}\).

Figure \(\PageIndex{4}\): Beta protein Sandwich - Mcg immunoglobulin light chain variable domain (4unu) (Copyright; author via source).
Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...4vcY5svrBveJp7

Alpha/Beta proteins

These are the most common class and contain many beta-alpha-beta motifs with mostly parallel beta strands surrounded by alpha helices. Again, these proteins have many variants, including the alpha-beta barrel. We will show two other common ones.

a. Two-layer sandwich. Example: HIV-1 Nef-SF2 Core Domain (4U5W), shown in the interactive iCn3D model in Figure \(\PageIndex{5}\).

Figure \(\PageIndex{5}\): Alpha-Beta Two layer sandwich - HIV-1 Nef-SF2 Core Domain (4U5W) (Copyright; author via source).
Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...6Fx6J3ecPJbF3A

b. Three-layer sandwich (aba). Example: Human biliverdin IX beta reductase (1hdo), shown in the interactive iCn3D model in Figure \(\PageIndex{6}\).

Figure \(\PageIndex{6}\): Alpha-Beta Three layer sandwich (aba) - Human biliverdin IX beta reductase (1hdo) (Copyright; author via source).
Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...s8GrdasiecmZC8

Little Secondary Structures

These proteins most likely can adopt different conformations on binding to target molecules. Here are two examples

a. HIV-1 TAT (Transactivating) Protein (1JFW), shown in the interactive iCn3D model in Figure \(\PageIndex{7}\).

Figure \(\PageIndex{7}\): Few Secondary Structures - HIV-1 TAT (Transactivating) Protein (1JFW) (Copyright; author via source).
Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...zVYofj3HFasWBA

b. Rat rat metallothionein-2 (4MT2), shown in the interactive iCn3D model in Figure \(\PageIndex{8}\).

.

Figure \(\PageIndex{8}\): Few Secondary Structures - Rat rat metallothionein-2 (4MT2) (Copyright; author via source).
Click the image for a popup or use this external link: https://structure.ncbi.nlm.nih.gov/i...fYY4xF2RWd7pD7

Summary

(Summary written by Claude, Sonnet 4.6, Anthropic)

This short but conceptually important chapter introduces a classification framework for proteins based on the predominant secondary structure content of their core, providing a practical overview of the major classes recognized within the CATH system: all-α, all-β, α/β, and proteins with little secondary structure.

All-α proteins have cores composed entirely or predominantly of α-helices, with hydrophobic side chains packed at the interhelical interfaces that form the protein's interior. Two major packing geometries are recognized. In orthogonal bundles, adjacent helices cross at approximately right angles (~90°), as seen in the Zβ domain of the RNA-editing enzyme ADAR1. In up-down bundles, adjacent helices run antiparallel and roughly parallel to one another — alternating up and down — as in phospholipase A2 from viper venom. Both arrangements achieve the same thermodynamic goal of burying hydrophobic surface area while maintaining the backbone hydrogen bonding network in the helix interior.

All-β proteins contain cores built predominantly from β-sheets, which are almost invariably antiparallel — consistent with the observation that short parallel β-sheets are thermodynamically disfavored and that antiparallel connectivity can be achieved by simple hairpin turns without the long-range crossovers required for parallel topology. The structural subtypes in this class are diverse, reflecting the many ways antiparallel strands can be arranged and interconnected in three dimensions. Rolls are compact, globular arrangements in which β-strands curve around a hydrophobic core, as seen in the SH3 domain from ponsin — a widely distributed protein-protein interaction module that recognizes proline-rich sequences. Sandwiches consist of two β-sheets packed face-to-face, with a hydrophobic interface between them; the immunoglobulin light chain variable domain exemplifies this architecture, which underpins antibody binding sites and cell-surface recognition modules throughout immunology.

Alpha/beta proteins are the most structurally diverse and abundant class, constructed from the β-α-β supersecondary motif, which is repeated to generate extended parallel β-sheets flanked by α-helices. The two-layer sandwich, exemplified by the HIV-1 Nef core domain, presents a single parallel β-sheet with helices packed against one face. The more common three-layer aba sandwich — exemplified by human biliverdin IXβ reductase — buries the β-sheet between two helical layers, with hydrophobic residues at both sheet-helix interfaces contributing to a well-packed core. This arrangement, which includes the Rossmann fold as a prominent example, is versatile enough to accommodate diverse enzyme active sites and cofactor-binding geometries while maintaining structural stability through its three-layer packing.

Proteins with little secondary structure represent the fourth class. Rather than adopting a fixed folded core, these proteins exist predominantly as irregular loops, extended chains, or loosely organized regions under physiological conditions. The HIV-1 TAT transactivator protein and rat metallothionein-2 exemplify this class. TAT is an intrinsically disordered protein that likely undergoes coupled folding and binding upon interacting with its RNA or protein partners, enabling it to engage diverse targets by adopting different conformations. Metallothionein acquires structure through coordination of multiple metal ions (Zn²⁺, Cd²⁺, Cu⁺) by clusters of cysteine residues, illustrating how metal binding can substitute for the hydrophobic core collapse that drives folding in typical globular proteins. These proteins challenge the classical structure-function paradigm and represent an important category of biologically active proteins whose function is predicated on flexibility rather than rigidity.