Honeybee Silks

11.08.2015 Innovative Biofibers from Renewable Resources

Honeybee • Fiber cross section • Nanofibril • Amino acid

Proteins produced by honeybees have distinct structure and properties compared to Bombyx mori or spider silks. Unlike the silkworm or spider silks that are composed of two filaments (brins) connected to each other, honeybee silk is formed by a single filament with a circular cross section and finer and smoother texture [10Zha]. Honeybee silks are formed by the assembly of 4-4.5 nm wide fibrils that consist of fine filaments of 2-2.5 nm in width similar to B. mori silks. These fibrils further formed tactoids that are 1-3 pm in width and 3-40 pm in length [11Sut]. To study the structure and properties of natural honeybee silk fibers, Italian honeybee (Apis mellifera) larvae were placed on glass plates and allowed to spin fibers at room temperature. Fibers formed were collected for analysis. Figure 44.1 shows a three-dimensional scanning probe microscope image of the honeybee and B. mori silk fibers. As seen from the SEM images in Fig. 44.2, honeybee silk has a circular and smooth cross section and did not show the presence of nanofibrils (dots in Fig. 44.2a) as opposed to the typical triangular cross section and nanofibrils seen in silkworm silk. The presence of a single filament in honeybee silk is evident from the cross section. X-ray diffraction studies have shown that honeybee silks predominantly contain a-helices in a coiled-coil form [06Sut]. In terms of primary structure, honeybee silks primarily contain high levels of alanine, serine, and aspartic and glutamic acid and considerably lower levels of glycine compared to regular silks. Six genes encoding silk proteins were identified in A. mellifera larvae that were named AmelFibroin 1-4. In addition, two genes (AmelSAl and 2) that are associated with silk were also identified [06Sut]. Table 44.1 lists the major differences between the four genes identified in the honeybee silks and silkworm (B. mori) silks. Tensile tests of the honeybee silk also showed substantial

Fig. 44.1 Scanning probe microscope images of silkworm (B. mori) and honeybee silk depict the distinct morphological structure [10Zha]. Reproduced with permission from Elsevier

Fig. 44.2 SEM images of the cross section of silkworm and honeybee silk fibers [10Zha]. Reproduced with permission from Elsevier

differences. Honeybee silk fibers had a nearly linear stress-strain curve until the fibers were broken. Breaking strength of the honeybee silk fibers was 1.4 g/den, elongation was 3.8 %, and modulus was 56 g/den. It was suggested that the considerably lower strength and elongation of the honeybee silk compared to silkworm or spider silks should be due to the functional differences of the silks. Honeybee silk is mostly secreted to act as reinforcement for the honeycombs and is not required to support heavy loads or strains.

Table 44.1 Comparison of the primary and secondary structures of honeybee silk and B. mori silk fibers [06Sut]

	Secondary structure (%)
Protein	a	в	Amino acids	Mol. wt. (kDa)
AmelFibroin 1	76	—	315	33
AmelFibroin 2	88	—	290	30
AmelFibroin 3	84	—	316	33
AmelFibroin 4	76	—	323	34
AFre1	75	—	552	61
AmelSA1	41	7	35	42
AmelSA2	21	18	4,262	490
B. mori heavy fibroin	—	64	5,242	392
Sericin	—	22	1,199	121