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The expanded amelogenin polyproline region preferentially binds to apatite versus carbonate and promotes apatite crystal elongation.

Gopinathan G, Jin T, Liu M, Li S, Atsawasuwan P, Galang MT, Allen M, Luan X, Diekwisch TG - Front Physiol (2014)

Bottom Line: Under calcium phosphate crystal growth conditions, only the carboxy-terminus augmented polyproline repeat peptide, but not the N-terminal peptide nor the polyproline repeat peptide alone, promoted the formation of thin and parallel crystallites resembling those of bone and initial enamel.In contrast, the amelogenin N-terminus bound to both carbonate and apatite, but preferentially to calcium carbonate.Our data suggest that the rise of apatite-based biominerals in vertebrates might have been facilitated by a rapid evolution of specialized polyproline repeat proteins flanked by a charged domain, resulting in apatite crystals with reduced width, increased length, and tailored biomechanical properties.

View Article: PubMed Central - PubMed

Affiliation: Oral Biology, University of Illinois at Chicago Brodie Laboratory for Craniofacial Genetics, University of Illinois at Chicago College of Dentistry Chicago, IL, USA.

ABSTRACT
The transition from invertebrate calcium carbonate-based calcite and aragonite exo- and endoskeletons to the calcium phosphate-based vertebrate backbones and jaws composed of microscopic hydroxyapatite crystals is one of the great revolutions in the evolution of terrestrial organisms. To identify potential factors that might have played a role in such a transition, three key domains of the vertebrate tooth enamel protein amelogenin were probed for calcium mineral/protein interactions and their ability to promote calcium phosphate and calcium carbonate crystal growth. Under calcium phosphate crystal growth conditions, only the carboxy-terminus augmented polyproline repeat peptide, but not the N-terminal peptide nor the polyproline repeat peptide alone, promoted the formation of thin and parallel crystallites resembling those of bone and initial enamel. In contrast, under calcium carbonate crystal growth conditions, all three amelogenin-derived polypeptides caused calcium carbonate to form fused crystalline conglomerates. When examined for long-term crystal growth, polyproline repeat peptides of increasing length promoted the growth of shorter calcium carbonate crystals with broader basis, contrary to the positive correlation between polyproline repeat element length and apatite mineralization published earlier. To determine whether the positive correlation between polyproline repeat element length and apatite crystal growth versus the inverse correlation between polyproline repeat length and calcium carbonate crystal growth were related to the binding affinity of the polyproline domain to either apatite or carbonate, a parallel series of calcium carbonate and calcium phosphate/apatite protein binding studies was conducted. These studies demonstrated a remarkable binding affinity between the augmented amelogenin polyproline repeat region and calcium phosphates, and almost no binding to calcium carbonates. In contrast, the amelogenin N-terminus bound to both carbonate and apatite, but preferentially to calcium carbonate. Together, these studies highlight the specific binding affinity of the augmented amelogenin polyproline repeat region to calcium phosphates versus calcium carbonate, and its unique role in the growth of thin apatite crystals as they occur in vertebrate biominerals. Our data suggest that the rise of apatite-based biominerals in vertebrates might have been facilitated by a rapid evolution of specialized polyproline repeat proteins flanked by a charged domain, resulting in apatite crystals with reduced width, increased length, and tailored biomechanical properties.

No MeSH data available.


Related in: MedlinePlus

Binding of amelogenin fragments to calcium minerals. (A,B) illustrate results from parallel experiments in which either an amelogenin N-terminal fragment (N33, A) or a C-terminus augmented amelogenin repeat fragment (PXXC, B) were incubated with three different calcium minerals, including calcium carbonate (CaCO3), calcium phosphate, and nanohydroxyapatite (NHAP). (C,D) illustrates our explanation for the differences in calcium mineral crystal growth when subjected to different protein environments. Here we propose that polyproline repeat regions (blue hooks) as they occur in amelogenin bind to the a- and b- axis of calcium phosphate crystals and allow for the growth of long apatite crystal through expansion mostly in c-axis direction (C). In contrast, proteins including polyproline peptides appear to affect calcium carbonate crystal growth equally in all directions, resulting in an overall compaction of the final crystal (D).
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Figure 4: Binding of amelogenin fragments to calcium minerals. (A,B) illustrate results from parallel experiments in which either an amelogenin N-terminal fragment (N33, A) or a C-terminus augmented amelogenin repeat fragment (PXXC, B) were incubated with three different calcium minerals, including calcium carbonate (CaCO3), calcium phosphate, and nanohydroxyapatite (NHAP). (C,D) illustrates our explanation for the differences in calcium mineral crystal growth when subjected to different protein environments. Here we propose that polyproline repeat regions (blue hooks) as they occur in amelogenin bind to the a- and b- axis of calcium phosphate crystals and allow for the growth of long apatite crystal through expansion mostly in c-axis direction (C). In contrast, proteins including polyproline peptides appear to affect calcium carbonate crystal growth equally in all directions, resulting in an overall compaction of the final crystal (D).

Mentions: The purpose of this experiment was to ask the question whether portions of the amelogenin molecule display preferential binding affinity to either apatite, calcium phosphate, or calcium carbonate mineral. In a first part of this experiment, the N-terminal N33 amelogenin peptide was incubated with calcium carbonate, calcium phosphate, and nanohydroxyapatite, and our binding assay revealed that N-terminal binding was approximately double as high between N33 and CaCO3 as it was between N33 and the two calcium phosphates employed in this study (Figure 4A). The second experiment (Figure 4B) was conducted in parallel and revealed that the C-terminus augmented polyproline repeat peptide preferentially bound to calcium phosphate and the nanohydroxyapatite minerals, while there was only faint evidence of carbonate binding. Together, this study demonstrated that the long amelogenin polyproline repeat region augmented by the charged C-terminus strongly binds to calcium phosphate and nanohydroxyapatite, while the amelogenin N-terminus preferentially binds to calcium carbonate.


The expanded amelogenin polyproline region preferentially binds to apatite versus carbonate and promotes apatite crystal elongation.

Gopinathan G, Jin T, Liu M, Li S, Atsawasuwan P, Galang MT, Allen M, Luan X, Diekwisch TG - Front Physiol (2014)

Binding of amelogenin fragments to calcium minerals. (A,B) illustrate results from parallel experiments in which either an amelogenin N-terminal fragment (N33, A) or a C-terminus augmented amelogenin repeat fragment (PXXC, B) were incubated with three different calcium minerals, including calcium carbonate (CaCO3), calcium phosphate, and nanohydroxyapatite (NHAP). (C,D) illustrates our explanation for the differences in calcium mineral crystal growth when subjected to different protein environments. Here we propose that polyproline repeat regions (blue hooks) as they occur in amelogenin bind to the a- and b- axis of calcium phosphate crystals and allow for the growth of long apatite crystal through expansion mostly in c-axis direction (C). In contrast, proteins including polyproline peptides appear to affect calcium carbonate crystal growth equally in all directions, resulting in an overall compaction of the final crystal (D).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4227485&req=5

Figure 4: Binding of amelogenin fragments to calcium minerals. (A,B) illustrate results from parallel experiments in which either an amelogenin N-terminal fragment (N33, A) or a C-terminus augmented amelogenin repeat fragment (PXXC, B) were incubated with three different calcium minerals, including calcium carbonate (CaCO3), calcium phosphate, and nanohydroxyapatite (NHAP). (C,D) illustrates our explanation for the differences in calcium mineral crystal growth when subjected to different protein environments. Here we propose that polyproline repeat regions (blue hooks) as they occur in amelogenin bind to the a- and b- axis of calcium phosphate crystals and allow for the growth of long apatite crystal through expansion mostly in c-axis direction (C). In contrast, proteins including polyproline peptides appear to affect calcium carbonate crystal growth equally in all directions, resulting in an overall compaction of the final crystal (D).
Mentions: The purpose of this experiment was to ask the question whether portions of the amelogenin molecule display preferential binding affinity to either apatite, calcium phosphate, or calcium carbonate mineral. In a first part of this experiment, the N-terminal N33 amelogenin peptide was incubated with calcium carbonate, calcium phosphate, and nanohydroxyapatite, and our binding assay revealed that N-terminal binding was approximately double as high between N33 and CaCO3 as it was between N33 and the two calcium phosphates employed in this study (Figure 4A). The second experiment (Figure 4B) was conducted in parallel and revealed that the C-terminus augmented polyproline repeat peptide preferentially bound to calcium phosphate and the nanohydroxyapatite minerals, while there was only faint evidence of carbonate binding. Together, this study demonstrated that the long amelogenin polyproline repeat region augmented by the charged C-terminus strongly binds to calcium phosphate and nanohydroxyapatite, while the amelogenin N-terminus preferentially binds to calcium carbonate.

Bottom Line: Under calcium phosphate crystal growth conditions, only the carboxy-terminus augmented polyproline repeat peptide, but not the N-terminal peptide nor the polyproline repeat peptide alone, promoted the formation of thin and parallel crystallites resembling those of bone and initial enamel.In contrast, the amelogenin N-terminus bound to both carbonate and apatite, but preferentially to calcium carbonate.Our data suggest that the rise of apatite-based biominerals in vertebrates might have been facilitated by a rapid evolution of specialized polyproline repeat proteins flanked by a charged domain, resulting in apatite crystals with reduced width, increased length, and tailored biomechanical properties.

View Article: PubMed Central - PubMed

Affiliation: Oral Biology, University of Illinois at Chicago Brodie Laboratory for Craniofacial Genetics, University of Illinois at Chicago College of Dentistry Chicago, IL, USA.

ABSTRACT
The transition from invertebrate calcium carbonate-based calcite and aragonite exo- and endoskeletons to the calcium phosphate-based vertebrate backbones and jaws composed of microscopic hydroxyapatite crystals is one of the great revolutions in the evolution of terrestrial organisms. To identify potential factors that might have played a role in such a transition, three key domains of the vertebrate tooth enamel protein amelogenin were probed for calcium mineral/protein interactions and their ability to promote calcium phosphate and calcium carbonate crystal growth. Under calcium phosphate crystal growth conditions, only the carboxy-terminus augmented polyproline repeat peptide, but not the N-terminal peptide nor the polyproline repeat peptide alone, promoted the formation of thin and parallel crystallites resembling those of bone and initial enamel. In contrast, under calcium carbonate crystal growth conditions, all three amelogenin-derived polypeptides caused calcium carbonate to form fused crystalline conglomerates. When examined for long-term crystal growth, polyproline repeat peptides of increasing length promoted the growth of shorter calcium carbonate crystals with broader basis, contrary to the positive correlation between polyproline repeat element length and apatite mineralization published earlier. To determine whether the positive correlation between polyproline repeat element length and apatite crystal growth versus the inverse correlation between polyproline repeat length and calcium carbonate crystal growth were related to the binding affinity of the polyproline domain to either apatite or carbonate, a parallel series of calcium carbonate and calcium phosphate/apatite protein binding studies was conducted. These studies demonstrated a remarkable binding affinity between the augmented amelogenin polyproline repeat region and calcium phosphates, and almost no binding to calcium carbonates. In contrast, the amelogenin N-terminus bound to both carbonate and apatite, but preferentially to calcium carbonate. Together, these studies highlight the specific binding affinity of the augmented amelogenin polyproline repeat region to calcium phosphates versus calcium carbonate, and its unique role in the growth of thin apatite crystals as they occur in vertebrate biominerals. Our data suggest that the rise of apatite-based biominerals in vertebrates might have been facilitated by a rapid evolution of specialized polyproline repeat proteins flanked by a charged domain, resulting in apatite crystals with reduced width, increased length, and tailored biomechanical properties.

No MeSH data available.


Related in: MedlinePlus