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Chi-Hing C. Cheng
proteins came from. What are their genetic ongms and the molecular
mechanisms that created them? When did they appear? The availability of
modem molecular techniques has allowed us to tackle these longstanding
questions and answer them in some cases. This chapter reviews the recent
findings on the evolution of the antifreeze glycoproteins of the Antarctic
notothenioids, and the near-identical antifreeze glycoproteins of the
unrelated northern cods.
Diversity of Fish Antifreeze Proteins
Four structurally different antifreeze proteins - antifreeze glycoprotein
(AFGP), and type I, II, and III AFPs (antifreeze peptides) from different
cold-water fishes - have been well characterized. These proteins are found
in abundance in the circulation of these fishes, 10-35 mg/ml depending on
the species and the severity of their cold environment. Despite their
diverse structures, they all perform the same function. The generally
accepted mechanism of antifreeze action is adsorption-inhibition. It is a
non-colligative mechanism involving binding of antifreeze molecules to
specific faces of the ice crystals that occasionally enter the fish, and inhibit
ice growth at the temperatures they live in, through the so-called Kelvin
effect whereby the thermodynamic driving force for ice expansion was
essentially removed. Thus the fish's body fluids are preserved in a liquid
state and the fish avoids freezing ([7,8] for reviews).
The first antifreeze protein discovered was the AFGP from Antarctic
notothenioid fishes by DeVries [9]. AFGPs are present in the notothenioids
as a family of size isofonns, all composed of repeating units of the simple
glycotripetide monomer:
(-~-J\la-J\la-)n
N-acetylgalactosamine
/
Galactose
Eight different sizes of AFGPs were initially described; the largest was
designated as AFGPI (molecular weight 34 kDa, n=55), and the smallest
as AFGP8 (molecular weight 2.6 kDa, n=4) [10,11]. This nomenclature
has persisted although better protein resolution techniques have now
revealed the presence of many other intermediate sizes, totaling at least 18
in the McMurdo species Dissostichus mawsoni and Pagothenia
borchgrevinki [12]. In addition to size variations, minor compositional
variations are found in the small AFGPs 6-8 in which the first J\la in the
repeats are sometimes replaced by Pro [12,l3]. Thus the notothenioid
AFGPs, although simple in primary structure, are complex in protein
Chi-Hing C. Cheng
proteins came from. What are their genetic ongms and the molecular
mechanisms that created them? When did they appear? The availability of
modem molecular techniques has allowed us to tackle these longstanding
questions and answer them in some cases. This chapter reviews the recent
findings on the evolution of the antifreeze glycoproteins of the Antarctic
notothenioids, and the near-identical antifreeze glycoproteins of the
unrelated northern cods.
Diversity of Fish Antifreeze Proteins
Four structurally different antifreeze proteins - antifreeze glycoprotein
(AFGP), and type I, II, and III AFPs (antifreeze peptides) from different
cold-water fishes - have been well characterized. These proteins are found
in abundance in the circulation of these fishes, 10-35 mg/ml depending on
the species and the severity of their cold environment. Despite their
diverse structures, they all perform the same function. The generally
accepted mechanism of antifreeze action is adsorption-inhibition. It is a
non-colligative mechanism involving binding of antifreeze molecules to
specific faces of the ice crystals that occasionally enter the fish, and inhibit
ice growth at the temperatures they live in, through the so-called Kelvin
effect whereby the thermodynamic driving force for ice expansion was
essentially removed. Thus the fish's body fluids are preserved in a liquid
state and the fish avoids freezing ([7,8] for reviews).
The first antifreeze protein discovered was the AFGP from Antarctic
notothenioid fishes by DeVries [9]. AFGPs are present in the notothenioids
as a family of size isofonns, all composed of repeating units of the simple
glycotripetide monomer:
(-~-J\la-J\la-)n
N-acetylgalactosamine
/
Galactose
Eight different sizes of AFGPs were initially described; the largest was
designated as AFGPI (molecular weight 34 kDa, n=55), and the smallest
as AFGP8 (molecular weight 2.6 kDa, n=4) [10,11]. This nomenclature
has persisted although better protein resolution techniques have now
revealed the presence of many other intermediate sizes, totaling at least 18
in the McMurdo species Dissostichus mawsoni and Pagothenia
borchgrevinki [12]. In addition to size variations, minor compositional
variations are found in the small AFGPs 6-8 in which the first J\la in the
repeats are sometimes replaced by Pro [12,l3]. Thus the notothenioid
AFGPs, although simple in primary structure, are complex in protein