Question

Outline the mature protein structure and function of the following four human molecules.                                                                                                                         /8 marks

a) transthyretin

b) mucin 2

c) catalase

d) calmodulin

Answer 1

ANSWER :

a) Transthyretin (TTR)

TTR is a 55kDa homotetramer with a dimer of dimers quaternary structure that is synthesized in the liver, choroid plexus and retinal pigment epithelium for secretion into the bloodstream, cerebrospinal fluid and the eye, respectively. Each monomer is a 127-residue polypeptide rich in beta sheet structure. TTR monomer is composed of 127 amino acid residues, forming 8 β-strands named from A-H, which are arranged in a β-sandwich of two four-stranded β-sheets and one small α-helix found between β-strands E and F.TTR monomers interact via hydrogen bonds between the antiparallel, adjacent β-strands H-H’ and F-F’ to form a dimeric species. The two dimers (A-B and C-D) predominantly form the tetramer through hydrophobic contacts between the residues of the A-B and G-H loops. The tetramer forms a central hydrophobic pocket (T4 channel) with two binding sites for hormones . Each TTR monomer contains one cysteine residue at position 10 and two tryptophan residues at positions 41 and 79, which can be used as a tool for monitoring TTR unfolding.

TTR transports retinol through binding to retinol-binding protein (RBP) and T4 due to the formation of a hydrophobic channel, which exists only when TTR is tetrameric. Although its function may vary, TTR is highly conserved from humans to bacteria.TTR is known primarily as a transporter, and TTR can also act as a protease and a neuroprotective molecule.Transthyretin (TTR) is an amyloidogenic protein with more than 120 TTR gene mutations that can lead to a heterogeneous group of disorders characterized by accumulation of polypeptide amyloid fibrils.

b) MUCIN 2

MUC2 is the major intestinal mucin, expressed by goblet cells of the small intestine and colon. Human MUC2 mucin has a protein core of approximately 5,179 amino acids, with a domain. MUC2 has cysteine-rich N- and C-terminal parts with four complete von Willebrand D domains in total. The central PTS domains are rich in serine, threonine, and proline, and these domains become heavily O-glycosylated to generate mucin domains. MUC2 forms dimers in the endoplasmic reticulum by disulfide bonds between the C termini. The dimer is translocated into the Golgi apparatus, where it is O-glycosylated, resulting in a size of ≈5 MDa. The MUC2 network is formed by disulfide-linked trimers connecting the N termini. The large polymers are stored in mucin granulae in the goblet cells before being secreted.

Its function include the Coats the epithelia of the intestines, airways, and other mucus membrane-containing organs. Thought to provide a protective, lubricating barrier against particles and infectious agents at mucosal surfaces. Major constituent of both the inner and outer mucus layers of the colon and may play a role in excluding bacteria from the inner mucus layer.

C) CATALASE

Catalase, an enzyme which catalyzes the reaction by which hydrogen peroxide is decomposed to water and oxygen. Found extensively in organisms that live in the presence of oxygen, catalase prevents the accumulation of and protects cellular organelles and tissues from damage by peroxide, which is continuously produced by numerous metabolic reactions. In mammals, catalase is found predominantly in the liver.

Catalase performs its rapid destruction of hydrogen peroxide in two steps. First, a molecule of hydrogen peroxide binds and is broken apart. One oxygen atom is extracted and attached to the iron atom, and the rest is released as harmless water. Then, a second hydrogen peroxide molecule binds. It is also broken apart and the pieces are combined with the iron-bound oxygen atom, releasing water and oxygen gas. The mammalian catalases are homotetrameric feriheme-containing enzymes whose molecular mass is approximately 240 kDa.

D) Calmodulin

The general function of CaM is to bind calcium ions and then bind a target protein, affecting its activity.Upon binding Ca2+, calmodulin changes its conformation to form complexes with a diverse array of target proteins. CaM is dynamic Ca2+ sensor, capable of responding to a wide range of Ca2+ concentrations, e.g. 10−12 M – 10−6 M, in Ca2+-dependent cellular signaling. The binding of calcium is achieved by the 4 EF-hand domains. A basic EF hand consists of two perpendicular alpha helices with a 12-residue loop region between them. Calmodulin has 148 amino acid residues. Its molecular weight is 16707.39 Da, and its isoelectric point (pI) is 4.09. Calmodulin is dumbbell-shaped. It has two globular domains connected by a central alpha-helix. Each domain has three alpha-helices and two calcium binding EF hands. The EF domains have a degree of sequence homology as low as 25%, but their three-dimensional structures are nearly identical.