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Choose one biologically important macromolecule (e.g: carbohydrate, protein, lipid, nucleic acid).
2. Describe the general structure of the macromolecule chosen. Also provide two specific examples of the macromolecule and explain how these two examples are structurally and functionally different.
3. Describe how these macromolecules are used in cellular respiration (Are they commonly used If not, explain why. If they are, explain how)
4. Describe one part of the cytoanatomy or other components of a cell (e.g: organelles,cytoskeleton, information storage) that this macromolecule contributes to, and detail what special abilities that macromolecule gives that part of the cell. In other words, what is it about the macromolecule structure that enables that part of the cell to complete its function.

Structure of Protein Molecules: Haemoglobin and Actin

Anatomy and physiology entail the structural and functional components of human body. Macromolecules are the essential part and parcel of the human physiological system. Comprised of carbohydrates, proteins, lipids and nucleic acids, macromolecules form various structural and chemical components of our body. Each cell in the body is made up of one or the other kind of macromolecule.

The chosen macromolecule is protein, which is widely acknowledged as the building blocks of life. Proteins are the major components responsible for forming enzymes and hormones, constructing blood and muscles, transporting various materials throughout the body (Buenrostro et al, 2013). The following essay tries to establish the key points addressing the importance of proteins in the structural reform and functional abilities of human body. 

Discussion

            Amino acid residues connected with peptide bonds make up proteins (Whitford, 2013). Chosen proteins for this essay are metalloprotein Haemoglobin and multifunctional globular protein Actin (Whitford, 2013).

Structure

Each protein is quite specific regarding their functions, and the function of each protein depends solely on their structural configuration. If the protein’s structure changes in some unfavourable condition, then that protein can no longer function properly. Generally, all proteins fall under one of the four structural categories, which are as follows:

  • Primary structure: includes sequence of amino acid chains.
  • Secondary structure: includes α-helix and β-pleated sheets, which are constructed with the help of hydrogen bonds between the peptide backbones.
  • Tertiary structure: includes different three-dimensional folds in the proteins due to side-chain interactions.
  • Quaternary structure: includes proteins which are composed of more than one amino acid.

            Red Blood Cells are comprised of protein pigment haemoglobin molecules which are responsible for its oxygen carrier function. Each RBC consists of about 250 million haemoglobin molecules. Haemoglobin has a quaternary structure and it is a protein with multiple subunits (Gleixner et al., 2016). Each subunit is made up of a protein part and a prosthetic group such as a metal ion or heme group. The porphyrin ring holds the heme group in place. The ring is made up of four pyrrole rings. This structure mostly contains alpha helices which are stabilized by hydrogen bonding. Approximately 150 amino acid residues make up the two alpha and two beta chains (Gleixner, 2016).

The structure of actin can be globular (like in G-actin) or filamentous (in F-actin) or it can be a multi-domain protein. It possesses the activity of ATP hydrolysing. A glutamate residue is also found at the active site of the ATP hydrolytic domain. Together with highly acidic N-terminus and alkaline C-terminus, it also comprises of 374 amino acid residues (Blanchoin et al., 2014). Tertiary and super secondary structures have been discovered. It is a highly conserved protein and is involved in many protein-protein interactions. Actins are responsible for several significant cellular functional properties including maintenance of cell shape and form, being a regulator of transcription, to most importantly supporting cell motility. Actin filaments, in conjunction with myosin, form the basis of muscle contraction.

Cellular Respiration and Proteins

Cellular respiration

Oxygen from lungs is carried by haemoglobin in blood to cells. It enables the tissues to perform cellular aerobic respiration cycles. The heme group, consisting of porphyrin ring and a ferrous atom, reversibly binds one oxygen molecule. In the deoxyhaemoglobin form, the globin units retain a tense conformation with electrostatic bonds among them. Conformational changes due to oxygen binding, allow them to attain a relaxed state which cooperatively binds 500times more oxygen than the deoxy-form (Rummer et al, 2013). Aerobic organisms like humans, generate 18times more ATPs from glycolysis during aerobic respiration, with the help of oxygen-utilizing TCA cycle and ETC. Proteins are indirectly used as energy sources, to run the cellular processes. Thus, haemoglobin only works as a respiratory gas transport medium and is not associated with the cellular respiratory processes directly.

Actin filaments, which are major structural components of muscles, are primarily responsible for muscle contraction and movements. Along with myosin, the globular protein molecule actin, drives motor functions by utilizing the chemical energy stored in ATP molecules, (Tang, 2015). Muscle cells produce ATP through the process of aerobic respiration.

Thus the catabolic pathways do not break down proteins who are not directly involved in cellular respiration. Glycolysis and TCA cycle, the pathways of respiration, only break glucose. Digestive enzymes like trypsin and chymotrypsin break down proteins into simpler forms like peptone or peptides. At last the peptone is broken down into amino acids. Several amino acids like glutamic acid and aspartic acid produce intermediates for the TCA cycle in the form of alpha keto glutarate by deamination or transamination reactions (Yang et al, 2014). Thus the by-products of amino acid metabolism enter the cellular respiratory pathway. Most importantly, proteins are not the first choice of macromolecules for energy production. If under any unfavourable circumstances the carbohydrate or lipid storage gets depleted, only then the cellular processes start relying on proteins for generation of ATP molecules.

Cytoanatomy

Cytoskeleton is made up of microtubules, microfilaments and intermediate filaments. Microtubule, made up of tubulin units, is the largest type of filament. Microfilaments, made up of polymers of F-actin filament, are the smallest type of filaments (Blanchoin et al., 2014). Septins, spectrins are also cytoskeleton proteins present in eukaryotes. Microfilaments have spiral filamentous structure, made up of globular actin protein molecules, which together with myosin protein, play a major role in muscle contraction.

Plasma membrane is provided with support and strength by these filaments which thus aid in cellular movement. Due to this property, actins play a major role in cell division, especially in cytokinesis. Cytoskeletons are important for maintaining a cell’s shape and organization. During mitosis the actin filaments help to maintain the cell shape by bearing the tension of centrosome positioning. It is due to the actin microfilaments that each cell is able to divide properly, without any imparity (Tee et al, 2015).

Cytoanatomy of Proteins

            Ribosomal RNAs (rRNA) and different proteins make up the cell organelle ribosome. The structural assembly of the organelle is mainly taken care of by both rRNA and ribosomal proteins. Eukaryotic ribosome has more than 80 proteins present on the surface and stabilizes the structure (Bai et al., 2013). Although some of these proteins do not have any role in the mature ribosome, due to enormous cooperativity of the ribosomal proteins it is quite difficult to distinguish the function of each protein.

Along with their role as house-keeping elements of the cells, these proteins attend to several functions which are independent of ribosomes. They possess significant objectivities in immunogenic, tumor-suppressor functions as well (Zhou et al, 2015). The essential process of translation accounts for the participation of these proteins, along with the rRNAs. Different steps of translation require binding of different ribosomal subunits which are specific for recognizing the core sequences.

Conclusion

            Therefore from the above discussion it can be concluded that proteins are the most diversely significant macromolecules of the human physiological system. The detailed structure of a haemoglobin molecule and the different forms of actin protein, enable these macromolecules to perform their roles in cellular processes. Any changes in their structures lead to severe impairment of the bodily functions. Every protein molecule has a unique function, such as haemoglobin acts as a transporter for respiratory gases whereas actin entails cellular structure and motility, as well as muscle contraction and movement. The proteins involved in translation holds the most importance, since any discrepancy in their function can lead to unavoidable consequences. Thus, proteins are of incredible importance to the human body.

References

Bai, X. C., Fernandez, I. S., McMullan, G., & Scheres, S. H. (2013). Ribosome structures to near-atomic resolution from thirty thousand cryo-EM particles. elife, 2, e00461.

Blanchoin, L., Boujemaa-Paterski, R., Sykes, C., & Plastino, J. (2014). Actin dynamics, architecture, and mechanics in cell motility. Physiological reviews, 94(1), 235-263.

Buenrostro, J. D., Giresi, P. G., Zaba, L. C., Chang, H. Y., & Greenleaf, W. J. (2013). Transposition of native chromatin for fast and sensitive epigenomic profiling of open chromatin, DNA-binding proteins and nucleosome position. Nature methods, 10(12), 1213.

Gleixner, E., Ripp, F., Gorr, T. A., Schuh, R., Wolf, C., Burmester, T., & Hankeln, T. (2016). Knockdown of Drosophila hemoglobin suggests a role in O2 homeostasis. Insect biochemistry and molecular biology, 72, 20-30.

Rummer, J. L., McKenzie, D. J., Innocenti, A., Supuran, C. T., & Brauner, C. J. (2013). Root effect hemoglobin may have evolved to enhance general tissue oxygen delivery. Science, 340(6138), 1327-1329.

Tang, D. D. (2015). Critical role of actin-associated proteins in smooth muscle contraction, cell proliferation, airway hyperresponsiveness and airway remodeling. Respiratory research, 16(1), 134.

Tee, Y. H., Shemesh, T., Thiagarajan, V., Hariadi, R. F., Anderson, K. L., Page, C., ... & Bershadsky, A. D. (2015). Cellular chirality arising from the self-organization of the actin cytoskeleton. Nature cell biology, 17(4), 445.

Whitford, D. (2013). Proteins: structure and function. John Wiley & Sons.

Yang, C., Ko, B., Hensley, C. T., Jiang, L., Wasti, A. T., Kim, J., ... & Rutter, J. (2014). Glutamine oxidation maintains the TCA cycle and cell survival during impaired mitochondrial pyruvate transport. Molecular cell, 56(3), 414-424.

Zhou, X., Liao, W. J., Liao, J. M., Liao, P., & Lu, H. (2015). Ribosomal proteins: functions beyond the ribosome. Journal of molecular cell biology, 7(2), 92-104.

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