Secondary, Tertiary, and Quaternary Structure

It is the sequence and types of amino acids and the way that they are folded that provides protein molecules with specific structure, activity, and function. Ionic charge, hydrogen bonding capability, and hydrophobicity are the major determinants for the resultant three-dimensional structure of protein molecules. The a-chain is twisted, folded, and formed into globular structures, a-helicies, and P-sheets based upon the side-chain amino acid sequence and weak intramolecular interactions such as hydrogen bonding between different parts of the peptide 

On a larger scale, the unique folding and structure of one complete polypeptide chain is termed the tertiary structure of protein molecules. The difference between local secondary structure and complete polypeptide tertiary structure is arbitrary and sometimes of little practical difference. 

Larger proteins often contain more than one polypeptide chain. These multi-subunit proteins have a more complex shape, but are still formed from the same forces that twist and fold 

aside from the covalently polymerized a-chain itself, the majority of protein structure is determined by weaker, noncovalent interactions that potentially can be disturbed by environmental changes. It is for this reason that protein structure can be easily disrupted or denatured by fluctuations in pH, temperature, or by substances that can alter the structure of water, such as detergents or chaotropes. 

RgurG 13 The a-cham structure of myoglobin illustrates the complex nature of polypeptide structure within proteins. 

Larger proteins often contain more than one polypeptide chain. These multi-subunit proteins have a more complex shape, but are still formed from the same forces that twist and fold the local polypeptide. The unique three-dimensional interaction between different polypeptides in multi-subunit proteins is called the quaternary structure. Subunits may be held together by noncovalent contacts, such as hydrophobic or ionic interactions, or by covalent disulfide bonds formed from the cysteine residue of one polypeptide chain being cross-linked to a cysteine sulfhydryl of another chain (Fig. 15). 

Hgure 15 Polypeptide chains may be bound together through disulfide linkages occurring between cysteine residues within each subunit. 

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Chapter 6 Proteins Secondary, Tertiary, and Quaternary Structure... 

Primary structure is determined, as we ve seen, by sequencing the pTotein. Secondary, tertiary, and quaternary structures are determined by X-ray crystallography (Chapter 22 Focus On) because it s not yet possible to predict computationally how a given protein sequence will fold. 

J 9 Describe the composition of proteins and distinguish their primary, secondary, tertiary, and quaternary structures (Section 19.13). 

Cross-linking constrains the conformational flexibility of biopolymers and, as a rule, stabilizes their secondary, tertiary, and quaternary structures against the denaturing effects of high temperatures.29 We used differential scanning calorimetry (DSC) to compare the heat-induced conformational transitions of selected RNase A samples that were characterized in Figure 15.2. A brief introduction to DSC is provided in Section 15.15.1 for those readers unfamiliar with this biophysical method. Trace 1 in Figure 15.3a is the heat absorption... 

The previous paragraph stated the most obvious structural role of calcium. However Ca2+, like several other ions, for example Zn2+ and K+, is also important in stabilizing a range of less rigid biological structures, such as membranes, and determining and maintaining the secondary, tertiary, and quaternary structures... 

PRIMARY, SECONDARY, TERTIARY AND QUATERNARY STRUCTURES OF PROTEINS... 

Primary, Secondary, Tertiary and Quaternary Structures of Proteins.47... 

Although PrP and PrP have identical amino acid sequences, they have profoundly different secondary, tertiary and quaternary structures. 

The aromatic rings in the protein absorb ultraviolet light at an absorbance maximum of 280 nm, whereas the peptide bonds absorb at around 205 nm. The unique absorbance property of proteins could be used to estimate the level of proteins. These methods are fairly accurate with the ranges from 20 p,g to 3 mg for absorbance at 280 nm, as compared with 1 to 100 p,g for 205 nm. The assay is non-destructive as the protein in most cases is not consumed and can be recovered. Secondary, tertiary and quaternary structures all affect absorbance therefore, factors such as pH, ionic strength, etc can alter the absorbance spectrum. This assay depends on the presence of a mino acids which absorb UV light (mainly tryptophan, but to a lesser extent also tyrosine). Small peptides that do not contain such a mino acids cannot be measured easily by UV. 

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