Biochemistry Histidine Properties

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Questions and Answers

What is the main role of histidine in proteins like hemoglobin?

Contributing to buffering capacity (correct)

Acting as a structural component

Facilitating oxygen transport

Serving as a source of energy

Which property uniquely distinguishes histidine among amino acids?

It is non-polar

It is the only amino acid that is acidic

It is the most abundant amino acid in proteins

It has the ability to ionize at physiological pH (correct)

Which amino acid has the one-letter symbol 'G'?

Glycine (correct)

Glutamate

Glycolic acid

Glutamine

What do the D and L designations refer to in amino acids?

Stereoisomers or enantiomers Signup and view all the answers

Why is glycine considered an exception among amino acids regarding chirality?

It has no asymmetric carbon atom Signup and view all the answers

Which letter is assigned to the amino acids aspartic acid and asparagine?

B Signup and view all the answers

What characteristic do stereoisomers of amino acids share?

They have the same connectivity but different orientations Signup and view all the answers

Which of the following amino acids can exist as both a positively charged and neutral form?

Histidine Signup and view all the answers

What is the primary structure of a protein?

The sequence of amino acids in the protein Signup and view all the answers

Which type of bond connects amino acids in a protein?

Peptide bond Signup and view all the answers

What is the dominant component of the amyloid plaque found in Alzheimer disease?

Amyloid β (Aβ) Signup and view all the answers

Which characteristic structure is formed by the aggregation of Aβ peptides?

β-pleated sheet conformation Signup and view all the answers

How are peptide bonds characterized structurally?

They have partial double-bond character Signup and view all the answers

What happens when amino acids are linked together through peptide bonds?

They create a polypeptide chain Signup and view all the answers

What are neurofibrillary tangles primarily composed of in Alzheimer disease?

Hyperphosphorylated tau protein Signup and view all the answers

What role does the amyloid precursor protein play in the formation of Aβ?

It is enzymatically cleaved to produce Aβ. Signup and view all the answers

What is the significance of knowing the primary structure of proteins?

It is essential for diagnosing genetic diseases Signup and view all the answers

Which of the following correctly describes how peptide chains are named?

All residues change suffix to -yl except the C-terminal Signup and view all the answers

Which disease is associated with the presence of prion proteins?

Creutzfeldt-Jakob disease Signup and view all the answers

What is the mechanism of the infectious agent causing scrapie in sheep?

It is a single protein species without nucleic acid. Signup and view all the answers

What condition is required to nonenzymically break peptide bonds?

Prolonged use of strong acids or bases at high temperatures Signup and view all the answers

What percentage of Alzheimer disease cases is considered familial or genetically based?

5% Signup and view all the answers

What role do peptide bonds play in protein functionality?

They maintain the integrity of the primary structure Signup and view all the answers

How do neurotoxic amyloid plaques contribute to Alzheimer disease?

They generate cognitive impairments. Signup and view all the answers

What distinguishes antiparallel β-sheets from parallel β-sheets?

The N-terminal and C-terminal ends of the β-strands alternate. Signup and view all the answers

Which type of bond stabilizes β-bends in polypeptide chains?

Hydrogen and ionic bonds Signup and view all the answers

How do β-sheets contribute to the structure of globular proteins?

They always have a right-handed twist when viewed along the backbone. Signup and view all the answers

What is the role of β-bends in protein structure?

They reverse the direction of a polypeptide chain. Signup and view all the answers

What percentage of an average globular protein is typically organized into repetitive structures?

50% Signup and view all the answers

What characterizes nonrepetitive secondary structures in proteins?

They have a less regular structure than α-helices and β-sheets. Signup and view all the answers

Which amino acid is commonly associated with forming kinks in the polypeptide chain within β-bends?

Proline Signup and view all the answers

Which statements about intrachain bonds and interchain bonds is true?

Intrachain bonds are formed when a single chain folds back on itself. Signup and view all the answers

What is the primary advantage of the sigmoidal oxygen-dissociation curve of hemoglobin compared to a hyperbolic curve?

It allows for more efficient oxygen delivery across varying partial pressures. Signup and view all the answers

Which condition enhances the release of oxygen from hemoglobin according to the Bohr effect?

Decreased pH and increased pCO2. Signup and view all the answers

What role does carbonic anhydrase play in the oxygen transport mechanism?

It converts CO2 into carbonic acid in tissues. Signup and view all the answers

What effect does a higher concentration of protons (H+) have on hemoglobin’s affinity for oxygen?

It decreases the affinity for oxygen. Signup and view all the answers

What happens to the oxygen-dissociation curve of hemoglobin when pH decreases?

The curve shifts to the right. Signup and view all the answers

How does the pH differ between the lungs and metabolically active tissues?

The pH is higher in the lungs than in the tissues. Signup and view all the answers

What is the primary function of bicarbonate in the blood buffer system?

To maintain a stable pH level. Signup and view all the answers

What is the main reason why myoglobin cannot effectively deliver oxygen to tissues like hemoglobin?

Myoglobin maintains high affinity across a range of oxygen concentrations. Signup and view all the answers

Study Notes

Histidine and Its Role in Proteins

Histidine is weakly basic and largely uncharged at physiological pH.
When incorporated into proteins, its R group can be positively charged or neutral, depending on the surrounding ionic environment.
Unique property of histidine allows it to play a crucial buffering role in proteins like hemoglobin.
Only amino acid with a side chain that can ionize within the physiological pH range.

Amino Acid Abbreviations and Symbols

Each amino acid has a unique three-letter abbreviation and a one-letter symbol.
One-letter codes prioritize unique letters for single amino acids (e.g., V = valine).
More common amino acids receive priority when multiple have similar initials (e.g., G = glycine).
Amino acids with similar-sounding names may take easily recognizable symbols (F = phenylalanine).
Remaining amino acids receive symbols close to their initial letter (K = lysine; B = Aspartic/Asparagine; Z = Glutamic/Glutamine; X = unidentified).

Optical Properties and Chirality of Amino Acids

Amino acids have an α-carbon attached to four different groups, making them chiral, except glycine (which has two hydrogens).
Exist as D and L forms, being stereoisomers that are mirror images.

Primary Structure of Proteins

The primary structure is the sequence of amino acids in a protein, crucial for understanding genetic diseases.
Peptide bonds link amino acids covalently, forming dipeptides (e.g., valylalanine).
Peptide bonds resist denaturing conditions; require strong acids/bases at high temperatures to break.

Characteristics of Peptide Bonds

Peptide bonds possess partial double-bond character, resulting in a rigid and planar structure.
β-sheets can be formed by either parallel or antiparallel arrangements of polypeptide chains, stabilized by hydrogen bonding.

β-Bends and Nonrepetitive Secondary Structures

β-Bends, typically containing four amino acids, reverse chain direction and create compact, surface regions on proteins.
Nonrepetitive structures constitute roughly half of globular proteins and are less regular but not random.

Amyloid Aggregation in Diseases

Accumulation of amyloids associated with diseases like Alzheimer's, Parkinson's, and Huntington's.
Alzheimer’s features amyloid β (Aβ) peptide, which is neurotoxic and causes cognitive impairment.
Aβ aggregates through enzymic cleavage of amyloid precursor protein and deposit in the brain.
Non-genetic factors predominate in most Alzheimer cases, while tau protein abnormalities contribute to neurofibrillary tangles.

Prion Diseases

Prion protein (PrP) is linked to transmissible spongiform encephalopathies (e.g., Creutzfeldt-Jakob disease).
Infectivity of prion diseases is associated with PrPSc, a misfolded form devoid of detectable nucleic acid.

Oxygen Transport and the Bohr Effect

Hemoglobin operates with a sigmoidal oxygen-dissociation curve, facilitating efficient oxygen delivery.
The Bohr effect describes how lowered pH or elevated pCO2 enhances oxygen release by decreasing hemoglobin's affinity for oxygen.
In metabolically active tissues, CO2 is converted to carbonic acid, lowering pH and promoting oxygen unloading, while higher pH in lungs favors oxygen loading.

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Description

This quiz explores the properties of histidine, an amino acid that exhibits unique buffering capabilities in proteins like hemoglobin. You'll learn about its ionic environment and how it can exist in either a charged or neutral form at physiological pH. Test your understanding of this essential biochemical component.