Biology: Proteins in Biological Systems

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Questions and Answers

What is the significance of the colloidal nature of proteins in biological systems?

They have a heavier density than water, allowing them to float.

They can form stable suspensions in solutions, affecting biochemical interactions. (correct)

They convert into gas when heated, leading to energy release.

They are primarily used for energy storage.

Which of the following classifications of proteins is distinctly characterized by their structural role for support and protection?

Catalytic proteins

Genetic proteins

Fibrous proteins (correct)

Transport proteins

Which type of proteins is known to perform metabolic regulation and maintain proper pH levels within cells?

Regulatory proteins (correct)

Defensive proteins

Transport proteins

Structural proteins

What defines the classification of proteins as complete proteins?

They have all essential amino acids in proper proportions. (D) Signup and view all the answers

What is characteristic of globular proteins compared to fibrous proteins?

They are generally more compact and spherical. (D) Signup and view all the answers

What is the primary significance of the tertiary structure of a protein?

It is crucial for the protein's function and is known as the active conformation. (B) Signup and view all the answers

Which interactions help stabilize the quaternary structure of proteins?

Ionic, hydrogen, hydrophobic interactions, and disulphide bridges (B) Signup and view all the answers

In sickle cell anemia, how does the substitution of glutamic acid by valine affect hemoglobin?

It reduces oxygen-carrying capacity and causes a clumping effect. (A) Signup and view all the answers

How many oxygen molecules can each hemoglobin molecule carry?

4 (D) Signup and view all the answers

Which of the following statements is true regarding simple proteins?

They contain only amino acids and no other components. (D) Signup and view all the answers

What is the relationship between primary structure and protein function?

A change in the primary structure can lead to the loss of function. (A) Signup and view all the answers

What does the term 'native conformation' refer to in proteins?

The three-dimensional structure that the protein assumes in its natural state. (B) Signup and view all the answers

What is the primary function of proteins in living cells?

Cell structure and function (D) Signup and view all the answers

Which of the following statements about amino acids is true?

There are 21 standard amino acids that form proteins. (B) Signup and view all the answers

What determines the three-dimensional structure of a protein?

The sequence of amino acids. (D) Signup and view all the answers

Which amino acid is incorporated through special translation mechanisms?

Selenocysteine (B) Signup and view all the answers

What characteristic is shared by all standard amino acids?

They possess an α-amino group and an α-carboxyl group. (C) Signup and view all the answers

What is the significance of pyrrolysine in relation to amino acids?

It is often regarded as the 22nd amino acid but not utilized by humans. (D) Signup and view all the answers

Which group is attached to the α-carbon of all common amino acids except glycine?

Four different groups (A) Signup and view all the answers

What does the term 'proteogenic' refer to in amino acids?

Amino acids that are coded in the genome for protein assembly. (D) Signup and view all the answers

Which characteristic ensures that amino acids can participate in peptide bond formation?

Presence of free α-NH2 and α-COOH groups (C) Signup and view all the answers

Which of the following amino acids is not coded in the standard genetic code?

Selenocysteine (B) Signup and view all the answers

Study Notes

Proteins

Proteins are the most abundant macromolecules in living cells.
These are nitrogenous organic compounds primarily composed of carbon, hydrogen, oxygen, and nitrogen.
In 1838, the term protein was derived from the Greek word "protos," meaning "primary" or "first."
The basic monomers of proteins are L-α-amino acids linked together by strong covalent peptide bonds.
Each protein has a unique sequence of amino acids that determines its three-dimensional structure and its specific biological function.
There are approximately 300 naturally occurring types of amino acids, but only 21 are involved in the formation of proteins—also called primary, proteogenic, standard or basic amino acids.
The rest are non-standard amino acids that are not used in the human body.
All 21 amino acids are biologically crucial, and their deficiency would interfere with protein synthesis.
The a–form of an amino acid is characterized by an amino group (-NH2) attached to a carbon atom (C) adjacent to a carboxyl group (-COOH).
Except for glycine, every amino acid has four different groups attached to a-carbon.
All amino acids possess a free a-amino (-NH2) group and a free α-carboxyl (-COOH) group.
Proteins have the L-configuration, which is defined by levorotatory counterclockwise behavior based on their optical activity.
Proteins form peptide bonds via the carboxyl group of one free amino acid and the amino group of another. This process is a dehydration reaction, releasing water.
Amino acids exhibit varying properties due to differences in their side chains (R groups).
Amino acid properties include shape, size, charge, hydrogen-bonding capacity, hydrophobic characters, and chemical reactivity.
Amino acids can be classified based on polarity and chemical nature. Polarities of R-groups can be hydrophobic or hydrophilic.

Levels of Protein Structure

Proteins have four levels of structural organization
- Primary: The amino acid sequence of a polypeptide chain (order). The backbone, [-N-Ca(R)-C(O)-] is repetitive. Only the R group side-chains vary.
- Secondary: Folded structures within a polypeptide chain. Includes alpha helices and beta-pleated sheets. Hydrogen bonds keep the secondary structures shaped.
- Tertiary: The final three-dimensional shape of a protein. Hydrophobic interactions between R groups primarily drive the structures. Other interactions such as ionic interactions, hydrogen bonding and covalent bonding via disulfide bridges create the final protein shape,
- Quaternary: The association of multiple polypeptide chains to form a functional protein complex. Can involve hydrophobic interactions, ionic interactions, hydrogen bonding, and covalent bonding (disulfide bridges).

Classification of Proteins

Simple: Contain only amino acids. Examples include albumin, globulin, globin, prolamines, histones, and protamines.
Conjugated: Contain a non-protein part (prosthetic group). Examples include nucleoproteins, mucoproteins, glycoproteins, lipoproteins, proteolipids, hemoproteins, and metalloproteins.

Properties of Proteins

Colloidal particles: Exist as colloidal particles, heavier than water in size varying from 5–100 µm.
Osmotic pressure: Protein concentration influences osmotic pressure.
Oncotic pressure: Generated by large proteins in blood plasma (e.g., albumin). Maintains fluid balance.
Solubility: Globular proteins are generally more soluble than fibrous proteins, Smaller molecules generally more soluble than larger.
Shape: Proteins like insulin are globular (sphere-like), albumin is oval-shaped, and fibrinogen molecules are elongated.

Amphoteric Nature & Isoelectric Point (pI)

Amphoteric: act as both acids and bases due to amino and carboxyl groups
Isoelectric Point (pI): The pH at which a protein is electrically neutral (net charge equals zero). This is crucial for protein solubility and behavior during electrophoresis.

Precipitation of Proteins

Factors affecting protein stability include charges, hydration of proteins
Techniques for protein precipitation include: Salting out (using neutral salts). • Isoelectric precipitation (at specific pH values). • Precipitation by organic solvents. • Precipitation by heavy metal ions and alkaloidal reagents.

Denaturation of Proteins

Denaturation: Protein unfolding caused by disruptive interactions (ionic bonds, hydrogen bonds, hydrophobic interactions), which alters its shape and function.
Factors leading to denaturation: heat, changes in pH, exposure to chemicals, mechanical stress.
Denaturation can be reversible or irreversible

Digestion & Absorption of Dietary Proteins

Digestion: Process of breaking down proteins into smaller peptides and amino acids.
Locations and mechanisms of digestion include the mouth, stomach, small intestine, through enzymatic cleavage by pepsin and others. The process usually involves several steps culminating in single amino acids or small peptides.
Absorption: Absorption of amino acids and short peptides into the bloodstream via active transport.
Different types of proteins require different types of enzymatic digestion, which is essential for the overall absorption of essential amino acids.

Metabolism of Proteins

Utilization of amino acids for energy, synthesis of new proteins, or the creation of other molecules.
Catabolism of proteins into amino acids when glucose or fat is not available as energy sources
Catabolism: a process where the body takes proteins and transforms them into energy through the use of various processes.
The process culminates in urea synthesis and excretion in the urine
Synthesis or further transformation and incorporation of amino acids into specific metabolites such as hormones, neurotransmitters and other compounds.

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Description

This quiz explores the significance of proteins in biological systems, including their colloidal nature, classification, structural roles, and functions. Test your knowledge on complete proteins and the differences between globular and fibrous proteins.