Protein Metabolism and Structure

Study Notes

Protein Metabolism Overview

Proteins are complex polymers of amino acids, essential for all living organisms, containing carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur, phosphorus, zinc, copper, and iron.
Amino acids (a.a) are the building blocks of proteins, featuring an amino group (NH2) and a carboxyl group (-COOH).
Protein molecules have a high molecular weight, ranging from 5,000 to 2,500,000 Da.

Amino Acids and Protein Structure

Peptides consist of short chains of amino acids, while polypeptides are longer chains formed through peptide bonds between amino acids by removing water (H2O).
R groups (side chains) determine the properties of amino acids and may be hydrogen, aliphatic, aromatic, or heterocyclic.

Biomedical Importance of Proteins

Serve as structural components of the cytoskeleton providing mechanical support.
Act as biochemical catalysts (enzymes).
Involve contractile proteins vital for muscle contraction and relaxation.
Function in defense against infections (immunoglobulins).
Essential for respiratory functions (e.g., hemoglobin).
Can be catabolized to provide energy.

Types of Amino Acids

Essential amino acids cannot be synthesized by the body and must be obtained through diet (e.g., valine, leucine, isoleucine).
Semi-essential amino acids, like arginine and histidine, are not synthesized in sufficient quantities during growth.
Non-essential amino acids can be synthesized by the body.

Digestion and Protein Breakdown

Proteolytic enzymes for protein hydrolysis are secreted by the stomach (HCl and pepsinogen), pancreas (trypsin, chymotrypsin, elastase), and small intestine (aminopeptidase).
Unlike carbohydrates and fats, amino acids are not stored, requiring supply from diet, synthesis de novo, or normal protein degradation.

Key Concepts in Protein Role

Amino Acid Pool: Refers to free amino acids present throughout the body in cells, blood, and extracellular fluids.
Protein Turnover: Continuous process of protein synthesis and degradation, maintaining a stable total protein amount in healthy adults.

Pathways of Protein Metabolism

Anabolism: Involves synthesis of tissue proteins, blood proteins, enzymes, hormones, and non-protein nitrogen compounds (e.g., creatinine, urea).
Catabolism: Involves breakdown pathways including transamination, oxidative deamination, decarboxylation, urea formation, and nitrogen utilization.

Major Processes of Amino Acid Catabolism

Transamination: Transfer of amine groups between amino acids and keto acids, forming new amino acids. Enzymes: GOT and GPT.
Oxidative Deamination: Removal of amino groups to create α-keto acids and ammonia; catalyzed by amino oxidase.
Decarboxylation: Removal of carboxyl groups to produce biologically active amines; involves decarboxylase.

Urea Cycle

The urea cycle occurs primarily in the liver and kidney, converting ammonia to urea through five key reactions:
- Carbamoyl Phosphate Synthesis: Ammonium ions react with carbon dioxide to form carbamoyl phosphate, catalyzed by carbamoyl phosphate synthetase I; consumes 2 ATP.
- Citrulline Formation: Carbamoyl phosphate combines with ornithine to form citrulline.
- Argininosuccinate Formation: Citrulline reacts with aspartate to produce argininosuccinate; ATP is consumed.
- Argininosuccinate Breakdown: Cleavage results in the formation of arginine and fumarate.
- Urea Formation: Arginine is converted to urea and ornithine by arginase, allowing ornithine to reenter the cycle.

Description

This quiz covers the basics of proteins, their composition, and structure. It also explores the fundamental units of proteins, amino acids, and their characteristics. Test your knowledge of peptides, polypeptides, and more!