Proteins

Structure and Composition of Proteins

Proteins are essential biological macromolecules composed of long chains of amino acids. There are 20 different amino acids used in human proteins, each with a central carbon atom bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a unique side chain (R group). The human body can synthesize 11 of these amino acids, while the remaining 9 must be obtained from the diet (essential amino acids).

• Amino acids are the monomers of proteins.

• Polypeptides are chains of 3–100 amino acids joined by peptide bonds via dehydration synthesis.

• Chains longer than 100 amino acids are typically considered proteins, which fold into complex structures for specific functions.

Functions of Proteins in Humans

Proteins perform a wide variety of functions in the human body, including:

• Structural support (e.g., collagen in connective tissues)

• Muscle contraction (e.g., actin and myosin in muscle fibers)

• Cell membrane components (e.g., receptors, channels, and transporters)

• Enzymes that regulate biochemical reactions

Enzymes

Role and Mechanism of Enzymes

Enzymes are specialized proteins that act as biological catalysts, speeding up chemical reactions without being consumed or permanently altered. They are crucial for sustaining life, as many metabolic reactions would proceed too slowly without them.

• Catalyst: A substance that increases the rate of a chemical reaction.

• Enzymes lower the activation energy required for reactions.

• Enzymes are highly specific for their substrates.

• Example: Humans can digest glycogen and starch due to specific enzymes, but lack the enzyme to digest cellulose.

Chemical Reactions in Macromolecule Synthesis and Breakdown

Dehydration Synthesis and Hydrolysis

Macromolecules are assembled and disassembled through two main types of chemical reactions:

• Dehydration synthesis (condensation): Joins monomers by removing a water molecule, forming a covalent bond.

• Hydrolysis: Breaks covalent bonds by adding a water molecule, splitting polymers into monomers.

These reactions are catalyzed by specific enzymes, ensuring efficient metabolism and cellular function.

Nucleic Acids

Types and Functions of Nucleic Acids

Nucleic acids are macromolecules that store and transmit genetic information. The two main types are:

• Deoxyribonucleic acid (DNA): Contains the genetic blueprint for the synthesis of RNA and proteins.

• Ribonucleic acid (RNA): Functions in the expression of genetic information and protein synthesis.

DNA and RNA are interrelated: DNA encodes instructions for RNA, which in turn directs protein synthesis.

Structure of Nucleic Acids

Nucleic acids are polymers of nucleotides. Each nucleotide consists of:

• A five-carbon sugar (deoxyribose in DNA, ribose in RNA)

• One or more phosphate groups

• A nitrogenous base (single or double ring structure)

There are four different nucleotides in DNA (adenine, thymine, cytosine, guanine) and four in RNA (adenine, uracil, cytosine, guanine).

DNA Structure

DNA is composed of two antiparallel strands forming a double helix. The strands are held together by hydrogen bonds between complementary bases:

• Adenine (A) pairs with Thymine (T)

• Cytosine (C) pairs with Guanine (G)

The backbone of each strand is formed by covalent bonds between the sugar and phosphate groups of adjacent nucleotides.

RNA Structure

RNA is typically single-stranded and contains ribose as its sugar. Uracil (U) replaces thymine as a base. RNA molecules are shorter than DNA and serve as templates for protein synthesis.

• Single-stranded

• Contains uracil instead of thymine

• Functions as messenger RNA (mRNA), transfer RNA (tRNA), or ribosomal RNA (rRNA)

ATP (Adenosine Triphosphate)

Structure and Function of ATP

Adenosine triphosphate (ATP) is a nucleotide that serves as the universal energy currency of the cell. It is structurally similar to an RNA nucleotide containing adenine, but with two additional phosphate groups, making a total of three phosphates.

• ATP stores potential energy in the bonds between its phosphate groups.

• Energy is released when the bond between the last two phosphates is broken by hydrolysis, forming ADP (adenosine diphosphate) and inorganic phosphate.

• ATP is regenerated from ADP and phosphate using energy derived from food or stored macromolecules.

Equation for ATP hydrolysis:

Additional info: ATP hydrolysis is coupled to many cellular processes, including muscle contraction, active transport, and biosynthesis.